UNL releases new nitrogen needs calculator for corn

Nutrient response trials were conducted from 2002 to 2004 for a total of 34 site-years throughout different agroecological zones of Nebraska, including most major irrigated corn areas. Of the 34 site-years, 13 were on research stations and 21 were in producers’ fields. The data set included 11 no-till sites, 9 with ridge-till and 14 with conventional tillage (disk or chisel plow). At 16 sites corn was grown following corn, at 13 corn followed soybean, and at five sites in western Nebraska corn followed dry bean. At most sites, maximum corn yields were in the range of 210 to 275 bu/ac.

Figure 1. Influence of the corn value ($/bu) to N price ($/lb N) ratio on the average economically optimal N rate (EONR) that should be used to maximize profit from corn production.

Economic analysis of corn yield response to nitrogen application revealed that at a corn value ($/bu) to N price ($/lb N) ratio of 10:1, the economically optimal N rates (EONR) were 155 lb N/acre for corn following corn or dry bean (CC) or 120 lb N/acre for corn following soybean (CS). The EONR is the amount of nitrogen that leads to maximum net return from nitrogen application for a given price scenario. The EONR declined with decreasing corn value to N price ratios, i.e., with increasing cost of nitrogen relative to the corn value (Figure 1).

Results showed that if it is correctly implemented -- using a realistic yield goal, good soil sampling and soil test results and full credits for previous crop, irrigation and manure -- the UNL N recommendation for corn results in profits that are close to the maximum profit for price scenarios in which corn is valued high relative to N. Adjustments of recommended N rates according to prices are of greatest importance in times when the corn value to N price ratio is below about 8 : 1. Therefore, two additional coefficients were added to the UNL equation that allow producers to adjust the recommended N rate according to different timing of N application (e.g., fall, pre-plant, split) and to the actual corn : nitrogen price ratio:

N-rate (lb/acre) = [35 + (1.2 EY) – (0.14 EY x SOM) – (8 NO₃-N)– other N credits] × f_A × f_R
EY = expected yield (bu/acre), e.g., 5-year average yield + 5%
NO₃-N = root zone soil residual nitrate-N in 2-4 ft depth, depth-weighted average (ppm)
SOM = soil organic matter content in 0- to 8-inch depth (%)
f_A = application timing adjustment factor
f_R = corn : nitrogen price ratio adjustment factor

The revised recommendation equation significantly improved the performance of the UNL algorithm for estimating the optimal nitrogen rate needed for maximizing profit in corn production. It provides more flexibility in terms of accounting for varying price ratios and the rigorous nitrogen response trials conducted during 2002 to 2004 have demonstrated that corn yield near the yield potential can be obtained with the recommended N rates.