Advancements in Sustainable Livestock Feed: Harnessing Drought-Tolerant Crops

1. Introduction

Climate change has brought devastating impact on agricultural activities that are central to humanity. Climate variability, such as unusual rainfall patterns and rising temperatures, negatively impact both food and livestock feed crops, leading to food and nutrition insecurity for both human and livestock [1]. Such developments have spurred research on alternative and sustainable livestock feed sources, with a focus on underutilized plants. The goal of this approach is to identify options that are simultaneously climate-resilient and nutritious for domesticated livestock. The selection of alternative feeds is intended to supplement existing feed sources during periods of shortage, while still ensuring optimal livestock performance parameters and improved profit margins for farmers. Livestock such as cattle, sheep, pigs, and chickens are economically crucial, providing nutritious food, income, and wealth significantly improving food and livelihoods of the citizens [2,3]. Therefore, solutions that support sustainable livestock production and alleviate the impact of feed shortages due to climate change are warranted.

The conventional livestock feed sources are not sufficient to mitigate feed shortages as their use and availability are further restricted by factors such as land and water use conflicts, feed cost inflations, and the continuous competition between humans and livestock for food sources [4]. The term “conventional feed” in this review covers a range of feedstuff that may differ depending on the region and farming system. Common, conventional feed sources include but are not limited to; forages (grasses and fodder), grains, oilseeds, soybean and sunflower [5]. Feed deficits, primarily due to the impact of climate change on these conventional feed sources, have resulted in the emergence of alternative sources of livestock feed including a wide range of products produced from drought-resilient crops, insects, tree leaves, agro-industrial fruit and vegetable wastes [6,7]. Among these different feed sources, drought-resilient crops are considered a more sustainable option, due to their availability during dry seasons, superior nutritional content, and suitability for small-scale farming systems [8,9].

The eco-friendly nature, biomass abundance, and affordability of drought resistant crops further enhance their appeal, particularly for small- to medium-scale livestock production businesses, especially in arid and semi-arid regions. Despite the highlighted benefits for livestock farming, drought resistant crops have been largely overlooked and underutilized as sustainable feed sources, limiting their usage during drought [10]. Limited utilization of drought resilient crops stems from limited available research that focuses on their inclusion in feed rations which can be an opportunity to provide farmers and feed industries with information that can help increase their options for feed alternative sources. Currently, in order to supplement livestock’ diets, farmers resort to importing feed, which is often more expensive and leads to unsustainable livestock production, especially those of small-medium status. Thus, addressing this important research gap is crucial, given the increasing prevalence of drought and the need for resilient and cost-effective livestock feed options.

Literature has highlighted the potential of several drought-resilient crops—sorghum (Sorghum bicolor (L.) Moench), pearl and finger millets (Pennisetum glaucum (L.) R. Br and Eleusine coracana (L.) Gaertn), cassava (Manihot esculenta Crantz), false banana (Ensete ventricosum (Welw.) Cheesman), and cactus pear (Opuntia ficus-indica (L.) Mill)—as sustainable alternatives for feed sources [11,12,13,14,15]. Findings from the above-mentioned studies demonstrate the need to consider drought resilient crops as valuable resources to support food and nutrition, food security and climate smart adaptation. However, the optimal quality of the feed from drought resistant crops warrants careful consideration of processing techniques, pre-harvest factors, and maturity stages. These factors interact to determine the nutritional value, digestibility, and overall suitability of the crops for livestock feed, providing critical information required by livestock feed processors. However, studies analyzing the synergistic effects on the quality of the feedstock are still limited.

This review critically discusses the role of key drought-resilient crops as alternative livestock feed, highlighting the main drivers such as crop usage, processing and application to improve livestock health. Furthermore, factors influencing their quality, challenges, and strategies to promote crop innovation as climate smart agriculture strategy is highlighted.

2. Main Drivers to the Use of Drought-Resilient Crops as Livestock Feed

The primary drivers for the increased use of drought-resilient crops as alternative livestock feed include the impacts of climate change, particularly the effect of drought on conventional feed sources, and the need for sustainable food production systems. Farmers are faced with reduced crop yields and quality due to extreme drought, leading to livestock feed shortages and increased costs of livestock production. Water scarcity and the rise in temperature have heavily impacted the production of conventional livestock feed sources resulting in a decline in livestock performance and productivity [16]. Various studies have reported on the negative effects of drought on livestock farming across the globe. For instance, Klinck et al. [17] reported a decline in forage and livestock production among small holder and semicommercial farmers in South Africa. Severe drought in the semi-arid regions of Ethiopia between 1985 and 2011 resulted in significant livestock deaths due to water and feed shortages [18]. In addition, Puerto Rico faced a severe drought that significantly hindered the growth of fodder and grasses, negatively affecting livestock production and farmers’ livelihoods [19].

Another example of a severe drought occurrence that caused undesired outcomes on livestock production is the 2015-2016 El Niño phenomenon which significantly impacted global crop and livestock productivity, with notable effects in regions like Southern African, South and Central American states [20,21,22,23]. This drought led to a drastic decline (8-78%) in maize production in Southern African nations such as Zambia, South Africa, Zimbabwe and Namibia (Figure 1A). In South and Central American countries like Bolivia, Brazil, Honduras, El Salvador, and Nicaragua, the drought resulted in 31-60% decrease in maize production (Figure 1B). The consequence of the El Niño drought significantly increased the prices of stockfeed, meat and other livestock products, impacting both farmers and consumers. Evidence from other drought related studies reported a 20-25% decrease of maize production in Thailand between 2019-2020, a 25.8% decline in livestock holdings and 8.4% decrease in milk production in Ethiopia, as well as reduced the nutritional value of crops like wheat, maize, and alfalfa [24]. Thus, beyond crop reduction, droughts cause serious disturbances to diets, with a significant depletion in protein content consumption. These factors highlight the far-reaching consequences of drought on food security and livelihoods. Faced with the highlighted challenges, integrating drought-resilient crops into livestock feeding is crucial for sustainable agriculture, especially for livestock productivity. However, understanding the mechanisms behind drought resilience and categorizing crops based on their drought response is essential for their effective utilization as stockfeed resources.

3. Classification of Drought Resistant Crops

The primary factor in categorizing crop varieties into drought-resilient classes is their ability to withstand different types and intensities of drought, coupled with their mechanism of action that enables them to execute such abilities. This classification aims to facilitate the selection of appropriate crops for sustainable agriculture, ensuring that the chosen varieties can thrive under the conditions in which they can be grown under. Although drought resilient ability of plants is a multifarious mechanism, literature has been able to summarize it into four broad categories- (i) drought avoidance, (ii) drought escape, (iii) drought tolerance and (iv) drought recovery. These classifications are derived from varying morphological, physiological and biochemical adjustments plants express to mitigate unwanted impacts caused by drought occurrences [25,26]. The drought resilience mechanism involves varying degrees of photosynthetic pathways, water use potential, antioxidant systems, regulation of hormones, leaf and root traits [27]. The previously noted categories are briefly explained as follows:

(i) Drought avoidance: Drought avoidance crops work by keeping water potential elevated at cellular levels enabling them to survive and function even with limited water availability [28]. This is achieved through a combination of drought avoidance and drought tolerance strategies.

(ii) Drought escape: These types of crops are known for rapid flowering which allows the completion of their life cycle before drought conditions become severe [27]. During this period, rapid plant organ development can be achieved due to high rates of metabolic processes that lead to efficient cellular growth and mitosis [29]. In addition to rapid flowering, these plants express early maturity and high photosynthetic rates [30].

(iii) Drought tolerance and recovery: These crops can withstand and survive in conditions of low water availability while maintaining the physiological processes necessary for growth and productivity [30,31]. This allows for the cultivation of drought-resistant crops in arid and semi-arid regions. These plants utilize various mechanisms to cope with water stress, ensuring survival and enabling economic viability.

From studying these various drought resilience mechanisms, certain plants have been reported to exhibit an outstanding recovery ability after experiencing severe drought [25]. It must be noted that drought occurrence and plant response mechanisms are a complex dynamic concept yet to be extensively researched. It has been reported that some plant species are able to execute integrated response mechanisms, signaling a broader adaptability during different drought intensities [25]. As such, the underlying mechanisms respective to each drought response strategy have been studied. Out of those varying biological processes, plants with phenomenal photosynthetic pathways have emerged as the next species for sustainable agricultural crop production. Therefore, the proceeding section discusses the types of drought tolerant crops with efficient photosynthetic pathways and their mechanisms of action against drought.

3.1. Types of Drought Tolerant Crops and Their Mechanisms of Action

Drought-tolerant crops are characterized by effective photosynthetic mechanisms that allow them to thrive in dry and hot conditions. Their ability to concentrate and fix carbon through the conversion of atmospheric inorganic carbon dioxide (CO₂) into essential organic compounds has been implicated in crop resilience against drought stress [25]. Based on the carbon fixation and water use efficiency (WUE) pathways, these crops are grouped into 3 distinct classes: C3, C4 and crassulacean acid metabolism (CAM) plants [32]. Although these three classes possess similar metabolites, such as malate and pyruvate, their carbon fixation efficiency varies, influencing their level of drought tolerance. Specifically, CAM plants possess the highest drought tolerance, when compared to C₃ and C₄ plant species [33]. Meanwhile, C₃ plants are generally considered to be least tolerant than C₄ [34]. Thus, C₃ plants can be considered less suitable for extremely drought-prone environments. Therefore, this review will concentrate on C₄ and CAM plants, exploring their unique drought tolerance mechanisms and potential use as alternative livestock feedstock in agricultural systems.

