Kevin Kuehner, Root River Field to Stream Partnership, Minnesota Department of Agriculture and
Jeff Vetsch – University of Minnesota Southern Research and Outreach Center
When choosing the best nitrogen (N) mangement practices, farmers and crop advisors must carefully consider the balancing act of achieving the most profitable N rate and timing practices while reducing risk for nitrate-N loss to surface and groundwater. Can the two co-exist with today’s high yielding corn hybrids? Table 1 summarizes the results of an on-farm nitrogen rate and timing study conducted in 2017. This high yielding corn following soybean site is located in Fillmore County. Seven preplant rates and three timing treatments were evaluated. Treatments ranged from 30 to 210 lb N/ac and included a 30 lb N/ac incidental N credit from fall-applied diammonium phosphate (DAP) and starter N at planting. The N rate recommendation when using the high end of the acceptable MRTN range was to apply 140 lb N/ac for C/S and 170 lb N/ac for C/C rotations. The maximum N rate guidance is to apply is no more than 160 lb N/ac for C/S and 200 lb N/ac for C/C. (Corn N Rate Calculator V.1.9, 2017, rounded to nearest 10, http://cnrc.agron.iastate.edu/)
Average corn yields at this site were excellent and peaked at 265 bu/ac. Remarkably, rates below 100 lb N/ac produced 72% to 95% of the maximum yield while rates between 120 and 150 lb N/ac produced 99% of the maximum yield. A significant yield increase occurred in one of three timing treatments when 90 lb N/ac was split applied as opposed to when all N was applied preplant. The economic return to fertilizer N (when using average 2017 fertilizer and corn price) was greatest at the 120 lb N/ac split and preplant treatments. It’s important to note that N rates with the greatest yield did not have the greatest profit. This is because the yield increase was not large enough to pay for the additional N fertilizer. When compared to the grower’s normal nitrogen program on this field, profits could have been improved by $25/ac when using the 120 lb N/ac preplant and split N rates. After three years of similar results, this producer has expressed interest in dialing back his nitrogen rates and shifting these savings into other aspects of his fertility program that will likely provide a better return on investment (i.e. liming and potash).
In addition to corn yield, soil samples were collected to a four-foot depth after harvest (early Nov.) to determine residual soil nitrate (RSN). High RSN increases nitrate leaching risk to ground and surface waters. RSN ranged from 15 lb N/ac to 121 lb N/ac across treatments, and the results showed that RSN was not significantly different among N rates less than or equal to 150 lb/ac. However, at 210 lb N/ac (40% greater than 150 lb), RSN increased 300%, clearly increasing risk for nitrate-N pollution to ground and surface waters.
In summary, this high-yielding site demonstrated that when economically optimum nitrogen rates were exceeded, profitability decreased while nitrate leaching risk to groundwater increased. How do your numbers compare? To help improve your bottom line while protecting water quality from controllable N losses, have a conversation with your crop retailer about your current N rate and timing program, and conduct your own on-farm nitrogen rate trial to see if there are opportunities to dial-in N management for improved profit and water quality.