Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/4/2006
Publication Date: 11/1/2006
Citation: Scharf, P.C., Kitchen, N.R., Sudduth, K.A., Davis, J.G. 2006. Spatially variable corn yield level is a weak predictor of optimal nitrogen. Soil Science Society of America Journal. 70:2154-2160.
Interpretive Summary: For most grain crops, farmers apply some form of nitrogen (N) to help meet the crop’s nutrient needs. With modern-day agriculture, N fertilization is essential to economically grow a crop. Historically, fertilizer recommendations have used the idea that you could calculate the amount of N to add to the soil based on the amount of crop you would expect to grow. This is commonly referred to as a yield-goal based N recommendation. This approach assumes you can reasonably estimate yield months before you harvest the crop. In recent years research has shown that this type of approach can be a poor predictor of how much N fertilizer is actually needed for corn production in the U.S. Midwest. However, most of these studies have either been conducted on small plots or are the averages of large fields. Our goal with this study was to evaluate the relationship between yield levels and the economically optimal N fertilizer rate (EONR) within large production fields. This information is needed to test the use of yield goal as a basis for N fertilizer recommendations. The amount of N needed to grow corn was highly variable within seven of the eight fields we investigated over three growing seasons. On average, corn yield variation explained only 15% of the variability in EONR. This means that even if a farmer was able to determine, very early in the growing season, how much his corn crop would yield and how yield would vary across a field, it still would not help him know how much N fertilizer should be applied. Although we observed considerable within-field variations in EONR, this was mainly due to differences in how much N was supplied by the soil and/or in N uptake efficiency by the crop, rather than to variations in crop demand for N. Our study showed that if the correct EONR could be determined early in the season when N fertilizer is applied, profit would increase by about $15 per acre, compared to a field-average yield-goal based fertilizer recommendation (this excludes costs associated with determining EONR and variable-rate fertilizer application). Importantly, these results showed that yield variability appears to be a small part of the information that must be used to make successful variable-rate N recommendations for corn. These results justify the development of new technologies and procedures that will help farmers determine EONR early in the growing season. Farmers will directly benefit from such strategies because N fertilizer costs have increased significantly in recent years. Also, if such strategies reduce excess applications of N fertilizer to fields, then the general public will benefit since N loss to lakes and streams will be reduced and the environment will be improved.
Technical Abstract: Historically, a mass-balance approach (yield goal times a factor) has been the dominant method for making N fertilizer rate recommendations. Although several states have moved away from the mass-balance approach for N rate recommendations for corn (Zea mays L.), much of the effort that has gone into variable-rate N research has focused on combining spatial yield predictions with a mass-balance approach. Our objectives were to evaluate, at field scale, the relationship between spatially variable yield levels and economically optimal N fertilizer rates (EONR)and to evaluate the performance of yield-based N rate recommendations. Eight experiments were conducted in three major soil areas (Mississippi delta alluvial, deep loess, claypan) over three years. Treatments were field-length strips of discrete N rates from 0 to 280 kg N/ha. Yield data obtained by combine yield monitoring were partitioned into 20-m increments and a quadratic-plateau function was used to describe yield response to N rate for each 20-m section. EONR varied much more widely than did plateau yield. Yield level explained on average only 0.15 of the variability in optimal N rate. Averaged over the eight fields, variable application of mass-balance-based N rates based on actual yields would have increased yield by only 31 kg/ha, and profit by $2/ha, relative to uniform mass-balance N rates based on field average yields. In comparison, variable-rate application of optimal N rates would have increased profit by an average of $38/ha. Of this, $14/ha could have been obtained by uniform application of the median optimal N rate for each field. We conclude that although we observed considerable spatial variability in optimal N rates, this was due mainly to variations in soil N supply and N uptake efficiency, rather than to variations in crop demand for N. Yield variability appears to be at best a small part of the information that must be used to make successful variable-rate N recommendations for corn.