Several studies in the Midwest region of the United States have documented yield increases as a result of manure application on soybeans. However, there is no single conclusion as to why this increase in yield occurs, and it is difficult to predict when and if a yield increase will occur.

Few studies have measured the water quality impacts of manure application on soybeans. Studies in Minnesota and in Iowa have found increased soil nitrate (Minnesota) or increased tile drainage nitrate levels (Iowa) when manure was applied prior to soybeans. However, the drainage studies in Iowa applied a fairly high rate of nitrogen as liquid swine manure prior to soybeans (~150-200 lb-N/acre). There have been no direct water quality measurements when manure is applied to soybeans at a rate that matches 50% of the total soybean nitrogen management requirement (~100 lb-N/acre).

Overall, based on the studies that have measured nitrate concentrations and losses, there is the potential that application of manure to both corn and soybeans in a corn/soybean rotation could increase the risk of nitrate loss.

Additional information about manure application to soybeans is available at the Livestock and Poultry Environmental Learning Center.

Author: Matt Helmers, Iowa State University