#07-135

Complete

Date Full Report Received

06/28/2011

Date Abstract Report Received

06/28/2011

Investigation

Institution:
Primary Investigator:
Co-Investigators: Matthew Helmers, Antonio Mallarino

In an earlier study conducted at Iowa State University from 2001 to 2006, liquid swine manure was applied to corn plots to give a total nitrogen application of 150 lbs/acre and the targeted nitrogen application rate of 200 lbs/acre to soybean plots under corn-soybean rotation production system. This study clearly showed that soybean plots (in rotation with corn in the previous year) and receiving liquid swine manure application resulted in much higher nitrate-nitrogen (NO3-N) leaching losses compared with soybean plots with no manure applications. Therefore, this study concluded that liquid swine manure application to soybeans at rates of 200 lbs-N/acre used in this study is not an acceptable practice. This study also concluded that more research should be conducted with lower nitrogen application rates from manure to soybeans so that farmers may have the option to apply liquid swine manure to soybeans and can still meet the nutrient loss criteria established for nitrogen leaching to groundwater. This resulted in initiating another six year (2007-2012) research study at Iowa State University in 2007 where the main objective of the study was to find an answer to the key question – “if applying swine manure to soybeans at a lower rate of 100 lbs-N/ac in the soybean year of corn-soybean rotation (but keep nitrogen application rates to corn at 150 lb/ac from swine manure in the corn year) can become an acceptable practice in Iowa”. In this study we are investigating the effects of lower nitrogen application rate to soybean plots on nitrogen leaching to groundwater. To this point we have collected three years (2007-2009) of nitrogen leaching data and preliminary results have shown that swine manure applications to soybeans at lower nitrogen rates of 100 lbs-N/acre do not necessarily result in increased nitrate-nitrogen (NO3-N) leaching losses to groundwater in comparison to swine manure applications to corn only at rates of 150 lbs-N/acre. Also, corn and soybean yields from this practice (when swine manure was applied to both corn and soybean plots) were among the highest. This limited data of three years indicate that swine manure application at the a rate of 100 lb-N/acre to soybeans can potentially become an acceptable practice if the remaining three years data of this study show similar trends as this practice of applying 100 lb/acre of nitrogen from manure does not seem cause additional leaching of nitrogen to groundwater and higher corn and soybean yields are expected. We definitely need to collect three more years of field data on water quality and crop yields before making final conclusions on this study. Once all six years of data on water quality are collected from this study, we would be able to conclude with confidence if swine manure applications to soybeans at a lower nitrogen rate of 100 lb/ac could be an acceptable practice for swine and crop producers of Iowa.