Nitrogen transfer from alfalfa to grass

Abstract

It has long been recognized that grass growing in association with alfalfa derives a benefit from that association. Grass growing adjacent to the legume tends to be greener and more productive than that further away, a growth response similar to that produced by applications of N fertilizer. This suggests that the grass is benefiting from N fixed by the alfalfa. As a result a mixture of grass and alfalfa may yield more than a pure stand of alfalfa. Typically the increase will average approximately 10 to 15 %. At Swift Current, water is the principle limiting factor in forage production. Several strategies have been devised to reduce the impact of moisture shortages on long-term productivity. These include seeding the grass and the alfalfa in separate, alternating rows, and spacing the rows as far as 1 metre apart. However, despite such a separation of the grass and the alfalfa, the grass/alfalfa combination will out-yield stands of either grass or alfalfa grown alone. Very few estimates of the amounts of N transferred from a legume to its associated grass have been published, even fewer obtained under field conditions. Those that have been published have dealt with transfers in solid grass/legume stands over very short distances and under moist temperate conditions. The study reported herein was established to determine the level of N benefit derived by grasses from the nitrogen fixed by alfalfa when grown in alternate rows at wide row spacings under the semi-arid conditions of south western Saskatchewan. In addition, an estimate of N-fixation by the alfalfa growing under dry land conditions was to be obtained and the extent to which the yields of such grass/alfalfa stands could be increased by N fertilizer applications was to be determined.