Plant‐Available Nitrogen Supply as Affected by Method and Timing of Alfalfa Termination

Abstract

Herbicide application may provide an alternative to intensive tillage for the termination of alfalfa stands, but might alter N release and N availability to subsequent crops. Our objective was to determine, under field conditions, the effect of timing and method of termination on the pattern of N release from perennial alfalfa, and on N uptake and yield of subsequent wheat crops. Four field experiments were initiated on perennial alfalfa (Medicago sativa L.) in southern Manitoba in 1992 and 1993. A factorial of three methods (herbicide, tillage, herbicide + tillage) and two times of termination (early summer, after first alfalfa cut, and late summer, after second alfalfa cut) was arranged in a randomized complete block design. A spring‐applied herbicide treatment was also included. Spring wheat (Triticum aestivum L.) was established after alfalfa termination. Soil NO−3 content, plant N uptake, and yield were then monitored for one to two years. In three of four experiments, plant‐available N in the spring after termination was higher in tilled treatments than in treatments receiving only herbicides. Regardless of method, plant‐available N in the spring after termination was reduced when termination was delayed from early to late season. Despite the lower short‐term plant‐available N supply in early‐ and late‐summer herbicide treatments, wheat yields in herbicide treatments were similar to or greater than those in tillage treatments. Differences in the N content among treatments diminished with time; by the fall of the second growing season after termination, differences in the cumulative available N supply were no longer evident. These results suggest that termination of alfalfa with herbicides may improve the synchrony between N release and N demand of a subsequent spring wheat crop, thereby improving N use efficiency.