3.1.1. Drought-Tolerant Crops Utilizing The C₄ Photosynthetic Pathway

The C₄ plants such as sorghum are highly recommended for livestock farmers in arid and warm climates due to their superior drought resistance strength [35,36]. These plants convert atmospheric CO₂ into malate or malic acid—a four-carbon compound—in the mesophyll cell, hence the name C₄, and transfer it into bundle sheath cell for decarboxylation (Figure 2). In addition to this exceptional photosynthetic pathway, their ability to thrive in drought-stricken environments has been attributed to reduced interaction and reaction with oxygen molecules, as it leads to unwanted photorespiration, an inefficient process that can result in energy and water loss [37]. Therefore, the significance of C₄ drought-tolerant plants as alternative feed sources lies in their extended availability during dry seasons, making them crucial for mitigating feed shortages, particularly among smallholder farmers in arid regions and developing states. Additionally, their efficient water and nitrogen utilization enhances their suitability for pasture production [38]. Compared to C₃ plants, the C₄ pathway possesses a higher energy conversion efficiency, which enables them to optimize solar radiation for the production of essential biochemical compounds [38]. This high energy conversion capacity translates into increased carbohydrate content, which is crucial for maintaining livestock metabolism and overall productivity. Additionally, the C₄ pathway enables these plants to execute photosynthesis even during low CO₂ concentration, and the presence of ribulose-1,5-bisphosphate carboxylase/oxygenaserubi (Rubisco), as illustrated in Figure 2. This allows for efficient use of nitrogen [39], as nitrogen is a vital element needed in plant growth, protein build up, nucleic acid, amongst many other benefits. Some of the commonly used livestock feed belonging to C₄ crops include some variety of maize, and millets [40]. These crops not only serve as primary feed sources but also complement the existing feed formulations in livestock production systems, further enhancing their utility in sustainable livestock nutrition [41].

3.1.2. CAM Drought-Tolerant Plants

Even though CAM and C₄ plants share similarities in their carbon fixation processes, CAM plants stand out because of their superior water conservation properties and their ability to completely prevent wasteful photorespiration [43]. Hence, through evolutionary adaptation, CAM plants have developed an efficient survival mechanism that enables them to thrive in extremely arid environments [33]. Their photosynthetic pathway operates through two primary processes: acidification at night and deacidification during the day, as illustrated in Figure 3. This unique mechanism allows CAM plants to open their stomata at night to absorb CO₂ while reducing water loss, making them highly efficient in moisture retention. Acidification occurs by fixing CO₂ when reacting with phosphoenol pyruvate (PEP); a process catalyzed by PEP carboxylase to form oxalo acetic acid (OAA). OAA is converted to malic acid that is later stored in vacuoles. During the day, the stored malic acid is converted to pyruvate that is transported to the mitochondria where it is catalyzed to OAA, by pyruvate carboxylase. OAA will enter the Krebs cycle to generate energy for the needs of the plant. It is worth noting that the stomata open at night and closes during the day.

Their water-use efficiency is further enhanced by root hydraulic conductivity, which, under wet soil conditions, can be up to six times higher than in other plant species [44]. These types of plants include Opuntia ficus-indica, Ananas comosus (pineapple), and Agave species which have gained industrial and agricultural significance in the recent years [45,46,47]. While these crops may have lower protein content than conventional feed sources, they are rich in vitamins, carbohydrates, and essential minerals, making them valuable in producing hybrid feeds [47]. Gusha et al. [48] report that CAM plants contain five times higher dry matter (DM) than other plants, which enhances their suitability as livestock feed. The Food and Agricultural Organization (FAO) associated publication recognizes and promotes CAM plants like Opuntia ficus-indica as valuable resources for nutritional properties and recommends their integration into livestock feeding systems as a sustainable alternative feed source during droughts and dry seasons [49,50]. This demonstrates how underutilized plants have garnered support across the globe and their potential to revolutionize the livestock feeding systems.

4. Recent Studies on The Utilization of Key Drought-Tolerant Crops in Livestock Feeding Systems

Several drought-tolerant crops are crucial in livestock feeding systems, particularly in arid and semi-arid regions. Sorghum (Sorghum bicolor), millets (Pennisetum glaucum and Eleusine coracana), false banana (Ensete ventricosum), and cactus pear (Opuntia) are notable examples, discussed in this section. The effect of processing on these crops and impact of the processed product on livestock nutrition and health is also discussed.

4.1. Sorghum Bicolor (L.) Moench

Sorghum (Sorghum bicolor) (Figure 4A) is a prominent drought-tolerant crop, recognized as the most widely cultivated crop for livestock and ranked fifth globally in production, with approximately 65 million tons produced annually [52]. This crop grows well in arid regions of different continents with Nigeria, Zimbabwe, India, China, United States, Mexico, Sudan, Ethiopia, Brazil, Argentina and Australia ranked as the top producers [53]. Besides its’ drought tolerant strength, sorghum is further appreciated for its rich nutritional composition, containing 9.5-10.4% protein, 1.5% fat, and 6.8% ash, which makes it a better alternative to other feed sources [15,54]. In addition, sorghum is a shorter seasonal crop, and its various parts (leaves, straws, and byproducts) are valuable as supplemental sources for livestock feed rations [54]. As shown in Table 1, various studies have demonstrated the potential of using different parts of sorghum, for livestock feed applications. Yusriani et al. [54] reported that using the ensilage technique can assist in extending the postharvest lifespan of sorghum leaves and stems which showed a positive response in livestock feed applications. Within the same study, sorghum grains were seen as a beneficial addition to chicken feed, potentially increasing survival rates at a 30% inclusion level compared to a control group.

Sweet sorghum (12%) produced higher villus heights on the intestinal compartments’ duodenum, ileum and jejunum of male geese [55]. Villus heights of the intestines are key indicators linked to nutrient absorption, as such, the greater the height, the more efficient the nutrient absorption process becomes, which signifies the quality of the feed produced [56]. Sorghum was also used to replace maize in broiler diets where positive results relating to feed intake, feed conversion and live weight gain during the early days of rearing were observed. The tested inclusion rates were noted to increase weight gains between day 21-48 [15]. In addition, the incorporation of sorghum in broiler feed at 30% increased broiler survival rates up to 95.6% compared to the control diet that produced 93.3%. With a similar approach of replacing maize with sorghum, Pontieri et al. [14] demonstrated that sorghum is a viable option for lactating cows and does not compromise livestock performance. High production of compounds such as succinic acid, 2-ethylacrylic acid and glutamic acid were observed during lactation in cows fed with sorghum diets [15,57]. These chemical compounds positively contribute to the quality and flavor of milk products. Besides the nutritional and health benefits, the inclusion of sorghum in livestock is related to its lower costs of production [14]. All these factors put together make sorghum a viable and sustainable crop for livestock feed. Additional studies demonstrating the potential of sorghum as livestock feed are presented in Table 1.

4.2. Pennisetum Glaucum (L.) R. Br and Eleusine Coracana (L.) Gaertn)

Pearl and finger millets (Pennisetum glaucum and Eleusine coracana) have been extensively studied as drought-tolerant forages, with various species demonstrating significant utility in livestock feed applications. Their application in the feed industry is attributed to the presence of desirable nutritional properties and bioactive constituents, which renders them ideal for inclusion in feed formulations [58]. For instance, a comparative analysis study by Gowda et al. [59] revealed that finger millet straw contains elevated levels of CP and various minerals, including phosphorus, calcium, copper, and magnesium, better than conventional rice straws. Several millet varieties are documented for livestock feed, with Eleusine coracana and Pennisetum glaucum being the most researched (Figure 4B and 4C), while millet husks, a byproduct of millet processing, are also less studied [60,61]. These two types are recognized as the predominant groups within the millet category, as they account for a substantial proportion of millet production and trade [58]. Mugula et al. [62] established that Pennisetum glaucum grains could fully replace maize in the diets of confined cattle, leading to increased DM digestibility and nutrient availability for grazing beef cattle during the dry season. Replacement of corn with various levels of Pennisetum glaucum resulted in an increased weight gain when fed to broilers [63].

A comparative evaluation between milled and grain diets made from a Gayamba Pennisetum glaucum showed that whole grains performed better than milled grains, when fed to chickens for 49 days [64]. During the finisher phase, dry feed intake of the treatment that contained 15% whole grain millet increased to 118.77 g compared with 45.18 g recorded during the starter feeding stage. This result is indicative of the adaptability whole grain pearl millet offers to broiler diets. The increase in dry feed intake was also linked to the increase in daily weight gain as well as feed conversion ratio (FCR). In another study, Oskey [65] demonstrated that Brown Midrib (BMR) pearl millet was preferred over conventional pearl millet when evaluating various nutritional parameters, including neutral detergent fiber (NDF), CP, sugars, and in vitro NDF digestibility. Among these parameters, BMR pearl millet exhibited superior performance in terms of in vitro NDF digestibility, achieving 66.3% compared to 63.6% from conventional pearl millet [65]. These results suggest high fiber in BMR pearl millet which was fermented by rumen gut microbiota into respective health beneficial byproducts. Using various inclusion levels, millet forms and enzyme addition in pearl millet-based diet, it was observed that 100% ground pearl millet diet resulted in increased intestinal weight of Japanese quails [66]. This was attributed to increased intestinal morphology due to enhanced functional nutrient absorption and digestion processes in the tested birds. In addition, high contents of fiber can also widen intestinal muscles to allow efficient breakdown of the fibrous components [66]. Other studies demonstrating the beneficial inclusion of pearl millet in livestock treatments with a focus on nutritional value, performance parameters, carcass and histological traits are presented on Table 1.

According to Gowda et al. [59] feeding finger millet straw to dairy cows resulted in a higher average daily milk yield (7.0 L) compared to feeding rice straw (6.3 L), enhanced digestibility of CP, DM, acid detergent fiber (ADF) and NDF. This indicates a potential benefit of finger millet for increasing milk production in cattle and nutrient metabolism. However, the same study found that incorporating finger millet into sheep diets negatively impacted digestibility coefficients including DM, organic matter (OM), total digestible nutrients and total carbohydrates. This implies that the effect of finger millet may vary depending on the livestock species and its digestive system [67]. Nonetheless, finger millet is still recognized for its high nutritional value, boasting rich protein content and essential minerals like zinc (Zn) [68].

