Nitrogen Management: 1.2 For Who?
Ken Janovitek, University of Guelph
Greg Stewart, OMAF Corn Specialist

Nitrogen fertilizer rates for corn are often based on the idea that it takes 1.2 Ib of fertilizer nitrogen (N) to produce 1 bushel of corn. However, countless research studies throughout Ontario, and surrounding corn-belt States, suggest that actual fertilizer N requirements are usually substantially less than the rates predicted using the 1.2 Ib-N/bu formula. In fact, current nitrogen fertilizer recommendations for Ontario and surrounding corn-belt States clearly suggest that fertilizer rates based on the 1.2 Ib-N/bu formula are rarely recommended. What is the historical basis for the 1.2 Ib-N/bu recommendation? Why is it that nitrogen rates based on the 1.2 Ib-N/bu formula are rarely appropriate for current day corn production? Research conducted throughout the 1960's and early 1970's indicated that 1.2 Ib-N/bu was a reasonable predictor of fertilizer N requirements for corn. During this time, the majority of corn fertilizer N research was conducted where corn had been planted
continuously. Current corn N fertilizer recommendations for surrounding corn-belt states are presented in Table 1. Current fertilizer N recommendations in surrounding states where corn follows corn are, for the most part, in the 1.0 to 1.3 Ib-N/bu range. Exceptions are made in some states where soils have relatively high N supplying potential and/or high organic matter levels where recommended fertilizer N rates fall below 1.0 Ib-N/bu even for corn after corn. Current Ontario recommendations were developed based on the idea that fertilizer N requirements are not consistent across the province. Only in the south-west region for corn planted following corn where fertilizer N is applied preplan! does the 1.2 Ib-N/bu formula closely predict fertilizer N requirements presently recommended in Ontario. Throughout the rest of the province, and/or when fertilizer N is sidedress applied, current Ontario fertilizer N recommendations are often less than 1.0 lb-N/bu. Most US corn-belt States apply a 30 to 40 Ib-N/ac credit for corn planted following soybeans. This has an effect of almost consistently reducing recommended fertilizer N rates to less than 1.0 Ib-N/bu for corn following soybeans (Table 1). In fact, for soils associated with high N supplying capabilities, the fertilizer N recommendations are often below 0.8 Ib-N/bu. Current Ontario recommendations suggest that soybean credits are about 13 Ib-N/ac in the southwest and 27 Ib-N/ac throughout the rest of the province. After applying these soybean credits, current Ontario N recommendations clearly suggest that fertilizer N rates are less than 1.0 Ib-N/bu. The Ontario Corn Producers' Association, in conjunction with the Agricultural Adaptation Council (CanAdapt program), provided funding for an examination of all Ontario corn N research for the purposes of updating/revising current Ontario recommendations. A preliminary analysis of the historic data suggests that:

1) a 20 to 30 Ib-N/ac credit for corn following soybeans is warranted compared to growing corn after grain corn
2) that sidedress rates are 10 to 20% lower than the preplan! recommended rate for the same crop
3) that actual fertilizer N requirements for corn in Ontario are often less than 1.0 Ib-N per bushel of expected yield when corn follows soybeans in rotation.

The 1.2 Ib-N/bu formula may have been a reasonable predictor of corn fertilizer N requirements 30 years ago when there was a tendency to plant corn continuously. However, now that most corn is planted in rotation, the 1.2 Ib-N/bu formula often over estimates (by double in many parts of Ontario) corn N fertilizer requirements. A fact that you may want to consider as you face the challenges of running a farm operation that is economically viable and environmentally sustainable.

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