Use of Variable-rate Technology for Agronomic and Environmental Phosphorus-based Liquid Swine Manure Management

Land application is the most common method for utilizing manure produced by the livestock industry. Manure applications at rates that exceed crop P removal are increasing soil-test P (STP) in many regions. Precision agriculture methods and variable-rate (VR) application can improve nutrient management and provide an environmentally responsible method to distribute manure. This study used a striptrial methodology and precision agriculture methods to compare fixed-rate (FR) and VR manure application based on STP for soybean [Glycine max (L.) Merr.] corn (Zea mays L.) rotations in two fields and two rotation cycles. Liquid swine manure was applied before planting soybean to supply the P requirement of the 2-year rotation, and uniform fertilizer N was applied to corn. Manure increased whole-field crop yield (P#0.05) in five site-years but the application methods did not differ. Analyses for field areas with contrasting STP showed frequent yield response to manure in low-testing areas and seldom in optimum or high-testing areas. Larger yield response to VR than to FR in low-testing field areas in one site-year did not influence the whole-field yield response significantly. On average, the VR method applied 11% more manure compared with FR. Use of VR reduced STP variability by increasing STP more than FR in low-testing areas and reducing or not affecting STP in high-testing areas. Use of VR may not result in increased economic benefits for producers, but allows for better management of liquid manure and reduces the risk of P loss to water resources from manured fields.