Other studies evaluated various forms of finger millet-based diets for Mandya lambs, Sahelia goats and cattle [60,61,69]. Performance indices revealed high CP intake of 66% g per day, from the extruded finger millet-based diet, compared to those made from area sheath and maize cob with values of 56.91% and 60.31% g per day, respectively, when offered to Mandya lambs [61]. Varying proportions of finger millet in silage production proved that the inclusion of 44% finger millet produced reliable results with regards to milk quality and production [60]. According to Tong et al. [69] feeding cattle with millet straw combined with maize showed an increase in fungal population of Basidiomycota compared to those fed with corn only. The existence of this rumen fungi in sufficient amounts is associated with an enhanced ability to degrade fiber within the ruminant digestive systems [69].

4.3. Ensete Ventricosum (Welw.) Cheesman

False banana (Ensete ventricosum) (Figure 4D), particularly found in Ethiopia is a drought-tolerant crop used as a feed component for lactating livestock, especially during dry seasons. However, there is a lack of extensive research supporting its widespread use as livestock feed beyond Ethiopia and exploring its full impact on livestock health and farming systems. In spite of this, Ensete ventricosum exhibits characteristics suitable for livestock feed due to its high-water content (85-90%), elevated amino acid levels, protein content of 13%, crude fiber (20%), and sugar content of 10% suggesting that it can be used as fodder or silage [8]. Fiber and stalks obtained from false banana showed considerable amounts of protein, mineral composition (K and Fe) supporting its suitability as a fodder crop [70]. Fekade [71] studied the impact of feeding white leghorn layers and broilers with different rations of Ensete ventricosum replacing maize at varying proportions (Table 1). The study revealed that a 30% maize replacement provided more energy for the chickens leading to increased egg production and profit margins. In another study, various treatments including wheat bran and Ensete ventricosum were offered to Doyogena sheep in a randomized complete design approach and parameters such as feed intake, body weight change, feed conversion efficiency, digestibility and chemical composition were investigated. Notably, treatments that contained higher contents of Ensete ventricosum produced higher intake of ME and in vitro organic DM [24]. Given these promising findings, further studies on the use of Ensete ventricosum in livestock feed are needed to better understand and explore this underutilized drought resilient resource in various livestock feeding systems.

4.4. Manihot Esculenta Crantz

Cassava (Manihot esculenta) (Figure 4E), known for its drought tolerance, can be cultivated with minimal inputs in arid regions, making it a valuable crop for food and nutrition security. Its leaves and roots are the main components used for livestock feed, further supported by its ability to regenerate after defoliation caused by prolonged drought conditions. Moreover, the main cause of cassava cultivation is that it can be achieved without the inclusion of fertilizers, as outlined by OECD et al. [72]. Cassava can be effectively integrated into livestock feed systems, with various processing and nutritional enhancement techniques such as fermentation. It has been reported that solid state fermentation of cassava and its residues with Pleurotus ostreatus (Jacq.) P. Kumm (oyster mushroom) mycelium significantly increased protein content of the crop from 4.29 mg/100 g to 7.91 mg/100 g after the fermentation process [73]. A cassava foliage that included banana flour and grass hay resulted in increased presence of bioactive compounds in comparison with a foliage that lacked the inclusion of cassava. Amongst the chemical constituents analyzed, chromatographic analysis revealed the presence of isoginkgetin; a major occurring compound known to possess anti-inflammatory potency [74]. Certain novel bacteria, such as Citrobacter freundii 5519, isolated from cassava waste have been noted to possess the ability to reduce the occurrence of a toxic cyanide compound, consequently promoting livestock feed safety [75].

The potential of cassava leaves, roots, and byproducts as livestock feed has been studied (Table 1). Ogbuewu et al. [76] found that substituting maize with cassava on the diet fed to chickens significantly impacted FCR and average daily gain, with a growth performance of approximately 10%. Evidence from previous studies also showed positive effects of cassava on in vitro gas production, synthesis of volatile fatty acids, antipathogenic effect, carcass characteristics and digestibility parameters (Table 1). Evaluation of hematological indices from five-week aged chickens revealed a significant increase in white blood cells (lymphocytes) as result of including biodegraded cassava root compared to a treatment that contained maize. A 48-h biodegraded cassava root meal recorded lymphocyte content of 47% while the control produced 42% [77]. Replacing up to 50% of feed concentrates with cassava tops and roots in beef cattle diets is a viable option, as it maintains feed intake, nutrient digestibility, and rumen fermentation without negatively impacting average daily gain. [78]. Yellow feather chickens fed 15% cassava root meal showed similar growth performance (thigh and breast muscle mass) to control birds, indicating that the meal is an ideal partial feed replacement [79]. However, to prevent suboptimal growth performance and carcass parameters, the inclusion levels of cassava in feed diets must be optimized.

4.5. Opuntia Ficus-Indica (L.) Mill.

Cacti (Opuntia ficus-indica) species are emerging as a valuable, drought-tolerant livestock feed alternative, offering solutions for livestock health, environmental sustainability, and economic viability, particularly in arid and semi-arid regions. Their high-water content, ability to thrive in dry environments, and potential to reduce reliance on traditional, often water-intensive, feed sources make them a promising area of research and application in livestock feeding systems [80]. Among the family of cactus crops (Cactaceae), the prickly pear cactus is the most extensively studied species for its potential as a sustainable alternative feed source in livestock systems [46,81,82,83]. Extensive explorations of Opuntia ficus-indica (Figure 4F) for various applications, employing diverse plant parts, varieties, and methodologies have been done [82]. As a plant exhibiting remarkable survival and adaptability, different varieties of Opuntia ficus-indica have demonstrated the ability to sprout, develop cladodes, and achieve acceptable weight in watershed areas [84]. A proximate analysis of various Opuntia ficus-indica varieties revealed CP content ranging from 5.38% to 6.02% [85]. In their assessment of nutritional feed diets, Gebremariam et al. [86] observed a notable presence of CP and soluble carbohydrates in Opuntia ficus-indica, with values of 83 g/kg DM and 251 g/kg DM, respectively. These figures exceed those of conventional basal diets, which contain 76 g/kg DM and 130 g/kg DM. This disparity proves the value of Opuntia ficus-indica as an alternative feed source, given its adequate CP and soluble carbohydrate content, which may be ideal nutrients for gut microbiota, and the energy needs of livestock.

The effect of Opuntia ficus-indica as feed on various livestock has been demonstrated producing positive results (Table 1). In goats, incorporating cactus pear silage at a 42% inclusion rate improved ruminating efficiency and water retention while reducing overall water intake without negatively impacting livestock performance [87]. A reduction in water intake is an ideal output when using cactus pear, especially during drought when water supply in farming systems is significantly limited. In the same study, evaluations of livestock productivity indicated that the inclusion of cacti in diets can lead to acceptable weight gains and satisfactory feed intake. Furthermore, the partial substitution of corn grain with Opuntia ficus-indica peel powder at inclusion levels of 5%, 10%, and 15% in Cobb chicken diets resulted in improved body weight, feed intake, and FCR compared to the treatment without inclusion of the prickly peel powder [88]. The incorporation of Opuntia ficus-indica in livestock feed diets is supported by various studies [12,82,83,89,90]. A recent investigation by da Silva et al. [12] proved that cactus-based diets are able to significantly reduce methane and ammonia nitrogen production (Table 1) better than alfalfa diets. These results suggest that cactus-based diets are sustainable and environmentally friendly as methane and ammonia nitrogen are considered hazardous to the environment.

Chequer et al. [91] found that adding Opuntia ficus-indica mucilage, at varying concentrations, to a lactose egg yolk extender significantly improved boar sperm quality after freezing and thawing. The study observed enhanced sperm motility, viability, and membrane integrity, suggesting that the mucilage is a beneficial addition to livestock reproduction. Moreso, adding cactus cladode powder to calf feed at a feed rate of 5 g/day significantly reduced the pathogenic bacteria Escherichia coli and Enterobacteriaceae, which are known to cause diarrhea in livestock. The antimicrobial effects were attributed to the high flavonoid content in the cactus cladodes [92]. While Opuntia ficus-indica is abundantly available in arid and semi-arid regions of Africa and could offer a solution to feed challenges, its use as a livestock feed, remains relatively under-researched. Thus, more research is needed to fully understand its role in livestock production, especially exploring the different components of the plants such as the fruit peel and various components from the cladodes such as fiber and mucilage. This strategy will assist in optimizing exploitation and realize the full potential of Opuntia ficus-indica in livestock feeding.

5. Factors Affecting the Quality and Functionality of Drought-Tolerant Crops

This section discusses pre-harvest and post-harvest factors that affect the quality and functionality of drought tolerant crops. It further highlights how pre-harvest factors such as cultivar and maturity stage, and postharvest factors like processing and storage techniques impact the quality and functionality of the drought tolerant crops as livestock feed. In addition, the section explores how these factors collectively inform the economic feasibility for small to medium-sized livestock production businesses, especially in developing nations, where the impact of drought is more severe. These factors provide the foundation for optimizing the quality of drought resistant crops-based livestock feed.

5.1. Preharvest Factors

5.1.1. Cultivar

While the effect of cultivar type on crop production is well-studied in horticulture, there is a noticeable research gap regarding its influence on livestock feed crops, particularly drought tolerant crops. Despite this, there is growing interest in developing drought-tolerant cultivars for sustainable livestock production, as cultivar choice can impact nutrition, livestock performance, and overall drought resilience. Miron et al. [111] compared three newly developed sorghum hybrids (Supersile 20, Silobuster, and Brown-Midrib Hybrid BMR-101) against a commercial variety (FS-5) and observed that the latter performed better than the experimental hybrids. In another study, a total of eight sorghum cultivars (Jawar-263, = F-1017, Jawar 2002, F-114, Hegari, Sandal Bar, Sorghum, MR-2011, Pak-China-1) were assessed for their fodder yield characteristics, revealing that parameters such as stem diameter, leaf weight per plant, DM yield, and forage yield were higher in the Puk-China and F-114 cultivars. Additionally, higher levels of crude fiber and CP were observed in the cultivars F-7017 and Sandal Bar [112]. Reports from Bean et al. [113], Pinho et al. [114], Neto et al. [115] and Sajimin et al. [116] have demonstrated the influence of cultivar on the nutrient composition, yield, digestibility, agronomic traits, and fermentation profiles of sorghum-derived forages. BMR cultivars exhibited the highest NDF digestibility, while cultivar BRS Ponta Negra showed high contents of WSC, produced greater contents of lactic acid compared to other cultivars [114]. Other studies showing the interaction between sorghum varieties, nutritional, chemical and digestibility parameters have been reported by Wahyono et al. [94] and Kapustin et al. [117].

