SEUIL DE NUISIBILITÉ DU CHIENDENT (Agropyron repens (L.) Beauv.) DANS LA LUZERNE (Medicago sativa L.)

Abstract

A field study was conducted at St-Augustin, Québec, in 1984 and repeated in 1985 to determine the influence of quackgrass on growth, stand counts, yield and quality of alfalfa established with or without oats as a companion crop. Four densities were obtained by treating quackgrass with glyphosate on 100, 50, 25 and 0% of the plot area. The results indicate that without oats, quackgrass reduced the growth of alfalfa. About 60 d after emergence, a density of 119 shoots m−2 reduced the root dry weight of alfalfa seedlings, but there was no effect of quackgrass on the shoot dry weight and the foliar area in 1984. In 1985, an infestation of 161 shoots m−2 reduced the above three parameters. In both years after establishment, alfalfa stands were thicker in quackgrass-free plots. In pure seedings, quackgrass increased forage yield but decreased the quality of the forage during both seeding years. As compared to the quackgrass-free treatment, a density of 275 shoots m−2 increased forage yields at first cutting of 1985, but there was no effect in 1986. Using oats significantly reduced the aerial growth of quackgrass in both seeding years and at first cutting of year after 1985 seeding. Forage quality, as evaluated by NDF concentration, was reduced at infestations of ca. 300 shoots m−2. Higher infestations were necessary to reduce crude protein concentration relative to that of quackgrass-free alfalfa. The ligno-cellulose content (ADF) was not influenced by quackgrass densities. The critical density of quackgrass in alfalfa lies between 150 and 300 shoots m−2. Above this level, quackgrass significantly reduces the quality of the forage.Key words: Quackgrass, alfalfa, critical density, forage yield, forage quality, companion crop