Plant-available nitrogen supply as affected by method and timing of alfalfa termination

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 NO3(-) 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.