QUADRATIC AND QUADRATIC-PLUS-PLATEAU MODELS FOR PREDICTING OPTIMAL NITROGEN RATE OF CORN - A COMPARISON

Based on quadratic response models, the University of Illinois recomme nds 22 kg fertilizer N Mg-1 corn (Zea mays L.) grain yield, with adjus tments for cropping history, manure, and planting date. Recent work su ggests recommendations based on the quadratic model are excessive. Thi s field study evaluated the quadratic-plus-plateau and quadratic model s and the current recommendation for a 10- to 12-yr, two-location, two -crop-sequence data set with 5 N fertilizer rates in Illinois. For all location-sequences, we detected a systematic bias with the quadratic model as compared with the quadratic-plus-plateau model. The quadratic response model predicted a maximum grain yield of 3 to 6% larger and an optimal N fertilizer rate 5 to 60% larger than that predicted by th e quadratic-plus plateau model and decreased profit from $0.61 to $17. 12 ha-1 yr-1. Current University of Illinois recommendations differed by from -6 to + 104 kg N ha-1 from the amount predicted by the quadrat ic-plus-plateau and resulted in a profit reduction of $0.01 to $31.42 ha-1 yr-1. These analyses indicate that the quadratic-plus-plateau is preferable to the quadratic model for predicting N fertilizer requirem ents of corn. Economic analysis indicates that the consequences of usi ng the quadratic model rather than quadratic-plus-plateau model can be large, but not in all cases. Furthermore, the general recommendation of 22 kg N Mg-1 of expected grain yield minus rate adjustments is not appropriate in all cases. Site-specificity plays a large role in deter mining optimal N rates.