Agronomic Performance of Lettuce (Lactuca sativa L.) Fertilized with a Whey-Based Biofertilizer: A Circular Approach to Nutrient Recycling

Dairy whey is an agro-industrial by-product with potential for agricultural nutrient recycling. This study evaluated fermented whey-based biofertilizers containing cattle manure, alfalfa, and baker’s yeast for lettuce (Lactuca sativa L.) production under semiarid conditions in Mexico. Three formulations containing 2.5, 5.0, or 7.5 L of whey were characterized and assessed through a germination bioassay and two 35-day field cultivation cycles. Increasing whey incorporation was associated with higher organic matter and nutrient concentrations, as well as greater electrical conductivity. Seed germination remained high (93.33–96.00%), without significant treatment differences; however, root and shoot lengths were lower than in the water control. Under field conditions, the intermediate formulation (T2; 5.0 L whey) supported strong vegetative development and produced fresh biomass statistically comparable to T3 in both cycles. T2 also showed the highest phosphorus concentration in plant tissue. Soil responses varied by property, with significant treatment effects on changes in pH, electrical conductivity, available phosphorus, and exchangeable potassium, but not on organic matter, total nitrogen, or moisture. These findings identify T2 as a promising formulation for further evaluation, while highlighting the need to monitor seedling growth and soil salinity during repeated applications.