The energy and Environmental Evaluation of Maize, Hemp and Faba Bean Multi-Crops

1. Introduction

Currently, high population growth and high demand for food and energy, especially produced from renewable resources (for example, biomass) [1].

Maize is a crop with a high potential for biomass production. Today, maize is one of the most important and productive crops in the world, with more than 206 million hectares being cultivated in 2021. In the USA, ethanol extracted from maize is mixed with standard fuel and the final product contains 10% of ethanol. They can be used for the production of feed, biofuels, synthetic plastics, chemical compounds, etc. [2,3]. The aim is to reduce the use of fossil fuels and regulate greenhouse gas emissions [4]. However, this fuel does not provide a significant amount of energy compared to the energy produced from sugar cane [5].

Faba beans are widely grown around the World. According to FAO data, faba beans were grown over 2.7 million hectares worldwide in 2021, and over 473 thousand hectares in Europe. The faba beans are mainly used for food, animal feed and soil improvement, but they can be used for energy purposes by converting biomass into pellets and using them as medium to low-power boilers [6].

Industrial hemp—it is great technical crop. Their ability to produce high biomass yields with relatively low water content is one of the main advantages. Industrial hemp is valuable for its high energy and biomass yield per hectare [7]. It is claimed that hemp can be used for energy production as a fuel source that does not emit sulphur either by direct combustion or by liquefying fuels such as bioethanol. Hemp oil is produced as a renewable biofuel because it emits less carbon monoxide when burned, thus helping to reduce global warming. It is important to emphasize that biomass by-products (straw and shivers) are a good raw material for energy production [8].

Multi-crops are a type of crop cultivations, where several species of plants are grown at the same place and time. Multi-crops ensure sustainable farming and increase crop productivity per unit area [9]. Multi-crops can increase biodiversity, reduce the damage caused by diseases, pests and weeds, reduce the need for mineral fertilizers and maintain soil fertility and quality, easier for plants to grow under adverse conditions [10]. Biomass plants are classified as renewable sources and their cultivation is very courageous, as the European Union wants to increase the energy supply from alternative sources by 2030 (up to 45%). However, despite the enormous hidden potential in this area, the cultivation of inter-crops has still not been sufficiently studied [11]. It is important to understand that the number of plants that can be cultivated as multi-crops is quite large and all crops have a different function and their benefits are not the same. In order to achieve the maximum harvest result, the advantages of each multilayer plant must be exploited [12].

Agro-technological operations have a significant impact on energy consumption and environmental pollution. Tillage technologies are one of the most important, but at the same time the most energy-intensive and expensive technological operations [13]. Reversible tillage technologies consume between 29 and 59% of total diesel fuel in technological process [14,15].

Fertilizers production and use account for the majority of energy expenditure (about 50%) in field crop production in Europe [16]. The production of nitrogen fertilizers is a particularly energy-intensive process, compared to the production of phosphorus and potassium fertilizers, about 10 times more energy is consumed [17]. Ghazvineh at al. [18] found that in maize cultivation, the total energy consumption and output were 50.485 and 134.946 MJ ha⁻¹, respectively. Nitrogen fertilizers, electricity and diesel fuel consume the largest part of energy with 35.25 and 20 percent, respectively. In barley production systems, the total energy input and output was 12416.62 and 43656.25 MJha⁻¹. The most energy consumed was diesel fuel energy (52.4%), nitrogen fertilizers (16.2%) and seed (23%). 3.51 and 0.11 KgMJ⁻¹ in energy efficiency and energy productivity of this agroecosystem. In wheat systems, total energy input was 14255.05 MJha⁻¹, total energy output—56161.7 MJha⁻¹. The biggest part of wheat made up the total energy input nitrogen fertilizers (31.8%), seed (27.2%) and diesel fuel energy (31.6%). In hemp systems, it was 17,945 MJha⁻¹ for the total energy input.