Several cactus species can be used as livestock feed, including Opuntia lindheimeri Engelm, Opuntia ficus-indica (L.) Mill., Opuntia stricta Haw, Opuntia engelmannii Salm Dyck., Opuntia ellisiana, Opuntia rastrera Weber, Opuntia chrysacantha Berg, Opuntia amyclae, and Nopalea cochenillifera Salm Dyck. According to Dubeux et al. [81] Nopalea cochenillifera Salm Dyck. exhibited superior WSC, DM, and IVOMD. The replacement of Miúda (Nopalea cochenillifera Salm Dyck) with a newly engineered genotype known to be Orelha de Elefante Mexicana (Opuntia stricta Haw.) managed to linearly increase microbial protein even though other traits such as DM digestibility, CP, total digestible nutrients and OM decreased with an increase in the replacement proportion of the diets [118]. Conversely, the replacement with Orelha de Elefante Mexicana (Opuntia stricta Haw) contributed to the maintenance of milk production in Girolando cows with a recorded measure of 12.5 kg per day. Assessment of nutritional variation amongst Opuntia varieties species established that higher levels of pectin was relatively found in erect prickly pear (Opuntia stricta Haw) than in Gigante (Opuntia ficus-indica), IPA-20 (Opuntia ficus-indica), F-08 (Opuntia atropes Rose) and African Prickly Pear (Opuntia undulata) [119].

In a comparative assessment of genotypes within the Opuntia ficus-indica species and those of Nopalea cochenillifera, Ramos et al. [120] noted that the cultivars Tamazunchale, Negro Michoacan, and California from Nopalea cochenillifera, as well as Orelha de Elefante Mexicana and Amarillo, performed better with respect to dry yield, green mass and WUE. The superior performance of these cultivars can be attributed to the distinct genotypic traits, which include the number of cladodes per plant, cladode dimensions (length, width, diameter, and thickness), as well as overall plant height and width [120]. In this sense, it can be postulated that a greater expression of specific genotypic characteristics correlates with improved performance outcomes. After assessing the performance of cladodes from sixteen cultivars of Opuntia based on digestibility and proximate parameters, it was observed that the cultivars Aloqa, Garao, and Opuntia robusta var. X11 exhibited the highest levels of similarity (65-98%) when multivariate analysis was conducted [121]. In a proximate analysis of Opuntia species, Keyih (Opuntia ficus-indica spp.) had the highest DM content recorded to be 91.88%. Ash content was highest in Opuntia ficus spp. cultivars Lemats and Sihuna, while Opuntia stricta var. Mexicana had the lowest ash content. Opuntia ficus and robusta species showed the lowest OM content. Opuntia stricta var. wild had the highest NDF (67.05%), and Opuntia stricta var. Mexicana had the lowest (37.21%). The reported fiber values were above the recommended ones in South African livestock feed (28%), demonstrating the potential of these studied species and cultivars in enhancing livestock nutrition [122].

Alves et al. [123] analyzed seven Opuntia cactus cultivars, focusing on cladode characteristics to understand morphological and nutritional variations. The study observed significant differences in traits like cladode number, length, width, area, and thickness across the cultivars, highlighting their diverse nature. In a rainfed region, the Orelha de Elefante, F-08, and Gigante cactus cultivars demonstrated superior photosynthetic performance compared to other tested varieties (Orelha de Elefante Mexicana, V-19, Redonda, F-08, and Orelha de Elefante Africana, Clone IPA-20) after propagation and harvesting [123]. The larger the total photosynthetic area of the cladodes, the more efficient they are in energy production and bioavailability for the feed livestock. Mineral content, in moderate amounts, of a feed source is essential to the overall nutritional value of a feed diet. For instance, phosphorus is known to be crucial for skeletal development, cellular signaling and nucleotide build up, when found in adequate concentrations [124]. Alves et al. [123] found that phosphorus distribution in certain cactus pear cultivars (Giganta, Redonda, V-19, and F-08) is significantly influenced by their genetic differences, with these cultivars showing the highest phosphorus concentration at cladode order 4.

The studies discussed above have demonstrated that morphological traits are positively linked to both dry and fresh mass production, suggesting a strong connection between cultivar and livestock feed outcomes. Thus, breeding programs focusing on these variations could help address feed shortages through optimizing the desirable traits. In addition, cultivar and harvest maturity are closely related because the maturity of a crop at harvest is heavily influenced by its specific cultivar. Meanwhile, different cultivars of the same crop can mature at different rates and reach their peak quality at different times, suggesting that they should be harvested at different stages of maturity.

5.1.2. Harvest Maturity

The maturity of a crop is intrinsically linked to its biological processes, nutrient composition, physical development, and overall plant growth [125]. Consequently, harvest maturity is typically employed as an indicator to ascertain the optimal timing for harvesting of some crops intended for human consumption [126]. Numerous studies have explored the relationship between nutritional composition and harvest maturity in general crops, yet there has been comparatively less focus on drought-tolerant crops, particularly those intended for livestock feed.

Four drought-resistant sorghum cultivars (Early Sumac, Leotti, Nes, and Rox) exhibited varying performance when evaluated at distinct harvesting stages: panicle emergence, milky, dough, and physiological maturity [127]. The study identified the physiological maturity stage as the optimal time for harvest due to the high yield and superior fodder quality by sorghum cultivars [127]. This is probably because the physiological maturity stage represents a period of continued plant development post-harvest, which results in an increased yield and fodder quality. However, lower values of ADF (53.9%) and NDF (30%) were recorded during the milky to dough maturity stages, which justified the ensilage of sorghum BRS-610 to improve these attributes as these harvesting periods ensure acceptable fermentation and good nutritive value [128]. During the assessment of different maturity stages of sorghum silage and fodder, the hard dough and physiological maturity stages exhibited better nutritive value and fodder yield [129]. More studies demonstrating the influence of harvest maturity stages on sorghum cultivars have been published by Terler et al. [130], Silva et al. [131], Alatürk [132] and Hakim [133]. Three varieties (categorized according to silage, biomass and grain) commonly utilized for producing whole crop sorghum silage for ruminants were evaluated for their differences in nutritional values (Table 2). It was found that, across all three tested maturity stages, DMY was significantly greater in the variety classified under the biomass group compared to the other varieties [130]. Harvesting during the 3-week stage or boot stage resulted in higher CP and total digestible nutrient content, and lower fiber, while the dough stage yields more DMY but lower nutritional value [130]. Nonetheless, the study recommended the dough stage as the optimal maturity stage for enhancing nutritional levels in grain sorghum varieties.

Four maturity stages, spaced over three weeks—boot stage, flowering stage, and dough stage—had varying effects on the quality and yield of Johnsongrass. In this study, Silva et al. [131] observed that the first harvest at three-week and boot stage maturities were characterized by elevated levels of CP concentrations and total digestible nutrients. The study recommended that Johnsongrass should be ensiled before reaching the flowering stage as this is the optimal phase that produces desired silage qualities [131]. An assessment of the quality of sweet sorghum and sorghum sudangrass hybrid cultivars harvested at early and late growth stages revealed that dry yield increased with plant maturity, recording a remarkable 172.2% increase in forage at the late growth stage [132]. Silva et al. [131] identified significant agronomic differences between sorghum green fodder and maize harvested on days 6 and 12, with the latter yielding the highest plant biomass, while the former produced the lowest. Furthermore, Moura et al. [134] expanded the investigation into sorghum maturity stages by assessing DM intake, digestibility parameters, and methane production in sheep. Organic and DM digestibility were observed to be induced with an increase in maturity stage that was measured at day 121 of harvest.

Despite some studies reporting that the chemical and physical properties of Opuntia ficus-indica are influenced by various maturity stages, they do not provide additional insights regarding its application in livestock feed. This presents an important research gap that needs to be addressed. These studies can serve as a foundational basis for further investigation into the effects of maturity on Opuntia spp in the context of feed utilization. Research conducted by Juhaimi et al. [135] demonstrated that prickle pear fruits harvested at 15-day intervals from June 15 to August 15 exhibited significant variations in their fatty acid composition and bioactive compounds. Notably, the highest concentrations of phenolic compounds, measuring 156.77 ± 0.09 mg GAE/100g, were recorded from the harvest on July 1, after which a decline to 121.61 ± 0.09 mg GAE/100g was observed [135]. In contrast, antioxidant activity, as measured by inhibition against 2,2-diphenyl-1-picrylhydrazyl (DPPH), increased with advancing maturity. This discrepancy may be attributed to the role of other bioactive compounds, in addition to phenolics, that might also contribute to antioxidant activity. Furthermore, the fatty acid composition revealed the emergence of compounds such as linolenic, behenic, and erucic acids from July 1 harvest through to August 15 [135].

