An Alternative Rationale for Corn Nitrogen Fertilizer Recommendations

Nitrogen recommendations based on corn (Zea mays L.) yield goal can overestimate N needs if yield expectations are not realistic, and yield goal‐based N recommendations may dot be well correlated with observed economic optimum N rates. This study investigated the feasibility of basing corn N recommendations on soil‐ and year‐specific data rather than on yield goals. Soil‐specific data from multiyear corn N rate studies, which included several soils and locations, were separated into high‐ and low‐yielding groups based on each year's maximum yield. Based on a 24‐yr corn N rate study on a Rozetta silt loam soil (finesilty, mixed, mesic Typic Hapludalfs) at Lancaster, WI, economic optimum N rates in high‐ and low‐yielding years were nearly identical (160 to 170 lb N/acre). Economic optimum N rates in individual years were poorly correlated (R2 = 0.02) with grain yields even though maximum yields ranged from 70 to 170 bu/acre. Similar results were obtained with other soils and locations. Greater N uptake with high yields does not require more applied N due to higher apparent recovery of available N in years with favorable growing conditions. A preplant soil nitrate test effectively adjusted optimum N rates for year‐specific variations in soil nitrate content and greatly improved the accuracy of N recommendations. This test also provides a strategy for minimizing potential nitrate losses to groundwater. Our results indicate that the optimum N rate for corn is a soilspecific characteristic and should not be adjusted according to yield goal. Corn N recommendations based on soil‐specific data and used in conjunction with a preplant soil nitrate test offer an improved approach for identifying optimum N rates. Research Question Ideally, N recommendations for corn should allow profitable production and minimize nitrate‐N losses to the environment. Nitrogen recommendations based on soil‐ and year‐specific data may provide a more accurate estimate of crop N needs than currently used yield goal‐based recommendations. The objective of this work was to investigate an alternative rationale for corn N recommendations in Wisconsin based on soil‐ and year‐specific data. Literature Summary Nitrogen recommendations for corn are based on yield goal in most of the USA, and a factor of about 1.2 Ib N/bu of expected yield is widely used to calculate N application rates. Excess N applications due to selection of unrealistic yield goals, lack of consensus on how yield goals should be determined, and reports of poor relationships between observed optimum N rates and current recommendations provide an incentive to investigate alternative approaches to development of corn N recommendations. Study Description To evaluate the relationship between corn yields and N recommendations, data from a 24‐yr corn N rate study on a Rozetta silt loam soil at Lancaster, WI, were separated into high‐ and low‐yielding groups based on the maximum yield attained each year. Years with a significant response to added N were identified using an analysis of variance and the response was characterized using a quadratic model. The economic optimum N rates in the high‐ and low‐yielding groups were calculated using a N/corn price ratio of 0.075 bu/lb of N. Results from the Rozetta soil were compared with optimum N rates found in high‐ and low‐yielding years for other soils and locations in Wisconsin. The year‐specific effects of preplant soil nitrate content on economic optimum N rates for corn were determined at all site‐years where N response was measured at uniform soil nitrate‐N values. Applied Questions Does the optimum N rate for corn vary with the yield obtained? Based on data from 22 N‐responsive years between 1967 and 1990 in the Lancaster experiment, optimum N rates were 160 to 170 lb N/acre in both high‐ and low‐yielding year groups (Fig. ). When data from each year were considered separately, optimum N rates were poorly correlated with grain yields even though maximum yields ranged from 70 to 170 bu/acre. These findings are consistent with results from other soils and locations, and indicate that the optimum N rate for a specific soil is not closely related to the corn yield obtained. Why are optimum N rates similar at high and low yield levels? Corn yield response to applied N and economic optimum N rates in high‐ and low‐yielding years. image High corn yields usually result in more crop N uptake than lower yields. Our results indicate, however, that the greater N uptake with high yields does not require more applied N due to higher apparent recovery of available N by the crop in years with favorable growing conditions. Conditions promoting high corn yields, such as adequate moisture and temperature, apparently improve the efficiency of available N use by the crop resulting in a constant requirement for added N to optimize yields. Is year‐specific information needed in a N recommendation system based on soil N response data? Soil N tests capable of detecting annual variations in N availability are a key component of N recommendations based on soil‐specific data. In our work, a preplant soil nitrate test was very effective for making these year‐specific adjustments and greatly improved the accuracy of the corn N recommendations. Recommendation The optimum N rate for corn is a soil‐specific characteristic and should not be adjusted for year‐to‐year variations in expected yield. Applying insurance N with the expectation of high yields or applying lower N rates with the expectation of a poor growing season probably will reduce profits. A preplant soil nitrate N test calibrated for specific cropping systems can be used to adjust the optimum N rate for annual variations in soil nitrate content.