Effect of Tillage Implement and Date of Sweetclover Incorporation on Available Soil N and Succeeding Spring Wheat Yields

A 3-yr field study was conducted on a Dark Brown Chernozemic soil to assess the impact of the date of sweetclover (Melilotus officinalis Lam.) incorporation and tillage implement used for incorporation on succeeding spring wheat (Triticum aestivum L.) yields. During year 1 wheat was underseeded to sweetclover. In year 2 the clover was incorporated on either 15 June, 1 July or 15 July. Five tillage implements were evaluated (a) mouldboard plough (b) rotavator (c) tandem disc (d) offset disc and (e) deep tillage cultivator. In the spring of year 3, each treatment was seeded to wheat. Over the duration of the study only small and usually insignificant differences (P ≥ 0.05) in year 3 wheat yields occurred related to incorporation implement. Date of sweetclover incorporation during year 2 was a significant factor when the 15 June incorporation resulted in 6 and 17% greater wheat yields than 1 July or 15 July, respectively. Sweetclover dry matter production, measured at the early date of incorporation was 32% less than the two later incorporation dates. Mineral nitrogen, determined to a depth of 60 cm in year 3 immediately following wheat seeding, was 82, 75 and 62 kg ha−1 for the 15 June, 1 July and 15 July incorporation dates, respectively. It was concluded that the implement selected for sweetclover incorporation is not critical, however the timing of plough down is important and should be relatively early in the growing season in order to maximize succeeding grain production.