Tillage and nitrogen rate effects on winter wheat yield in a wheat-sorghum rotation

The objectives of this study were to quantify long-term tillage practice and nitrogen (N) fertilizer rate effects on yield and N use in a winter wheat (Triticum aestivum L.)-grain sorghum (Sorghum bicolor L. Moench)-fallow (W-S-F) rotation. The experimental design was a randomized complete block with a split-split-plot arrangement. The main plot treatments were crop rotation phases W-S-F, S-F-W, and F-W-S. The sub-plots were tillage practices, i.e., conventional tillage (CT), reduced tillage (RT), and no-tillage (NT). And the sub-sub-plot treatments were N rates of 0, 45, 90, and 134 kg ha-1. Wheat yield increased at rates of 15.6, 9.3, 22.8, and 25.7 kg ha-1 for a kg N ha-1 increase in very low-, low-, high-, and very high-yielding environments (average yields of similar to 2000, 2500, 2800, and 4400 kg ha-1), respectively. On average, winter wheat yields were 7%-9% greater for CT compared with both NT and RT. Winter wheat removed about 52 kg N ha-1 from the unfertilized control treatment, but N uptake varied by N rate and growing conditions. Nitrogen use efficiency, N agronomic efficiency, and applied N recovery decreased as the N rate increased. Across environments, wheat yield increased by 16, 20, and 17 kg ha-1 for each additional kg ha-1 N applied under CT, NT, and RT, respectively, and additional 2-2.5 kg ha-1 yield increases for a mm increase in fallow precipitation. We concluded that wheat yield response to N is highly dependent on growing conditions, and NT required greater N fertilization than CT and RT for similar yields.