QUACKGRASS (ELYTRIGIA-REPENS) INTERFERENCE AND CONTROL IN SEED ALFALFA (MEDICAGO-SATIVA)

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.