Compatibility of reed canarygrass with alfalfa and birdsfoot trefoil
Perennial grass-legume mixtures are an important component of the hectareage harvested for hay and by grazing ruminants in the U.S.A. An ideal perennial grass-legume mixture has an intermediate botanical composition, but competitive exclusion of one component by the other is a common problem. Our objectives were to (1) characterize competitive relationships between reed canarygrass (Phalaris arundinacea L.) and alfalfa (Medicago sativa L.) and birdsfoot trefoil (Lotus corniculatus L.) in binary reed canarygrass-legume mixture, (2) describe mechanisms of competition displayed by these species, and (3) define traits amenable to selection which could improve legume compatibility of reed canarygrass. Both legumes dominated reed canarygrass following an initial establishment period. Niche divergence, apparent in reed canarygrass-birdsfoot trefoil mixtures, was responsible for their stability relative to reed canarygrass-alfalfa mixtures, which did not exhibit niche divergence. At early harvests, when competitive forces were rapidly changing botanical composition, legumes were taller and more mature in mixture than in monoculture, resulting in increased shading of reed canarygrass. Height and early maturity of reed canarygrass genotypes were associated with competitiveness against legumes. Tiller density, the more important reed canarygrass yield component in our study, exhibited a greater competitive effect against legumes than tiller weight. Mechanisms of competition changed over time. Increased height and advanced maturity of legumes in mixture relative to monoculture were present only at early harvests. Likewise, residual variation for legume dry-matter yield (DMY) among mixtures with 14 reed canarygrass clones, beyond that explained by the clones' mixture reed canarygrass DMY, was important only at early harvests. Both at early harvests, when reed canarygrass dominated, and at late harvests, when legumes dominated, suppression (defined as DMY reduction of a given component per unit DMY of its competitor) of the dominant component exceeded suppression of the dominated component. Selection for high DMY, height, early maturity, and low rhizomatous spreading ability should improve legume compatibility of reed canarygrass.