There is still no formal agreement on a methodology for calculating greenhouse gas (GHG) emissions from crop production systems. The authors conclude that further research and development is still needed to improve crop production sustainability [19]. It is argued that the large number of units used to measure agricultural inputs makes it very difficult to compare carbon inputs. Points out that it is useful to convert different units of measurement into carbon dioxide emissions for agricultural activities [20].

Agriculture’s direct contribution to GHG are estimated to be 10−15% of total anthropogenic GHG emissions which non-CO₂ anthropogenic GHG emissions consist of 48%. Sustainable agriculture, which reduces greenhouse gas (GHG) emissions, can contribute to climate change mitigation [21]. Also in organic agriculture, in contrast to traditional farming, is associated with a lower average of GHG emissions per hectare [22]. Bručienė et al. [23] pointed that in organic agriculture the most important factor in controlling GHG emissions is reducing the use of organic fertilizers and diesel fuel.

The aim of our study was to evaluate the biomass production capacities of differently biodiversed multi-crops, to analyse the possible for these technologies machinery, to evaluate the inputs of energy for main materials, fuels, working time and biomass energy outputs, to ascertain the impact of technologies on the GHG emissions according CO₂ equivalent. We hypothesize that cultivation of a ternary crop will not only produce more plant biomass, but will also use energy more efficiently, and GHG emissions will not be the highest.

2. Materials and Methods

2.1. Experimental Site

A short-term stationary field experiment was performed in 2020–2022 at the Vytautas Magnus University, Agriculture Academy, Experimental station (54°52′ N, 23°49′ E). In this study, data of crops biomass from 2020 is presented because the yield was the highest and the most suitable for fuel production in comparison with continue cropping in 2021 and 2022.

The soil of the experimental field is a deeper gleyic saturated loam (45.6% sand, 41.7% silt, 12.7% clay) Planosol (Endohypogleyic-Eutric, Ple-gln-w [24]. Soil pH_HCl varies from 7.3 to 7.8; total nitrogen content, 0.08 to 0.13%; humus, 1.5 to 1.7%; available phosphorus, 189 to 280 mg·kg⁻¹; available potassium, 97 to 118 mg·kg⁻¹; available sulphur, 1.2 to 2.6 mg·kg⁻¹; and exchangeable magnesium, 436 to 790 mg kg⁻¹.

Lithuania is in a zone of surplus moisture. In the experimental site, the annual average precipitation rate varies from 600 to 650 mm or approx. 300–400 mm of precipitation per vegetative season. According to the climatic norm, the coldest month is January, and the warmest and the most humid is July. In nova days, January became much soft, negatives temperatures are recorded less often. In July, droughts occur more and more often, and the peak of precipitation moves to the beginning of August. In 2020 vegetative season, average air temperatures were the same as long-term average, but rainfall was 10 times less. May was colder, but with surplus humidity (Table 1). June was warmer than usually, but precipitation rate was approximately 30% higher than long-term climatic rate.

July was colder than average and slightly arid, but August was warmer and little more humid. In summary, it can be said that the vegetation conditions of the year were favourable for crop development, as moisture was usually sufficient.

2.2. Treatments and Agronomic Practice

Maize (Zea mays L., Pioneer hybrid P8105), hemp (Cannabis sativa L., cultivar Austa SK) and faba bean (Vicia faba L., cultivar Vertigo) were grown in the experiment as three types of cultivations: single, binary and ternary (Table 2).

An experiment was performed with three replications. 21 experimental plots (the size was 8 m²). In 2019, the pre-crop of multi-cultivations was oat. In 2021–2022, cultivations were grown continuously.

In 2019 autumn, disc-cultivated and ploughed (Table 3). In spring 2020, and cultivated in spring before sowing. In April, before seeding, the soil of experiment was shallowly tilled. Before the sprouting of crop, plots were fertilized with NPK 15:15:15 (300 kg·ha⁻¹). There were used no more fertilizers.

The experiment more closely followed the model conditions, so the crops were sown manually according to predetermined schemes. Sowing schemes are shown in Figure 1.

According to the schemes of seeding, the seed rates are presented in Table 4. The idea of experiment was to increase usual seeding rates for more rapid weed control and higher potential of biomass production. For better balance of energy inputs, seeding rates could be decreased, especially faba bean.