Cladodes from various Opuntia species, cultivated at different developmental stages for use in ruminant feeding, demonstrated that parameters such as DM, NDF, and ADF increased with maturity across all tested varieties, while CP exhibited an inverse relationship [119,136]. The elevated levels of NDF and ADF may indicate reduced digestibility of the feed, attributable to the composition of plant cell wall structures, specifically cellulose and lignin, which are typically resistant to breakdown by digestive enzymes and gut microbiota, especially in monogastric animals. A high DM content reflects the availability of other nutrients in the forage, which may enhance the overall nutritive value of the feed. Notably, young cladodes from Opuntia ficus-indica (IPA-20) and Opuntia atropes Rose (F-08) exhibited high mineral matter concentrations, with respective values of 95.5 g/kg DM and 133.5 g/kg DM. The erect prickly pear (Opuntia stricta Haw) also demonstrated substantial mineral matter content, recorded at 120.0 g/kg DM during the intermediate maturity stage [119]. Furthermore, the study conducted by [136] confirmed that the Opuntia stricta Haw cultivar contained significant phosphorus levels at the mature stage. Mineral matter analyses by Pessoa et al. [119] and specific phosphorus content assessments by Silva et al. [136] revealed that mineral content increased with maturity in African prickly pear cladodes. Maiuolo et al. [137] investigated the influence of three phenological stages of Opuntia ficus-indica cladodes on their antioxidant and anti-apoptotic properties. The findings indicated that early or young cladodes exhibited the lowest antioxidant activity compared to other developmental stages, while medium-aged cladodes demonstrated the highest antioxidant levels and these results were corroborated by the low DPPH inhibition values observed at the same developmental stage. Additionally, the maturity of Opuntia ficus-indica showed potential in mitigating apoptosis via the pretreatment of cells with extracts from late-aged cladodes, prior to exposure to lipopolysaccharides for 24 h, which resulted in increased cell survival [137]. These results suggest potential implications for Opuntia ficus-indica for enhancing livestock health at a cellular level.

5.2. Processing Factors

The quality of livestock feed from drought tolerant crops cannot be fully optimized without the consideration of postharvest management and processing techniques [138]. Properly managed postharvest practices, activities and carefully implemented processing techniques are crucial, as they serve as primary determinants of whether the desired qualities of products derived from fresh produce can be retained, enhanced or degraded [139]. As such, this section focuses on how postharvest factors such as storage, and processing factors such as pulverization, drying and ensiling impact the quality and functionality of the developed livestock feed products.

5.2.1. Drying Techniques

Numerous studies have demonstrated that drying techniques can impact on the quality attributes of various food and feed products. In the context of livestock feed production, however, the application of drying methods to drought tolerant crops appears to be less prevalent, as evidenced by the scarcity of studies addressing research on drying technologies applied in drought tolerant forage with the objective of improving feed quality. Among the available drying methods, sun-drying is the commonly utilized technique in the processing of livestock feed. The prevalent application of sun drying in most farming operations can be due to limited accessibility to innovative drying techniques and machinery. In spite of this, sun drying technique has long proven the capacity of producing livestock feed worth of desired qualities in various farming systems due to its convenience and access. The supplementation of sun-dried Atriplex halimus L. foliage with exogenous enzymes exhibited a superior recovery of secondary metabolites in the gastrointestinal passage when compared to fresh foliage [140]. This investigation posits that sun-dried foliage provides a greater substrate availability for enzymatic action than its fresh counterpart. Furthermore, the increased levels of secondary metabolites in sun-dried feed indicate enhanced bioavailability and bio-accessibility of compounds, which may confer various health benefits, including antiparasitic properties, improved immunity, and methane reduction, among others [140].

Ramsumair et al. [141] investigated the effect of five different drying techniques (oven drying at 70 and 60 °C, sun drying, shade drying and freeze drying) on drought-tolerant species including Trichantera gigantea (Bonpl.) C. Mart, Leucaena leucocephala (Lam.) de Wit, Morus alba L, and Gliricidia sepium (Jacq.) Walp. Their study recommended shade drying over sun drying for the preservation of feed quality, while freeze drying or oven drying were recommended for laboratory analysis of feed based on the differences in quality attributes observed [141]. Different drying methods produce varying results that could also be influenced by the type of crop processed, on parameters such as ash content, NDF, ADF and acid detergent lignin (ADL). For instance, the freeze-drying option led Morus alba to record the lowest values of NDF, ADF and ADL, compared to other plant species dried by the other techniques previously mentioned. Freeze-drying technique could not reduce the NDF of Trichantera gigantean as it was the highest amongst all other techniques with shade drying exhibiting the lowest value of this element. Sun drying was observed to lower the ADF on Trichantera gigantean species better than other methods. Oven drying at 60 °C produced the lowest ADF values when dehydrating Gliricidia sepium plants [141]. The study observed that the interaction between the drying technique and the type of plant species studied played a role in the quality of the final product. Even so, these drying techniques were able to reduce the content of fibrous elements in the samples studied, which is a key indicator of efficient digestibility of livestock forage.

Proving how a drying technique is a direct link to feed intake and digestibility, solar drying of Gliricidia sepium, Leucaena leucocephala, and Cenchrus purpureus (Schumach.) Morrone demonstrated greater palatability than fresh forage [142], while it exhibited lower DM content (17.86%) compared to sun-drying (21.47%) and oven drying (24.45%). These results allude to the fact that drying can enhance the composition of a forage or a feed. A study by Suwignyo et al. [143] found that oven-dried commercial alfalfa for poultry feeding had higher levels of amino acids compared to fresh alfalfa. Essential amino acids such as L-histidine, L-isoleucine and L-valine were relatively similar between the two treatments (oven-dried and fresh), suggesting that temperature (55 °C) did not compromise protein concentration. These findings suggest that a carefully chosen drying process can improve the nutritional composition of forage or feed. However, sufficient evidence to support such findings is lacking as research on drying technologies such as freezing drying, hot air drying, oven drying or their combination in livestock feed is limited.

This scarcity can be attributed to the inaccessibility of modern drying equipment in rural farmsteads and the high procurement costs associated with such machinery. Nevertheless, these contemporary drying methods are recognized as the most efficient and convenient agro-processing technologies capable of meeting the demands of the feed industry. For instance, freeze-dried wild cactus cladodes have demonstrated excellent dietary fiber values when processed into flour [144], even though the flour was not intended to be used in livestock feed formulation. Furthermore, freeze-dried wild cactus cladodes exhibited higher concentrations of phenolic compounds and flavonoids compared to other drying techniques, such as tunnel, fluidized bed, and spray drying [144]. Recent research by Ferreira et al. [145] indicated that the food dehydration method could recover higher phenolic content of 0.5 mg GAE/mL in prickly pear, compared to 0.16 mg GAE/mL achieved through microwave drying. Oven-dried (50 °C) cladodes from spineless Opuntia ficus-indica f. inermis and spiny Opuntia amyclae yielded comparable chemical compositions, volatile fatty acids, and gas production parameters during ruminal fermentation studies [146]. Although this investigation did not specifically emphasize the effects of drying concerning ruminant fermentation, it can be reasonably inferred that drying cactus cladodes at 50 °C yields acceptable nutritional values. Notably, a consistent increase in gas production was observed during the ruminant fermentation experiments between winter and summer oven-dried cladodes, with the highest recorded value of 136.8 mL/0.5g from the summer samples after a 72-h period.

Aruwa et al. [147] concluded that freeze-dried Opuntia ficus-indica pulp and peel exhibit superior antioxidant activities compared to oven-dried extracts. In the same study, the methanolic freeze-dried extract demonstrated an 18.8 mm zone of inhibition against gram-negative methicillin-resistant Staphylococcus aureus (MRSA), a microorganism recognized as detrimental to livestock health. The observed inhibition of MRSA by the freeze-dried peel supports the notion that drying technology enhances essential biological activities of Opuntia ficus-indica within the feed industry. Gouws et al. [148] reported superior performance of microwave-dried prickly pears over freeze-dried samples in terms of total phenolic content, recording 149 µg GAE and total flavonoid content of 76.6 µg CE. A recent comparative study of silage versus solar-dried cactus forage revealed that the latter is more beneficial, as solar-dried cactus forage exhibited higher protein content than ensiled forage while preserving excellent nutritional quality [149]. Studies by Pastorrelli et al. [46] and Maniaci et al. [150] elucidates the significance of dried Opuntia ficus-indica in ruminant diets and its substantial contribution to daily milk quality, particularly focusing on cladodes. It is important to note that the drying of drought-tolerant crops remains underexplored, despite the common practice of forage drying on farms. The limited research on the application of drying techniques in feed formulation underscores the need for researchers to initiate investigations aimed at improving forage quality through various methods, including freeze drying, oven drying, and solar drying, either individually or in combination.

5.2.2. Grinding and Pulverization

Mechanical processing during the postharvest phase, both before and after storage, significantly impacts the techno-functional properties, chemical composition, and biological integrity of livestock feed. Grinding has been used as the most suitable mechanical technique in feed technology since it converts a coarse feed into a finer particle sized powder through physical separation of the fibrous matrix that holds the material intact [151]. The influence of grinding feed from drought resistant crops has been studied in relation to feed intake, gut health, nutrient digestibility, growth performance, feed palatability and quality by several authors [152,153,154,155,156,157]. Overall, grinding or pulverizing produced positive results towards livestock such as poultry and pigs. For instance, sorghum and barley ground to 0.3-0.9 mm and 0.43- 1.10 mm particle sizes, respectively, managed to reduce stomach ulcers in pigs [153]. Pulverizing increases surface area which enhances availability of nutrients for the digestive system and commensal microbiota found within the gastrointestinal passage [158].

Feed particle size reduction through grinding was reported to reduce proventricular weight of chickens, preventing the occurrence of a deadly proventricular dilation disease that severely impairs nutrient absorption efficiency in poultry [154]. Research is also exploring the use of varying grinding technologies, and their combination that can turn livestock feed ingredients into powder with better functional properties to livestock [157,159,160,161]. Grinding plant waste materials such as beet pulp and sunflower husks have resulted in acceptable increments of essential amino acids and reduction of fiber to appreciable values [160]. In addition, using different designs of crushers (open and closed), has shown that changing sieve parameters has a significant impact on the quality of the processed grain [159]. A normally ground diet with low fiber received high feed intake (76.5 g/ day) compared to a low fiber coarse ground that expressed feed intakes of 67.4 g/day. Also, growth rate and liveweight per day were better in this dietary treatment during the weaning days of the tested rabbits [161]. These studies solidify the effect of various grinding and pulverizing methods on livestock feed, performance, and products.

