Spring wheat response to tillage and nitrogen fertilization in rotation with sunflower and winter wheat

Spring wheat (Triticum aestivum L.) is a major crop in the northern Great Plains that is generally grown following a 21-mo fallow period. A 12-yr study was conducted to determine the effects of tillage system [conventional-till (CT), minimum-till (MT), and no-till (NT)], N fertilizer rate (34, 67, and 101 kg N ha(-1)), and cultivar (Butte86 and Stoa) on spring wheat yields within a dryland spring wheat (SW)-winter wheat (WW)-sunflower (Helianthus annuus L.) (SF) rotation. Grain yield responses varied with tillage system, N fertilizer rate, cultivar, and year as indicated by significant tillage x N rate x year and N rate x cultivar x year interactions. In years with >260 mm total plant available water (TPAW) but <400 mm TPAW, NT grain yields were greater than those with CT at the highest N rate, with similar trends at the medium and low N rates. When TPAW exceeded 400 mm, grain yields for CT were generally greater than for NT at the medium N rates. The greatest 12-yr average grain yield (1727 kg ha(-1)) was obtained with NT and application of 101 kg N ha(-1). Grain yields were lowest during years when TPAW was <300 mm, with only small responses to tillage and N treatments. Cultivars responded similarly to N fertilization in years with >300 mm TPAW, with Butte86 yielding more than Stoa in 6 out of the 12 yr, Soil NO3-N levels increased in the root zone following three consecutive drought years, but had declined to initial year levels by the end of the study. These results indicate that farmers in the northern Great Plains can produce SW following SF in annual cropping systems that do not include a fallow period, particularly if NT or MT systems are used with adequate N fertilization.