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ARS Home » Midwest Area » Ames, Iowa » National Laboratory for Agriculture and The Environment » Agroecosystems Management Research » Research » Publications at this Location » Publication #262273
Title: Economically optimal N rates of corn management zones delineated from soil and terrain attributes

Author
item Jaynes, Dan
item Kaspar, Thomas
item COLVIN, THOMAS - Retired ARS Employee

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/28/2011
Publication Date: 6/1/2011
Citation: Jaynes, D.B., Kaspar, T.C., Colvin, T. 2011. Economically optimal N rates of corn management zones delineated from soil and terrain attributes. Agronomy Journal. 103:1026-1035.

Interpretive Summary: Much of the nitrate found in Midwest streams comes from leaching of nitrogen fertilizers applied to corn. To reduce this leaching, nitrogen use efficiency must be improved. Dividing fields into smaller areas that respond more uniformly to management is one approach for improving efficiencies, but crop responses to nitrogen management within proposed management zones have been mixed. The objective of this study was to determine and compare economically optimal nitrogen fertilizer rates for corn in a corn/soybean rotation within yield zones in three producer fields. The yield zones were determined based on previous studies using soil and landscape characteristics that were correlated with multi-year spatial patterns of corn yield. Delineated zones were roughly consistent with landscape position resulting in Toe, Foot, Back, and Shoulder yield zones. Nitrogen was applied at six rates between 0 and 200 lb/acre within replicated small plots (15.1 x 4.5 m) located in each yield zone. Corn yields increased with increasing nitrogen rate in all years. Maximum yields among the yield zones typically followed the pattern of Toe = Foot = Back >> Shoulder. Conversely, economically optimal nitrogen fertilizer rates followed the reverse pattern and varied from 20 to 220 lb/acre meaning that maximum corn yield is a poor predictor of optimal nitrogen rate within these yield zones. Based on six field-years of data, the optimal nitrogen rate could be reduced by applying relatively less nitrogen fertilizer to the Toe and Foot yield zones and relatively more to the Shoulder to Back yield zones. Thus, dividing fields into yield zones may be a viable method for improving nitrogen fertilizer rates to corn and potentially reducing nitrate losses to surface and ground waters. These results will help researchers design and interpret nitrogen fertilzier use efficiency experiments resulting in improved recommendations for farmers.

Technical Abstract: Much of the nitrate found in Midwest streams comes from leaching of nitrogen (N) fertilizers applied to corn (Zea mays L.). To reduce this leaching, N-use efficiency must be improved. Dividing fields into areas that respond to management more uniformly is one approach for improving efficiencies, but crop responses to N management within these management zones have been mixed. The objective of this study was to determine and compare economically optimal N fertilizer rates for corn in a corn/soybean [Glycine max (L.) Merr.] rotation within yield zones in three producer fields. The yield zones were determined based on previous studies using soil and landscape characteristics that were correlated with multi-year patterns of corn yield. Yield zones were roughly consistent with landscape position resulting in Toe, Foot, Back, and Shoulder yield zones. N was applied at rates of 0, 45, 90, 135, 179, and 224 kg ha-1 within replicated small plots (15.1 x 4.5 m) located in each yield zone. Yield v. N rate for each yield-zone were fit to the Mitcherlich equation using nonlinear methods. Final Mitcherlich parameters and economically optimal N-rates with their correct confidence limits were computed for each yield zone. Corn yields increased with increasing N-rate in all years. F-tests showed that the individually fitted yield-zone parameters for the Mitcherlich C were not significantly different among yield-zones within any field-year or among all field-years so a single value for C was fit to all response curves. Maximum yields among the yield zones typically followed the pattern of Toe = Foot = Back >> Shoulder with the exception of 2003 when a wetter than normal June may have led to increased losses of soil nitrate from the root zone by leaching or denitrification compared to other years, especially in the lower areas of the field represented by the Toe zone. Conversely, economically optimal N rates followed the reverse pattern to maximum yields, again with the exception of 2003 and varied from 23 to 247 kg ha-1. Thus, maximum yield is a poor predictor of optimal N rate for these yield zones, but based on the 6 field-years, the optimal N rate could be reduced by applying relatively less N fertilizer to the Toe and Foot yield zones and relatively more to the Shoulder to Back yield zones.