5.2.3. Ensiling

Silage production, also called ensiling, is a long-standing feed preservation method that is crucial for maintaining forage quality and ensuring livestock feed, especially in areas affected by droughts [162]. Due to the adverse edaphoclimatic conditions exacerbated by drought, ensiling of drought-tolerant crops or other forages remains a critical strategy, particularly in agricultural operations that lack adequate processing machinery and storage facilities [163]. The ensiling process is facilitated by natural anaerobic fermentation phenomena largely dominated by lactic acid bacteria (LAB) and yeasts microorganisms during the four main phases, which are initial aerobic, intense anaerobic fermentation, stable and feed (Figure 5). These microbes are able to convert OM into various products, predominantly lactic acid, assisted by the absence of oxygen and an acidic pH environment [164]. In addition, ensiling converts unpalatable residues into health-beneficial chemical compounds for livestock, improving rumen degradability of cereal starch and allowing the amalgamation of different feed sources without compromising quality or introducing undesirable complexities [163].

The impact of ensiling on drought tolerant crops has been studied. Lima et al. [41] demonstrated that ensiling two varieties of sorghum in combination with soybeans yielded superior silage results by improving quality characteristics such as pH, fermentation acids and ammonia nitrogen calculated from total nitrogen, compared to silage produced solely from soybeans. These findings demonstrated that favorable fermentation parameters were significantly enhanced due to the incorporation of sorghum. The mixed silage produced a more favorable pH outcome (4.03) compared to soybean silage alone (5.47). Lower pH due to higher concentrations of lactic acid is generally preferred for proper silage quality [41]. The amount of ammonia nitrogen derived from total nitrogen was significantly less in mixed silage (3.22 gNH3 -N/100gN) than the silage made from soybeans only (18.4 gNH3 -N/100gN). It must be noted that high ammonia levels indicate potential toxicity of the feed. In addition, the duration of ensiling process has been studied to contribute to a significant influence on the final characteristics of silage. For instance, ensiling sweet sorghum for 30 days resulted in marked increase in DM and CP in comparison to fresh samples [165]. Further increasing the ensiling process to 120 days increased ADF and ADL, which decreased in vitro digestibility of the feed. Prolonged ensiling also enhanced the fermentation process, producing increased levels of lactic and acetic acid contents. In a silage prepared from stylo (Stylosanthes guianensis (Aubl.) Sw.), a drought tolerant crop commonly used for livestock nutrition, low levels of ammonia-nitrogen and pH after 45 days of ensiling were noted [166]. Optimization of this ensiling technique concluded that the addition of an engineered LAB xg significantly increased in vitro DM digestibility of the forage with values of 68.57% compared to control that had 59.78% [166]. Insignificant degrees of acetic acid are considered an excellent measure of good silage quality. Ensiling varieties of alfafa (Medicago sativa L.) have been proven to result in augmented chemical and fermentation qualities of the forage [167]. These studies demonstrate that ensiling duration and addition of supplements results in varying feed features that can be used to optimize nutritional characteristics of livestock feed.

It is worth highlighting that pathogenic bacteria may grow during ensiling process, and therefore developing additional precautionary measured is advised. In an attempt to address this, Forwood et al. [168] demonstrated that ensiling drought-tolerant sorghum in conjunction with various types of common vegetables such as pumpkin or carrot can foster a beneficial microbial population conducive to livestock health, while suppressing pathogenic microbes. The microbial diversity observed as a result of increased vegetable inclusion was predominantly characterized by Lactobacillus species, with a lesser fungal population that included Kazachstania humilis, Monascus purpureus, Issatchenkia orientalis, and Fusarium denticulatum [168]. High levels of Lactobacillus lead to high lactic acid production which is an efficient compound that can inhibit spoilage- causing microorganisms in livestock feed. Even so, it is crucial to emphasize that the existing literature on the ensiling of drought-tolerant crops underscores the need for further research focused on optimization, integration of diverse feed sources, prevention of microbial spoilage, and the influences of harvest maturity and ensiling duration. These factors may significantly enhance current ensilage practices.

Figure 6. Common silage making process where a crop or silo is collected and wilted for a period of 24-36 h before the addition of fermenting additives. Created using BioRender.

Figure 6. Common silage making process where a crop or silo is collected and wilted for a period of 24-36 h before the addition of fermenting additives. Created using BioRender.

5.2.4. Storage

Precise storage conditions after harvest are crucial for maintaining the quality of agricultural commodities because plant metabolism continues even after the produce is separated from the soil. Proper postharvest handling, including storage, significantly impacts the physiological state and shelf life of the produce [169]. As such, the fundamental aim of any postharvest storage method must be to delay physiological processes that lead to quality deterioration through the limitation of metabolic pathways and pathogenic invasion that favor quality decline, while extending shelf life and maintaining ideal nutritional value [170]. Therefore, the effects of various storage conditions on the quality attributes of drought tolerant crops require investigations. Temperature is a key factor in post-harvest storage, significantly impacting the nutritional value and safety of a feed. High temperatures can accelerate spoilage, reduce nutrient content, and even lead to the growth of harmful microorganisms, affecting livestock health and productivity, while low temperature slows down respiration and metabolic processes [171].

Wang et al. [172] discovered that low temperatures of 5 °C are able to prevent microbial spoilage in wet brewers’ grain by-products from barley that are usually used as livestock feed. Low temperatures have been reported to alleviate the growth and proliferation of Aspergillus parasiticus, known to cause the generation of aflatoxin in livestock feeds [173]. According to Mannaa et al. [174] temperatures around 8 °C are known to inhibit the growth of aflatoxins in grains during storage. During the storage of barley grain feed, high temperatures around 35 °C exhibited the worst surface spoilage after 36 h of storage. Further increase in temperature significantly decreased CP and WSC [172]. The study demonstrated that high storage temperatures can deplete amino acid content in livestock feed and further cause unwanted growth of fungi, bacteria and insects [175]. A temperature range of 26-37 °C was observed to cause insect infestation, particularly Rhyzopertha spp, in sorghum ingredients that are usually used as raw materials for cattle feed [175]. In addition to insect infestation, aflatoxins may also grow in feed stored under high temperatures posing a threat to livestock health, performance and productivity [176]. Kirigia et al. [177] has drawn conclusive evidence-based perception on how ambient temperature is one of the factors that principally contributes to the decline of quality attributes specific cowpea. During the analysis it was observed that storing cowpea leaves at ambient temperature range of 20-25 °C resulted in a decline of starch and sucrose while there were no noticeable changes observed under 5 °C storage temperature. Alternatively, ambient and high temperatures have been reported as safe for the storage of dry livestock feed sources. Using a closed system, a temperature range of 15-25 °C produced suitable nutritional parameters of lentil grains [178]. Based on the few studies found in literature, there is a noted research gap regarding the studies on effects of various storage conditions on the final quality of livestock feed made from drought tolerant crops.

6. Challenges of Using Drought Tolerant Crops as Livestock Feed

Drought-tolerant crops face significant challenges despite their resilience: they remain susceptible to disease infestation, are often characterized by low protein levels and high fibrous content and further suffer from low awareness which hinders their widespread adoption as climate-smart agricultural solution. The subsequent section of this review explores these limitations regarding the full integration of drought-resistant crops into daily farming operations.

6.1. Disease and Mycotoxins Infestation, and Feed Safety

Just like any type of agricultural crop, drought tolerant crops are vulnerable to disease-occurrences. For instance, Opuntia ficus-indica cladodes are unfortunately prone to diseases that are difficult to pinpoint to specific pathogens. The high-water content of these cladodes is believed to be a major contributor to disease susceptibility, as it creates a favorable environment for microbial growth [179]. Fungi such as Alternaria tenuissima, Lasiodiplodia theobromae, and Fusarium have been isolated from South African Opuntia ficus-indica cladodes, extensively studied and discovered to exhibit symptoms such as chlorosis, necrosis, and black gum exudation [179]. Molecular analyses of fungi isolated from Opuntia ficus-indica cladode lesions have confirmed the presence of Alternaria alternata, Colletotrichum gloeosporioides, Fusarium lunatum, and Curvularia lunata [180]. Recently, black spot disease has been identified in cactus cladodes cultivated in the Mexican region. Among six fungi responsible for inducing black spot symptoms, three have been classified as significant pathogens: Alternaria alternata, Neoscytalidium dimidiatum, and Corynespora cassiicola [181]. Black spot diseases are typically exacerbated by environmental factors such as high temperature and humidity, conditions that are intrinsic characteristics of drought. These findings underscore the vulnerability of Opuntia ficus-indica to various fungal pathogens, which limits the plant’s yield potential and increases disease transmission to the livestock and possibly humans. During drought conditions, certain diseases may be favored while others are suppressed; however, pathogenic infections remain a significant threat to the diversity of drought-tolerant plants. For instance, charcoal rot disease, caused by the fungus Macrophomina phaseolina, has been reported in crops such as sorghum and maize during drought periods [182]. These fungi have developed survival mechanisms that enable them to thrive in extreme heat conditions within dry soil and subsequently migrate towards their target plants. Another fungus, Aspergillus flavus, has been implicated in causing premature growth and size reduction in drought-tolerant crops, while fungi from the genera Fusarium and Bipolaris have been associated with dryland foot and root rot, respectively [182].

In addition to diseases, toxic compounds have also been reported in drought tolerant crops meant for livestock feed implicated as the major causes of decreased survival rate of these plants. Groundnut shell from Arachis hypogea (L.) has been observed to accumulate aflatoxin contaminant, as investigated by [183], through subjecting the seeds to drought intense conditions. The study concluded that the drought tolerant ability of groundnut seeds does not help alleviate aflatoxin concentrations, posing a threat to livestock health. Drought resistant sorghum is also considered to be vulnerable to mycotoxin accumulation as analysis of 1533 samples revealed that a portion of 33% contained alarming levels of fumonisins, zearalenone, sterigmatocystin, Alternaria toxins and aflatoxins [184]. Additionally, factors such as sorghum color, collection period, source and country of origin were seen to significantly contribute to the type, amount and the spread of the analyzed mycotoxins [184]. The rationale behind drought tolerant crops being susceptible to mycotoxins, especially aflatoxins, can be explained by the climatic conditions of drought, which are extremely hot temperatures and heat stress, that support the growth of mycotoxigenic fungi. Existing edaphoclimatic conditions, characterized by low water availability, reduced nitrogen levels, poor soil quality, and increased toxic metal accumulation, contribute to oxidative stress, phosphate starvation and salt stress, ultimately disrupting the survival, nutrition, and growth of plants [185].

