Variable nitrogen (N) rate application is one of the emerging technologies in an effort to improve nitrogen use efficiency (NUE) of grain crops. However, it is important to determine the optimum field element size when variable treatments result in a greater impact on yield and NUE. This study was conducted to determine the effect of treating field spatial variability in winter wheat at different resolutions on grain yield and NUE. The treatments included variable N rate application at 3 resolutions (0.836 m2, 13.37 m2, 26.76 m2), a fixed N rate at 100 kg ha-1 applied pre-plant and top-dress, and a check plot. Treatments were arranged in a complete randomized design (CRD) with 3 replications at two locations (Tipton and Chickasha, OK). Plots treated variably were sensed at Feekes 5 using the Green SeekerŪ hand held sensor prior to fertilization. The N rate for each microplot was calculated using the present algorithm in wheat and that is composed of predicted yield potential, response index (RI) and coefficient of variation (CV). While yield did not show significant differences across treatments, NUE significantly increased by an average of 50% at both locations when N was applied variably compared to fixed rate application. The highest NUE, 60% occurred at the 0.836 m2 resolution for the Tipton site and 27% at the 26.67 m2 resolution for the Chickasha site.
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