Swine Effluent Application Timing and Rate Affect Nitrogen Use Efficiency in Common Bermudagrass

Bermudagrass [Cynodon dactylon (L.) Pers.] hay production is integral to manure management on southeastern swine farms. But swine effluent timing must be synchronized with crop nitrogen (N) demands to decrease the potential for soil N accumulation and nitrate (NO 3 ) leaching. Field studies were conducted on a Prentiss sandy loam (coarse‐loamy, siliceous, semiactive, thermic Glossic Fragiudult) to determine N‐use efficiency (NUE) and residual soil NO 3 –N. Two rates of 10 and 20 cm yr − 1 (∼260 and 480 kg ha −1 N, respectively) were applied in four timing treatments: April to September (full season), April to May, June to July, and August to September. Plots were harvested every 7 to 9 wk beginning in June, and soil was sampled in fall after a killing frost and the following spring. Annual uptake of N and P were least in the August to September timing treatment. Doubling the effluent rate increased N uptake 112% in 2000 (from 130 to 276 kg ha −1 ) and 53% in 2001 (from 190 to 290 kg ha −1 ), suggesting 10‐cm did not meet crop N demands. Due to low rainfall and decreased forage yield in 2000, doubling the effluent rate led to increased soil NO 3 –N to 30‐cm depth in fall 2000 and spring 2001. Averaged across timing treatments, soil NO 3 –N at 5‐cm depth ranged from 8.5 mg kg −1 in non‐irrigated controls to 39.6 mg kg −1 with 20‐cm effluent. Results indicate low NUE in the order of 30 to 38% for applications in August to September increase the risk to surface and ground water quality from excess N remaining in soil.