Impact of in-Season Variable Rate Nitrogen Management on Corn Yield and Nitrate Losses in Minnesota.

There is a perceived economic benefit of site specific nitrogen management in the Midwest of the US where the saving is based on field variability, crop type, rainfall time and intensity, commodity price, fertilizer cost and application cost. However research is needed to demonstrate the effect of site specific nitrogen management on nitrate pollution reduction especially in rural areas where domestic wells are a main source of water supply. In this field scale study, rainfed corn (Zea mays) was planted in the spring of 2016 at the Ecology and Agricultural Research Farm of the University of Minnesota in Waseca, and a variable rate in-season nitrogen treatment was compared to the conventional pre-plant uniform rate nitrogen treatment under two drainage intensities. The first intensity represented the conventional drainage rate of the region while the second represented a lower drainage intensity. Conventional nitrogen treatment consisted of applying the economically optimum nitrogen rate (EONR of 151 kg/ha) one day before planting, while the in-season variable rate treatment consisted of a rate of 50 kg/ha of nitrogen applied before planting and the remaining applied at growth stage V7. The variable rate side-dress application rate was derived from Crop Circle Green NDVI sensing at V6 in conjunction with sensing a zero applied nitrogen strip and an EONR applied nitrogen strip. Data were collected from a total of 8 sub-fields, each drained and monitored individually. Drainage samples were collected weekly the entire growing season and analyzed for nitrates allowing for flow weighted concentrations.  At the end of the growing season, this research will answer not only the economic impacts of the in-season variable rate nitrogen management, but more importantly its environmental benefits.