Challenges of Managing Liquid Swine Manure

Liquid swine manure can be a resource for providing essential plant nutrients and building soil resources, but its management for maximum productivity with minimum environmental impact is often very difficult. Failure to understand the challenges may result in expectations or even regulations that are impossible to achieve. Our objective is to provide producers and policy makers with information regarding agronomic and environmental challenges of liquid swine manure management based on results from a 6-year study with continuous corn (Zea mays L.) and a corn-soybean [Glycine max (L.) Merr.] rotation in Iowa. Challenges that we encountered are openly discussed to increase the awareness of those people who may ultimately be developing policy and writing manure management legislation. Drainage volume, NO3-N concentrations, and NO3-N leaching loss were spatially variable, being influenced by seasonal precipitation as well as current and past manure application rates. Annual manure applications for continuous corn increased soil-test P and K values because more nutrients were supplied than removed through grain harvest. Biennial applications also increased soil-test P but not K. We suggest developing local manure nutrient databases to guide application rates, using total Kjeldahl N (TKN) values to estimate plant-available N, monitoring manure loads to quantify variability and actual loading rates, and routinely using soil-testing to determine if application rates should be based on P rather than N content in the manure