For the energy and environmental evaluation, we selected machinery that are mostly used in agriculture in Lithuania. The inter-rows of crops were loosened 1–2 times. No pesticides were used. In experiment, crops biomass production was harvested by hands after a short 103-days vegetative season. At that time, faba bean have been reached full maturity. The biomass of maize and hemp had not yet reached its maximum, but there is a lack of plant biomass for fuel production in Lithuania in August.

In our calculations of energy and environmental evaluation of multi-cropping agrotechnologies, we used the data normative of agricultural machinery from Lithuanian Institute of Economics and Rural Development [25,26]. For calculations we used field size as 2–10 ha. Tractors power varied from 45–102 kW, power of biomass harvester was 250 kW (Table 5). The data of mechanical seeders are given in the calculations, when about 200 kg ha⁻¹ of seeds are sown. Double sowing is carried out with a combined sowing machine, in which separate seed boxes or the sowing unit of the one next to it could be installed. For sowing in two passes, we have provided both of the previously mentioned seeders. Of course, a ternary crop seeder can be made to seed such a crop in one pass. Then the energy and environmental calculations would change.

We chose 6-furrow biomass harvesters. Single and binary crops without hemp are classified as low harvester load. For difficult conditions, we classified single and binary crops with hemp. Biomass harvesting in two passes is provided in ternary crops, in which the upper juicier part of the biomass is cut first, and the more woody lower part is cut in the second pass.

2.3. Methodology

For crop biomass productivity assessment, plants were cut in a row of 1 m length with at least in 5 spots per plot. Samples for each species of plant were taken separately, so a total of 36 samples were formed. Biomass was dried at a temperature of 105 °C to absolutely dry humidity. The results of dried biomass are presented in this study.

A biomass chemical composition was tested in the laboratories of Lithuanian Research Centre for Agriculture and Forestry in Kaunas, Lithuania (Table 6). We would like to show that biomass of crops is rich in elements important for biofuel production and plant nutrition (as pellets or ash). We took advantage of these features by registering patents for the production of solid biofuel from faba bean residues and ternary crop biomass at the Lithuanian National Patent Office [27,28].

Equivalents of energy inputs and outputs in single and multi-crops biomass production are presented in Table 7.

The energy ratio (energy use efficiency), energy productivity, net energy were calculated based on the energy equivalents of technological inputs and outputs [30,36]:

Energy output (MJ ha⁻¹) = Dried biomass yield (kg ha⁻¹) · Energy equivalent (MJ kg⁻¹)

(1)

Energy efficiency ratio = Energy output (MJ ha⁻¹)/Energy input (MJ ha⁻¹)

(2)

Energy productivity (kg MJ⁻¹) = Biomass yield (kg ha⁻¹)/Energy input (MJ ha⁻¹)

(3)

Net energy = Energy output (MJ ha⁻¹) − Energy input (MJ ha⁻¹)

(4)

All agricultural technological operations, machinery and materials can be evaluated as environmental impact by CO₂ equivalent (Table 8).

In our study, we calculated the conditional GHG emission of fuel, machinery, sowing material and mineral fertilizers according to equation 5 [23]:

GHG emission (kg CO_2eq ha⁻¹) = Agro-technological inputs · CO₂ equivalents (kg CO_2eq unit⁻¹)

(5)

An experimental data were analysed using one-factor analysis of variance (ANOVA) from the statistical software SELEKCIJA (vers. 5.00, author dr. Pavelas Tarakanovas, Lithuanian Institute of Agriculture, Akademija, Kedainiu distr., Lithuania). The significance of differences between treatments was estimated by the least significant difference (LSD) test. Lower-case letters mean significant differences between treatments at p ≤ 0.05.

3. Results and Discussion

3.1. Energy Inputs

Energy is an important driver of development, but it is particularly influential in the agricultural sector, as agriculture is both a consumer and a producer of energy [42]. Complex mechanized technological processes such as tillage, sowing or harvesting are widely used in agriculture and have a significant impact on energy consumption [30].

