Alfalfa seed yield and quackgrass density were measured in seven irrigated fields in southern Alberta to assess the effect of quackgrass on seed yield. Six herbicides were tested to determine their effect on quackgrass growth and accompanying alfalfa seed yield. The effect of quackgrass on seed yield was adequately described by a hyperbolic model. Quackgrass densities of 50 and 100 heads per m2 reduced seed yields by 56 and 74%, respectively. The economic threshold density was 3 quackgrass heads per m2. The most effective herbicide treatments, quizalofop and propaquizafop, resulted in good initial quackgrass suppression and yield increases similar to those predicted by the hyperbolic model.

Alfalfa seed yield and Canada thistle densities were measured in 10 fields in southern Alberta to assess the effect of thistles on seed yield. Thistle density and seed yield from each field were fitted to the Weibull, hyperbolic, and linear equations and a random coefficient and linear equation were used to obtain an “average yield loss curve”. Estimated average losses in alfalfa seed yield were >34% and >48% at thistle densities of 10 and 20 shoots m–2, respectively.

Field experiments were conducted during 1998, 1999, and 2000 in Santa Fe, Argentina, to evaluate chlorimuron, clethodim, flumetsulam, 2,4-DB, glyphosate, and haloxyfop for control of broadleaf and grass weeds in established alfalfa. Herbicides were applied to alfalfa, 6 to 10 cm in height, after first or second cuttings. Clethodim and haloxyfop were highly effective against bermudagrass, barnyardgrass, and knotroot foxtail, and alfalfa yields were increased. 2,4-DB was effective against Scotch thistle, swinecress, turnipweed, and birdsrape mustard, but no effects were observed on alfalfa yields. Chlorimuron, flumetsulam, and glyphosate were effective against burning nettle, common chickweed, common lambsquarters, spiny cocklebur, and Persian speedwell, but they injured alfalfa and decreased yields.