Herbage production as influenced by stand arrangement, nitrogen fertility and supplemental water

Nitrogen (N) fertilizer is usually applied to grass-legume herbage stands at a rate representing a compromise between the low N tolerance of the N-fixing process of legumes and the high N requirement of grasses. The productivity and N responsiveness of a bromegrass-alfalfa association was compared for stands in which the components were arranged at random (conventional grass-legume mixture, GLM) with those in adjacent, 1.5-m-wide strips of grass and legume (strip-culture, SC). One experiment compared three stand arrangements (GLM, and SC stands with grass:legume land areas of 1:1 and 2:1) receiving two rates of supplemental N (0 and 60 kg ha−1 N). A second experiment compared GLM to the 1:1 SC stand at five rates of supplemental N (0, 40, 80, 120, and 160 kg ha−1 N restricted to grass-containing areas of each stand treatment) under both natural and supplemental soil-moisture conditions. Physically separating the grass and legume plants in the SC stands reduced total herbage dry matter (DM) yield, particularly when little or no N was applied. In the SC stands, the reduction in DM yield caused by physically separating the grass and legume plants of the herbage mixture was compensated for by an increase in the apparent efficiency of N fertilizer use by the grass plants in that association. Herbage DM yield was enhanced greatly by supplemental water but the effects of N fertilization and/or stand arrangement were not modified by soil-moisture conditions. Strip-culture stands facilitate independent optimization of inputs for components of grass-legume associations. Key words: Grass-legume mixtures, nitrogen response, stand arrangement, herbage production, irrigation, bromegrass-alfalfa