An energy input-output analysis was performed to compare the efficiency of maize, hemp, faba bean production under different multi-cropping systems. The energy inputs for maize, hemp, faba bean multi-crops were calculated based on the mechanized operations and agricultural machinery used, working time, fuel consumption, the sowing of seeds and the application of fertilization. After the maize, hemp, faba bean crop biomass production, energy outputs were determined and recalculated per hectare. Energy inputs and outputs were assessed separately for each maize, hemp and faba bean production treatment (Table 9).

Analysing the indicators of multi-cropping and other technological operations presented in Table 9, it was found that the lowest labour capacity was for the maize single crop (8.8 MJ ha⁻¹) and faba bean (8.8 MJ ha⁻¹). The highest capacity was for the ternary crop (11.0 MJ ha⁻¹) because of wider agro-technological operations.

Biodiesel has been gaining growing importance in terms of environment and energy. Other researchers have obtained opposite trends in the high labour and fuel costs of growing bean crops [30]. Biodiesel can be produced of processed vegetable oils from different feedstock like sunflower oil [43], rapeseed oil [44,45], soybean oil [46,47], palm oil [48] and hemps (Table 9).

In our experiment, the highest fuel consumption is obtained when growing ternary crop (5815.8 MJ ha⁻¹), and the lowest—in single maize (3980.4 MJ ha⁻¹) and faba bean (3980.4 MJ ha⁻¹). Ali et al. [49] stated that the most energy consumption of diesel fuel accounted for about 54% of the total energy used for single faba bean cultivation.

Similar trends have been identified in agricultural machinery as well as in diesel fuel. The most intensive use of agricultural technique was found in the ternary (M+H+FB) crop (2000.3 MJ ha⁻¹). Compared to other crops, it was by 14.3 to 19.6% higher (Table 9).

Maize (600.9 MJ ha⁻¹) and hemp (315.1 MJ ha⁻¹) seeds were the lowest in the ternary crop (M+H+FB) compared to other crops. The highest amount of energy from seed of maize (1505.6 MJ ha⁻¹) was determined in the binary (M+FP) crop, and seed of hemp (512.9 MJ ha⁻¹), in the single (H) crop. Seed of faba bean (8355.9 MJ ha⁻¹) had the highest amount of energy input in the binary (M+FB) crop, and the lowest (7620.9 MJ ha⁻¹)—in the single (FB) crop compared to other crops.

Fertilizer use has increased exponentially worldwide over the past few decades. The intensive use of mineral fertilizers and the use of conventional methods of fertilization have a negative impact on the soil, the environment and human health [50]. In our experiment, the highest amount of energy input (2727.0 MJ ha⁻¹) was determined for N fertilizer, while the lowest (301.5 MJ ha⁻¹) was calculated for K₂O fertilizer (Table 9).

The total energy consumption is the highest (20162.9 MJ ha⁻¹) in the ternary crop (M+H+FB), and the lowest (10176.6 MJ ha⁻¹) in the single crop (H) (Table 9). In ternary crop (M+H+FB) the most energy was used, because more human labour is added, more diesel fuel is consumed and agricultural machinery is used several times. This is because single crop require much less energy than other multi-cultivations [51].

3.2. Biomass Yields

Maize and hemp biomass is the main resources for biofuel production [52,53,54]. The highest biomass of maize was grown in single cultivation—4461 kg ha⁻¹ and the lowest one—in ternary crop (1358 kg ha⁻¹) (Table 10) because maize crop density in ternary crop was more than twice less (about 134 thousand plants per ha) according to the seeding scheme. Maize with faba bean inter-crop produced second higher yield of biomass—3519 kg ha⁻¹. According findings of other scientists, maize mix with Fabaceae crops warranted higher maize yields [55], but we found the converse conclusions [56].

The highest dried hemp biomass was in the binary hemp and maize crop (9824 kg ha⁻¹) and the lowest one was in the ternary crop (7239 kg ha⁻¹).