Ultimately, the noted diseases and accumulation of mycotoxins compromise feed safety standards which lead to limited applications of drought tolerant crops as alternative feed sources during feed scarcity. The appearance of pathogenic fungi and bacteria in a silage increases the risk of disease transmission to the livestock, consequently affecting the health of livestock and livestock product quality [186]. Institutions such as FAO and International Feed Industry Federation strongly recommend the application of proper storage, preservation and packaging techniques since they hold a valuable position in determining the final quality of the produced feed [187]. Despite silage being a common practice to produce feed for livestock, the silage method, however, has been reported as the main host of pathogenic microorganisms, especially when poorly monitored and managed [186]. Frequently occurring disease-causing microorganisms in silage are predominantly Listeria, Salmonella enterobacteria and Clostridium spp [186]. When silage is subjected to high amounts of moisture, Clostridium spp species bacteria develop and convert available carbohydrates into butyric acid instead of lactic acid leading to ketosis that can decrease milk production and weight gain in lactating cows [188]. Thus, when the safety of livestock feed is compromised, it poses health risks not only to the livestock themselves but also to the individuals who consume their products.

6.2. Low Crude Protein Content

CP amount is a critical nutritional element for livestock feed as it is crucial for nearly all biological processes in humans and livestock [189,190]. CP is a vital feed ingredient for efficient growth performance, tissue repair, milk production, weight gain, and ME, to name a few [191,192,193]. As a result of its implications in these crucial functions, minimum concentration of CP in livestock feed has been established. For ruminants, the minimum CP required is reported to be around 7.5%, lower concentrations than that are not advised since they may compromise rumen fermentation [194]. With lactating cows, a diet is recommended to contain about 160 g/kg CP in order to achieve metabolizable protein and reduced nitrogen excretion [198]. In bird husbandry, according to the veterinary manual, CP requirements for broiler rearing ranges from 18-23%, depending on the type and age of a bird fed. Using a fitting model, CP requirement was estimated to be around 21% for Jint Tint chicks during a six-week rearing period [55]. For laying hens to achieve desired egg production, CP should be 0.18 g CP/g egg mass [199]. Recently developed CP standards contain contents of 14.7, 22.7 and 35% for swine, poultry and fish feed evaluation, respectively [200]. These studies cite the importance of a feed source to contain sufficient amounts of CP as part of the recommended dietary attributes, highlighting the point that elevated CP is indeed desirable for livestock nutrition.

However, as reported in Table 3, various studies have shown that a number of drought tolerant crops meant for livestock feed might fail to meet the minimum requirement for CP content due to the effects of drought and antinutritional compounds, such as tannins, that affect protein synthesis. Nutritional profiles of sorghum varieties (Pant Chari 5, PKV 809 and CSV 17) revealed low CP content (45.3 g/kg) Pant Chari 5 which expressed a decline in IVDMD [201]. In vivo analytical measurements also exhibited low CP intake when sheep were fed Pant Chari 5 variety. In other studies, it has been revealed that millets are also characterized by low protein content (7.2-7.8%). Pearl millets have been revealed to have low quantities of tryptophan and lysine, which are key amino acids that contribute to protein content [202]. Recent nutritional analysis of millet varieties proved finger millet contains the lowest CP of 7.24%, similarly little and kodo millets exhibited respective protein values of 7.6 and 7.8%, making their utilization limited only to ruminants at the minimal CP content highlighted above [202]. Varieties of sweet potato tubers of have been proven to contain CP range of 4-6% on DM basis which can be attributed to the occurrence of trypsin inhibitors [203]. Ensete ventricosum tuber revealed CP of 3.33% while the whole plant was reported to contain about 5.98% [8].

These findings suggest the need to blend drought tolerant crops with conventional livestock feed that contain high CP content. For example, supplementation of finger millet straw with various proportions of noug seed cake resulted in improved body weight gain compared to the control that contained finger millet straw only [195], emphasizing the need of protein supplementation in crops such as finger millet. A substitution of rice bran with 9% cassava generated a ruminant meal with approximately 14% CP in DM plus a better protein intake [196]. Likewise, Opuntia ficus-indica peel and 12% wheat bran yielded approximately 12% CP in DM [197]. Other approaches include adding fermenting microorganisms to low CP containing drought tolerant crops, as detailed in a recent review by [47] that the supplementation of Trichoderma viride, Aspergillus niger and Saccharomyces cerevisiae increased CP content in pineapple residues observed from various studies. These studies further demonstrate the importance of feed processing techniques like fermentation and ensiling which can be employed to optimize and enrich the CP in drought tolerant crops.

6.3. High Lignin Content and Limited Digestibility

The survival strategy of drought-tolerant crops, which primarily relies on high lignin composition to conserve water during periods of aridity, adversely affects feed digestibility by enzymes and the gut microbiome in livestock [207]. Although lignin is a necessary cell wall component whose function is needed during hot temperatures, heat stress, transportation of minerals and mechanical support [208], its presences become a physical barrier hindering microbiome from reaching needed polysaccharides, cellulose, hemicellulose, starch and protein. The chemical make-up of lignin determines its digestibility. Lignin chemical composition includes two major monolignols (Figure 7): dimethoxylated syringyl (S) and monomethoxylated guaiacyl (G). As a result, the ability of these monolignols to form bonds with other units is directly related to a measured indigestibility of plant material in livestock. An increased concentration of G units is believed to enhance plant indigestibility, which may also be associated with greater maturity. It should be noted that lignin and plant digestibility are complex and challenging phenomena as lignin varies across plant species, also the method to measure the influence of lignin on digestibility cannot be easily applied due to various factors such as crop age, environmental conditions and plant morphology, to name a few [207].

The inability of rumen bacteria to efficiently degrade lignified plant cell wall limits the conversion of cellulose and hemicellulose to short fatty acids which play a crucial role in energy provision for livestock whereby lignin itself contains energy that is 30% higher than cellulose [207]. Highly lignified feed formulations extend the passage time of feed within the ruminal digestive system, thereby reducing feed intake. This reduction in feed intake can be correlated with poor feed quality and suboptimal livestock performance. The influence of lignin on feed digestibility has been extensively researched over the years, with various studies demonstrating a negative correlation between lignin concentration and digestibility [209,210,211]. For instance, increasing lignin concentration (9.59-13.3%, NDF basis) in feed diets reduced the digestibility of NDF, CP, and starch in lactating cows [212]. Supporting the notion that feed digestibility may also depend on the digestive system of the livestock, an in vivo study involving buffalo demonstrated excellent ADL degradability, presumed to be facilitated by microorganisms such as Rikenellaceae RC9 gut group, Ruminococcaceae UCG-011 and Prevotella1, 226 [213]. The extent to which the lignin undergoes degradation and affects digestibility may also depend on its structural composition, specifically highlighting which subunit predominates. Feeds with a high presence of the sinapyl unit exhibited enhanced digestibility of ADF, NDF, and cellulose, suggesting that the linearity of sinapyl promotes efficient lignin degradation [214].

High concentrations of lignin and their detrimental effects on feed digestibility of drought tolerant crops have led to the development of pretreatment techniques aimed at reducing or removing lignin to make cellulose and hemicellulose more accessible for gut microbiota [215,216,217]. Several authors have reported on methods to lower lignin concentrations specifying pretreatments such as comminution, application of rot fungi, the use of chemicals such as potassium hydroxide, sodium hydroxide, calcium hydroxide, sulphuric acid and physical pretreatments such as soaking and mechanical grinding [218]. Escapa et al. [219] and Jędrzejczyk et al. [217] have further reported on the use of grinding, pyrolysis (using temperature greater than 300 °C), microwave oven, hot water, acid or alkaline pretreatment. Pretreatments created from enzymatic systems are also reported as a viable option able to decrease lignin content in livestock feed [220]. The common objective of these cited pretreatment applications is to disrupt the cross-links present in lignocellulosic compounds to facilitate the availability of essential nutrients, such as sugar polymers, for further efficient digestion in livestock.

6.4. Socio Economic and Financial Challenges

The adoption of drought tolerant crops faces several interconnected challenges. These include insufficient knowledge and training, lack of awareness, limited availability support from extension services, restricted access to improved seed varieties, limited land for food production, and lack of capital for investment.

Livestock owners in arid regions lack the necessary knowledge needed in order to fully understand the implications of drought tolerant crops in livestock feed systems. For example, in Uganda, it has been reported that the limited adoption of drought tolerant crops by smallholder farmers included lack of awareness about the crops which further resulted in low crop production [221]. With respect to the cacti plants, farmers are unaware of the difference between spiny and spineless cactus, leading to the perception that both plants are the same [222]. Meanwhile, in some areas like Pakistan the cultivation of cacti species is not valued as it is perceived that they are invasive species, rendering their farming fruitless. This perception highlights the lack of awareness, knowledge and training on the use of drought tolerant crops, for smallholder farmers. The use of drought tolerant crops for livestock forms part of the climate smart agricultural approach, however, the use of climate smart approaches is hindered by insufficient technical knowledge amongst smallholder farmers [223].

The access to technical information is one of the challenges facing the extended use of drought tolerant crops in livestock feed. Research results from drought tolerant crops fail to reach the ultimate beneficiaries. For instance, limited access to improved seed systems for enhanced sorghum varieties by smallholder farmsteads in regions such as Nigeria has been reported [224]. Besides limited access to research information, the available engineered or certified seeds are more expensive compared to the traditional ones [225]. Acevedo et al. [226] reported that both access and lack of information on the certified seed varieties were principal barriers that hindered farmers from adopting climate resilient crops, especially those with low-income status. These findings underscore a lack of support from agricultural extension services, whose duties include providing support, guidance and information to local farmers.

