Fertilizer is one of the largest variable input costs to wheat (Triticum aestivum) producers and nitrogen is applied more than any other nutrient. In order to increase nitrogen use efficiency and lower production costs, this study was designed to assess the effectiveness of residual soil nitrate crediting and identify the optimal nitrogen rate for wheat grain yield across multiple environments in the Southern Great Plains. The study was conducted in a three replicate RCBD split plot design at five locations across Texas (Lubbock, Vernon, Commerce, McGregor, and Louise) during the 2017 and 2018 harvest seasons. Six nitrogen rates (main plots) were applied based on yield potential of each site (0, 0.8, 1.7, 2.5, 3.3, 4.2 kg N 100 kg-1 of expected grain yield) with estimated yield potentials ranging from 2,021 to 4,715 kg ha-1. Main plots were then split into subplots based on residual soil nitrate crediting depth treatments (0, 0-15, 0-30, 0-60 cm depths). Wheat grain yield only responded to nitrogen rate in three out of seven site years, increasing yield anywhere from 0 to 188%. Crediting residual soil nitrates down to 60 cm did not lower yield in any site-year. In some cases, higher nitrogen rates decreased grain test weight as much as 12.5% while soil nitrate crediting tended to increase test weight. These differences were most pronounced in 2017. Grain protein responded positively to nitrogen rate in three out of the four site years measured, even in cases where yield did not respond. In summary, the lack of yield response to nitrogen rate in four out of seven site years indicated that wheat was utilizing residual soil nitrates from below 60 cm in the soil. Findings support that wheat producers can safely credit residual soil nitrates down to 60 cm with no yield penalty though grain protein may decline slightly. Yield was maximized at rates between 2.7 and 4.3 kg N 100 kg grain-1.

This wide range requires further investigation to fine tune recommendations to producers.