DELTA-YIELD ESTIMATION A shortcut to the multi-rate N trial is the delta-yield estimation method using the “N-Rate Evaluator” tool at www.gocorn.net (Figure 2). This method only requires two rates: a zero-N rate and a non-limiting N rate (Figure 3). The yield difference (“delta-yield”) between these treatments allows us to estimate the optimum N rate based on relationships established in historical Ontario N datasets. Pros • Requires only two N rates • Provides a reasonable estimate of the optimum N rate Cons • Requires a zero N rate (yield loss) THE “+/- 30-LB” TEST The simplest method is likely the “+/- 30-lb” test. This test involves testing N rates slightly higher and lower than the normal field N rate to see if yields respond. It will not deliver an optimum N rate like the first two methods but can indicate whether normal field N rates might be high, low, or about right. Pros • Require only two additional N rates • Does not require low N rates (minimal yield loss) Cons • Does not provide an optimum N rate Optimum N rates vary year-to-year and spatially across fields. One test in one year indicates what worked in that spot for that year, but replication of N tests over time and space is critical for confident decision-making. l THERE ARE MANYtools available to guide nitrogen (N) rate decisions, but nothing beats good on-farm N response data. These three methods might assist in determining how close your corn N rates are to the optimum. MULTI-RATE N TRIAL The gold standard is the multi-rate N trial. Several N rates are applied from zero to a non-limiting rate (e.g., 200-250 lb-N/ac), and an N response curve is generated (Figure 1). Optimum N rates typically occur a little before yields plateau. Pros • Most accurate method Cons • Requires several N rates (lots of plots) • May include N rates with yield losses • Statistical software required to fit yield response curves Optimum Corn N Rate CROP SIDE WITH... Crop side ONTARIO GRAIN FARMER 23 DECEMBER 2022 / JANUARY 2023 FIGURE 1. YIELDS, RESPONSE CURVE AND OPTIMUM N RATE FROM AN ONTARIO MULTI-RATE N TRIAL. PHOTOS COURTESY OF BEN ROSSER. Ben Rosser, Corn Industry Program Lead, Ontario Ministry of Agriculture, Food and Rural Affairs FIGURE 3. ZERO-N (LEFT) AND NON-LIMITING N (RIGHT) STRIPS AT AN ONTARIO CORN N TRIAL. 250 200 150 100 50 0 Yield (bu/ac) 0 50 100 150 200 250 Nitrogen Rate (lb-N/ac) 10-year average cost/price scenario MERN (price ratio 6.7)
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