The Effect of 15 N Fertilizer Rate and Application Time On Fertilizer Nitrogen Uptake Efficiency in Soft Red Winter Wheat.

World-wide, N is the most heavily applied nutrient in production agriculture, both in terms of tonnage and acreage. Currently, N fertilizer represents 31% of the input costs associated with Arkansas wheat (Triticum aestivum L.) production. The majority of Arkansas wheat research to date has been conducted on well-drained soils, but the majority of production is located on poorly-drained silt loam soils. Therefore, a study was conducted during the 2011-2012 growing season to help establish the N fertilizer needs and efficiency of wheat produced on poorly-drained soils using the stable isotope N¹⁵ to determine fertilizer N uptake efficiency (FNUE). Trials were conducted at the Pinetree Branch Station in Colt, Arkansas on a Calloway Silt Loam. The experimental design was a 3 x 5 factorial in a randomized complete block design with four replications. Three different application times were included to represent the current strategies employed by producers: single early, single late, and split application. Five different N fertilizer rates were applied by hand ranging from 0 to 224 kg N ha^-1 in 1.5 x 1.74 m microplots using 2.6 atom% N^15-labeled urea. There was a significant application time by rate interaction (p<0.0001). The greatest FNUE was achieved with the early and split applications at the 90 kg N/ha rate, and were 98.3 and 98.5%, respectively. Highest yields were achieved with the early and split applications with rates of 135 kg N/ha, and the FNUE of these treatments were not significantly different, but were numerically lower. The late application across all N rates resulted in lower FNUE and yield and may be attributed to the shorter time frame allowed for N uptake from application to sampling. These results highlight the importance of proper rate and application time for maximizing FNUE and yield in winter wheat production.