Faba bean as companion of maize and hemp in ternary crop showed the best results (14331 kg ha⁻¹ of dried biomass) although the density of the faba bean crop in the crops of different diversification varied little—from 560 to 614 thousand plants per hectare. In addition, the hemp and maize crops densities were the lowest one in ternary cultivation. Faba bean less interact with rarer crops. Similarly, Ewansiha et al. [57] concluded that reducing the maize shade increases the productivity of inter-crops.

The significantly highest total dried biomass of crops was found of ternary crop—22,928 kg ha⁻¹ or 2–5 times higher than it other tested single and binary cultivations (Table 10)

3.3. Fuel Consumption and Energy Indices

Diesel fuel consumption, energy input, energy output, energy efficiency, energy productivity, net energy of the various mechanised technological operations for tillage, sowing, fertilising and harvesting are presented in Table 11.

The highest consumption of diesel fuel was found in a ternary crop (M+H+FB) (103.3 L ha⁻¹), due to more technological operations. When growing maize and faba bean single crops, diesel fuel consumption was the lowest at 70.7 L ha⁻¹, which is less than 1.5 times that of ternary crop (M+H+FB). Šarauskis et al. [30] obtained the same trends with monocropped faba beans.

In our experiment, the highest energy output (387831.0 MJ ha⁻¹) was obtained from the ternary (M+H+FB) crop, the lowest one—from the single maize crop (M) (7859.7 MJ ha⁻¹). The energy efficiency ratio and the energy productivity was less from by 2.8 to 2.9 times compared to the ternary crop. In terms of energy productivity, the best result (1.14 MJ ha⁻¹) was shown by the ternary crop. The highest net energy (367668.1 MJ ha⁻¹) was also obtained in the ternary crop. Other researchers claim that growing more plants in cultivation results in higher energy productivity and net energy [14].

3.4. Environmental Impact

Agriculture makes a significant contribution to GHG emissions, with as much as 24% of total net GHG emissions coming from Forestry, Agriculture, and Other Land Use (AFOLU) [58]. Agriculture is itself a consumer of energy and a supplier of energy in the form of bioenergy [59]. Efficient use of energy is one of the main requirements of sustainable agriculture [60]. All production processes need to manage energy use and greenhouse gas (GHG) emissions for sustainable development [61]. Mitigation is an intervention aimed at reducing anthropogenic net greenhouse gas emissions [62]. Although the majority of anthropogenic emissions originate from industrial processes, agriculture is one of the most polluting sectors in the world [63,64]. The constant need for increased food production has led to the intensive use of chemical fertilizers, pesticides, agricultural machinery, electricity and other natural resources; however, the intensive use of energy causes problems that threaten public health and the environment [60]. Environmental mitigation in agriculture requires technological changes to reduce the consumption of energy resources and emissions per unit of agricultural production [62]. Efficient use of energy in agriculture will reduce environmental problems, prevent the destruction of natural resources and develop sustainable agriculture as an economical production system [65]. In the period from 2000 to 2017 global GHG emissions from the agricultural sector ranged from 5.0 to 5.8 Gt CO_2eq per year [66]. Scientists have found that the largest share of pollution in crop production is related to the use of machinery, diesel, electricity, fertilizers, seeds and pesticides [67,68,69]. Lu et al. [67] showed that nitrogen fertilizers emit the most greenhouse gases (40.9–65.7%), followed by diesel. According to the authors, in order to mitigate the impact of agriculture on the environment, it is necessary to increase nitrogen efficiency and reduce diesel consumption [14]. Reducing fuel consumption by 1 L reduces GHG emissions by 2.76 kg CO_2eq [37].

Taking into account the different units of measurement, the GHG emissions for the total agricultural inputs were recalculated into a unified CO_2eq system using the conversion equivalents (Table 12).

The highest amount of CO_2eq ha⁻¹ from diesel fuel was obtained in the cultivation of ternary M+H+FB crop (285.1 kg CO_2eq ha⁻¹), and the lowest in single maize (M) and faba bean FB—195.1 kg CO_2eq ha⁻¹ because diesel consumption was 1.2 to 1.5 times higher. Pishgar-Komleh et al. [61] estimated that the total GHG emissions from cucumber production were 82,724 kg CO_2eq ha⁻¹, with the highest emissions from diesel (61%), electricity (19%), and manure (14%).

