Determining the appropriate rate of fertilizer nitrogen (N) for corn production is one of the most important nutrient management decisions a farmer makes. Of the 4 R’s, rate of fertilizer N is the “Big R,” exerting the greatest impact on N availability to corn. The Maximum Return to N (MRTN) is the University of IL’s guidance and recommendation tool for fertilizer N application rates in Illinois for corn and is continually updated via on-going N rate trials throughout the state with NREC funding. Based on the price of corn and N fertilizer, the MRTN currently recommends approximately 190 lbs of N/A to corn following soybean in central Illinois. A recent evaluation of data from the Precision Conservation Management program (PCM) suggests that more than 50% of farmers apply rates above the MRTN (Farmdoc Daily).
As corn grain yields have increased, so too has the perception that more fertilizer N is needed to maximize yield since many farmers still mentally utilize a “yield goal” calculation for making nitrogen fertilizer rate decisions. However, advanced breeding efforts have resulted in corn hybrids today that require about the same amount of N as old hybrids 40 years ago but are able to provide much more grain per acre. Overapplication of fertilizer N will lead to decreased profits and will increase the likelihood of tile nitrate loss. An on-going NREC study allows farmers to establish an MRTN test strip in their field to compare corn yield and economics of their own N rate against those of the MRTN-recommended rate. Building on this study, we propose to validate the MRTN by evaluating corn yield and tile nitrate loss from 3 N rate treatments (MRTN rate plus and minus 40 lbs of N/A) using a replicated tile drainage site in Macon County. The study will simultaneously accommodate both phases of the rotation for a robust evaluation of tile nitrate loss in a corn-soybean rotation.
The value of this investigation towards the objectives of the IL Nutrient Loss Reduction Strategy will be clearly demonstrated as we observe the effect of potential N deficiency as well as excess N on grain yield and tile nitrate loss. This study will evaluate the impacts on grain yield and tile nitrate loss resulting from an 80-lb range of spring-applied N application rates centered around the MRTN rate range.
As our previous NREC studies have demonstrated, tile nitrate is not simply a matter of how much fertilizer N was applied during the corn phase of the rotation; we have found tile nitrate loss following soybean to be substantial, especially during warm, wet springs. Agronomists have long recognized that the fertilizer N requirement for corn is less following soybean than following corn. This rotation effect was known as the “soybean N credit” for many years and valued at about 40 lbs N/A. However, the term is now recognized as a misnomer since soybean grain removes more N from the field than is added via N fixation and is no longer used. The importance of the preceding crop, corn vs. soybean, is important from a net N mineralization standpoint, however, and the rotation effect is factored into the MRTN based on the algorithm developed for each region of Illinois. With both crop phases present in our proposed study, we will be able to annually measure the effect of previous crop on grain yield and tile nitrate loss across the 80 lbs/A range of fertilizer N application (low rate=150 lbs/A; MRTN =190 lbs/A; high rate=230 lbs/A) to evaluate the effect of weather on net N mineralization/plant available N.
Overwintering grass cover crops can act as a N catch crop and greatly reduce tile nitrate loss; however, the MRTN was not developed with cover crops as a management factor. Therefore, we propose to evaluate the MRTN recommendation with and without cover crops (winter barley ahead of corn and cereal rye ahead of soybean). We speculate that the added carbon from the cover crop may affect the fertilizer N requirement, possibly increasing the requirement of fertilizer N to corn while still reducing tile nitrate loss.