Another significant barrier to using drought-tolerant crops for livestock feed is the limited land for food production. This challenge is further intensified by water scarcity, land degradation and arability, which limit the resources available for sustainable feed crop cultivation. For instance, South African arable land is only about 12% of the total surface area while the rest is considered for grazing [227]. Unfavorable edaphoclimatic variations are reported to contribute to the observed limited land use for crop production, allowing the irrigation of rain fed crops only. Lack of capital and monetary support from financial institutions is also hindering the adoption of climate smart agriculture practices, especially for small-medium scale farmers. Olabanji et al. [228] reported that smallholder farmers in South Africa often face challenges in accessing credit from financial organizations, limiting their participation in climate smart agriculture interventions. Addressing these issues requires increased investment and support systems to encourage the use of these crucial crops, ensuring food security and sustainable agricultural practices.

7. Key Strategies for Sustainable Innovation for Drought Tolerant Livestock Feed

7.1. Increased Awareness

To effectively address the challenge of limited awareness regarding the adoption of drought-tolerant crops, relevant organizations and institutions (across private, non-governmental, and government sectors) need to establish strong relationships with smallholder farmers in rural settlements. Awareness campaigns and outreach programs, especially in rural farmsteads, are important to strengthen and yield increased adoption of drought-tolerant crops in arid environments. The outreach programs should focus on informing the farmers about available agricultural extension services, knowledge on climate-smart technology, where and how to access drought-resilient certified seeds, the latest research findings, financial support, and the importance of drought-tolerant crops as livestock feed [226]. Partnerships with key stakeholders such as telecommunication companies, government, and non-governmental organizations will help facilitate the awareness and widespread adoption of drought-resilient plants in livestock [226]. Zougmoré et al. [229] emphasized the need to increase awareness of climate-smart technologies, especially for vulnerable communities. In some parts of Southern Africa (Zimbabwe and South Africa), there is evident lack of awareness of drought tolerant crops as a climate smart agriculture for improved crop production. The adoption of drought resilient maize varieties by smallholder farmers has shown increased crop yield compared to those who did not cultivate these varieties [230,231]. Furthermore, their incorporation resulted in increased profits of US$240/ha with no additional cost incurred. This is proof that continuous awareness programs with a specific focus on the adoption of drought tolerant crops for livestock feed hold a potential that can help improve the performance of livestock in arid regions. This approach will help to disseminate information about climate-resilient plants and their implications in livestock productivity to help small-scale farmers to make informed decisions.

7.2. Policy Reforms

Policies supporting innovation in drought-tolerant crops for livestock feed are crucial as a vehicle that will allow small-scale farmers to access the needed financial support, research and development, and regulatory frameworks. Besides, reforming the existing policies is further needed to ensure inclusion and widespread adoption of drought resilient crops as climate smart agriculture in smallholder farming operations, especially livestock producers. The current policies are reported to be unfavorable for small-scale livestock producers, especially in African countries [232]. Acevedo et al. [226] purported that the existing seed policies have created challenges in accessing the improved seed varieties by the majority of farmers. Wongnaa [232] stresses that for livestock farms to be sustainable during unpredictable climatic events, policy reforms must be prioritized. On the other hand, Ghana and Nigeria have successfully developed policies planned at implementing climate-smart agricultural practices for farmers vulnerable to drought [229,233]. In Mali, policy reforms have enabled the implementation and provision of eco-friendly solutions, such as bio-pesticides, organic fertilizers, fallow and zero tillage, as part of climate change adaptation strategies [234]. Although these examples support the notion that inclusion of drought-resilient crops for livestock feed during policy reform is possible, there is an urgent need for the development of policies that support and prioritize innovation of drought-tolerant crops for livestock feed.

Policy frameworks also need to promote financial support for farmers interested in the production and processing of drought tolerant crops for livestock feed. This is critical to help the farmers to afford the machinery needed and land for the planned projects [232]. For instance, FAO, in collaboration with the Canadian government, introduced a project that financially supports smallholder farmers to afford certified seeds of drought tolerant fodder, livestock breeding tools and equipment [228]. In addition, government investment in research on innovation of drought tolerant crops in livestock feed is fundamental to developing new, higher-yielding, and nutritious drought tolerant varieties suitable for local conditions. This includes utilizing advanced techniques such as molecular breeding and gene-editing tools. Public-private partnerships, such as the Drought Tolerant Maize for Africa program, an initiative led by International Maize and Wheat Improvement Center and the International Institute of Tropical Agriculture, with an aim to develop and distribute drought-tolerant maize varieties in sub-Saharan Africa is one good example of a successful model for future initiatives. Such models are also needed for innovations that focus on livestock feed, ensuring that policy interventions adequately support innovation in drought tolerant crops for livestock feed.

7.3. Strengthening of Research Capacity

Strengthened research, integrating traditional breeding with cutting-edge biotechnology such as marker-assisted selection and gene editing, is vital to accelerate the development and use of superior drought-tolerant varieties. Research on Sudan sorghum has proven that genetic manipulation, through traditional breeding techniques, is able to produce varieties with optimized growth abilities as it can be propagated in various soil types without the loss of its nutritional value. Research on cultivar breeding of forages is an ideal approach to improve plant resistance, durability and nutritional value of the crops of interest [235], Lolium, Trifolium and Medicago spp are crops that have shown greatest improvements through breeding technology [235]. The continuous research on these species has attracted the participation of various institutes across Canada and United States, showcasing how joint cooperative efforts improve the quality of research findings that benefit both research and industry. Similar plant breeding projects exist in countries such as Uganda, Tanzania and Mozambique which aim to explore ways to improve crop varieties for livestock feed [236].

Despite advancements in research on drought-tolerant crops, a significant disconnect persists among academia, industry, and government, hindering support for research into these crops as alternative feed sources during droughts. This gap is particularly evident in South Africa, where research on alternative feed and superior forage options is limited [5], and public evidence of collaboration between the government and the private sector is lacking. Although industry and government maintain active breeding programs, academic involvement is less common. Consequently, research institutions contribute minimally to generating the data needed to encourage the wider adoption of drought-tolerant crops among smallholder farmers. By increasing governmental funding for academic research, scientists can create the essential tools needed to help small farmsteads transition to using drought-tolerant feed crops. Thus, collaboration among researchers, government, industry, and farmers is an effective way to optimize the use of drought-tolerant crops for animal feed, which is becoming increasingly important due to unpredictable climate change. Crucially, research investments focused on enhancing the quality, processing efficiency, shelf life, and overall quantity of alternative feed sources (drought-tolerant crops) will boost their adoption by small-scale farmers.

8. Conclusions and Prospects

This review critically discussed the role of key drought-tolerant crops —including Sorghum bicolor, Pennisetum glaucum, Manihot esculenta, Ensete ventricosum, and Opuntia ficus-indica — as alternative livestock feed. It highlighted the main drivers for the use of drought-tolerant crops and provided an in-depth discussion on their processing and application, factors influencing their quality, challenges, and strategies to promote their innovation as climate smart agriculture strategy. The impact of climate change is the main driver for the application and research of drought-tolerant crops for livestock feed as climate unpredictability continues to threaten traditional feed resources and livestock productivity. Various techniques such as drying, ensiling, milling, and pelleting have been applied to process drought-tolerant crops. Sun drying is still the predominant method for drying drought-tolerant crops, and there is a noticeable absence of advanced drying techniques, indicating a need for research and innovation in this field. The incorporation of alternative feedstuffs such as Sorghum bicolor, Pennisetum glaucum, Manihot esculenta, Ensete ventricosum, and Opuntia ficus-indica into livestock diets to partially or wholly replace conventional feed has shown promising results in enhancing animal health and nutrition. However, optimizing these formulations is essential for achieving the best outcomes. Cultivar and maturity stage are the main preharvest factors affecting the quality of drought tolerant crops and their livestock feed, whilst after harvesting, drying, grinding and pulverization, ensiling and storage processes are the key determinants of quality preservation and enhancement. There is a need for further research to comprehensively study these crucial quality determinants, especially how they interact with each other and across a diverse range of drought-tolerant crops.

The development of drought-tolerant crops faces several key challenges, including susceptibility to disease and mycotoxins, low protein levels, and issues with digestibility due to high lignin content. Socio-economic and financial obstacles also limit their effectiveness. To fully leverage the potential of these crops for food and nutrition security and climate-smart agriculture, a comprehensive, integrated strategy is necessary to address all these issues. While progress is being made in developing drought-tolerant crops, several key research areas remain under-explored. Firstly, there is a need to expand research on various drought tolerant crops as the current research focuses on a limited number of crops mainly, Sorghum bicolor, and Pennisetum glaucum which are crops for human food. Though research has expanded to other drought-tolerant crops like Opuntia ficus-indica species, this particular crop’s vast potential as a solution to livestock feed shortages in drought-prone areas remains entirely underutilized. Current research on Opuntia ficus-indica has primarily focused on the nutritional profiling of the whole cladodes, despite the plant’s overall importance and potential of fruit waste. This approach overlooks the potential for significant nutritional and techno-functional differences in specific components of cladodes like mucilage and fiber, which could greatly influence the properties of feed derived from cladodes. Additionally, studies lack diversity in investigating the influence of cultivar and maturity stage. Although the Opuntia ficus-indica is globally distributed, research is geographically sparse. Addressing these critical factors is essential for driving innovation related to this plant. While advanced drying methods such as freeze-drying are not commonly used, they remain critical as the gold standard for preservation. This benchmark is essential for assessing the impact of other processing techniques and underscores the necessity of advanced technology in developing drought-tolerant crops.

The short-term future research strategies for innovating drought tolerant crops for livestock feed should focus on: 1.) identifying more drought tolerant species, optimizing agronomic and water management practices, as well as intensifying research on preharvest and processing factors 2.) application of inorganic and organic nutrients and chemicals to support their growth or enhance the drought resistance and antioxidant defense systems, 3.) policy and financial support, 4.) integrated research platforms, 5.) create more awareness on the potential of the drought tolerant crops as sustainable livestock feed, 6.) carry out techno economic and environmental impact of producing drought tolerant crops and processing them into feed. Meanwhile, the long-term future research strategies should focus on 1.) advanced plant breeding and genetic engineering, and 2.) exploring novel drought tolerant crops. Bulleted lists look like this: