The MRTN rates used were 140 lb N/acre for corn following soybean and 188 lb N/acre for continuous corn in the main area of Iowa and 154 lb N/acre for corn following soybean and 204 lb N/acre for continuous corn in the Southeast area of Iowa. Precipitation totals during those periods were evaluated for each of the site-years against the occurrence of an economic optimum N rate (EONR) more than 20 lb N/acre greater than an MRTN rate. The analysis indicates that the Southeast area of Iowa should use accumulated March through June precipitation, and the rest of Iowa April through June precipitation. The database from those long-term trials provides 216 crop rotation/site-years of data. Either March through June in Southeast Iowa or April through June in the Main area of Iowa (areas delineated within the CNRC). The “tool” developed from that long-term research is the accumulated total springtime precipitation. That is, a way (“tool”) to adjust applications when a greater than normal (MRTN) rate would be appropriate. A summary of long-term N rate trials with continuous corn and corn rotated with soybean conducted at seven ISU research and demonstration farms in Iowa from 1999 to 2016 has been an on-going basis for indicating the potential for needing an N rate greater than the MRTN rate suggested by the Corn Nitrogen Rate Calculator (CNRC). Or the question, do I need to apply additional N to my cornfields? At this time we do not know what the weather/precipitation will be for the entire springtime, but the extended period of wet conditions this spring is of concern. Wet springtime conditions typically raise questions about the status of applied nitrogen (N).
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