The same trends remained in agricultural machinery. In the ternary crop, CO_2eq of agricultural machinery was to be 1.3 times higher compared to other cultivations. Seeds of maize (553.9 kg CO_2eq ha⁻¹) and hemp (256.2 CO_2eq ha⁻¹) had the lowest emissions in the ternary M+H+FB crop compared to other crops. Maize seeds (1387.7 CO_2eq kg⁻¹) had the highest amount of CO_2eq in the binary M+FP crop, and hemp seeds (417.0 kg CO_2eq ha⁻¹)—in the single hemp crop. Seeds of faba bean (393.9 kg CO_2eq kg⁻¹) had the highest amount of CO_2eq in the binary (M+FB) crop, and the lowest (359.3 kg CO_2eq ha⁻¹)—in the faba bean single crop compared to other crops.

The same amount of fertilizers was used. So the CO_2eq was the same in all tested cultivations. Higher fertilizer rates are associated with higher greenhouse gas emissions [70]. In our study, we found that N fertilizer released the highest amount of 58.5 kg CO_2eq ha⁻¹, and K₂O fertilizer released the lowest amount of 6.85 kg CO_2eq kg⁻¹. In terms of GHG emission CO_2eq ha⁻¹, the best indicators were shown by the single faba bean crop. Faba beans also play an important role in crop rotation due to the ability to biologically capture nitrogen (N). Therefore, beans are classified as efficient nitrogen sources [71].

The lowest total GHG emissions were calculated in single faba bean and single hemp crops (629.02 and 709.63 CO_2eq ha⁻¹) (Table 12). The highest ones were obtained in binary M+H and M+FB cultivations (1729.84 and 2067.33 CO_2eq ha⁻¹). Carraretto et al. [40] having assessed the technological operations of mechanized maize cultivation technologies on the basis we would come to the conclusion that the emissions of CO₂ into the environment related with all technologies would be reduced about five times. Alimagham et al. [68] investigated the GHG emissions of faba bean. Cultivation and found that the total GHG emissions ranged from 1265.1 kg CO_2eq ha⁻¹ to 2969.2 kg CO_2eq ha⁻¹) ha⁻¹. In our experiment, ternary cultivation with the highest energy inputs conducted average CO_2eq—1541.90 kg ha⁻¹.

4. Conclusions

The highest consumption of diesel fuel was found in a ternary crop (103.3 L ha⁻¹) or 31–46% higher than in single cultivations and 22–35% higher than in binary cultivations because of double crop sowing and harvesting operations. For this reason, the energy input was the highest in the ternary crop or near twice higher than in M, H, M+H crops, but similar with M+FB and H+FB. Despite this, the productivity of the ternary crop and, at the same time, the energy output were 2–5 times higher than in other treatments. This compensated for higher energy inputs and the energy efficiency ratio of ternary crop reached 19.23. In ternary crop, energy productivity was from 1.1 to 2.8 times higher and net energy was by 1.9–5.3 times higher than in other tested cultivations.

The highest total GHG emissions were obtained in binary M+H and M+FB cultivations (1729.84 and 2067.33 CO_2eq ha⁻¹). As we expected, ternary cultivation with the highest energy inputs initiated average GHG emission—1541.90 kg ha⁻¹ CO_2eq.

In general, this study showed that the multi-cropping in agriculture can be an effective method to warrant high biomass production level, to optimize efficiency of energy consumption, while GHG emissions remains average. In order to make the cultivations of multi-crops even more efficient, it is necessary to improve the sowing and harvesting systems so that the ternary crop can be sown and harvested in one machine pass. In addition, faba beans should be included in the ternary crops, as their biomass takes a significant part of the total biomass produced. Also to review the intercropped faba bean seeding rates as faba bean seeds have a high energy equivalent.