Effect of Fall Frost on the Activity of Glyphosate on Alfalfa (Medicago sativa) and Quackgrass (Agropyron repens)1

The effect of fall frost on the activity of glyphosate (N- (phosphonomethyl)glycinel on alfalfa (Medicago sativa L. 'Vernal') and quackgrass (Agropyron repens (L.) Beauv.) was studied in a room of the Biotron, a controlled environment facility of the University of Wisconsin, and in the field in the fall. In a Biotron time-course study, the most effective day of treatment for quackgrass control was the day after the first frost. Quackgrass plants treated the morning after frost had significantly higher uptake and translocation of glyphosate (93%, 55%) than plants treated 4 days before frost (71%, 32%) and greater translocation than plants treated 5 days after frost (24%). In the field, glyphosate was most active on quackgrass when applied the morning after the first frost of the fall. Control of quackgrass treated with 0.4 kg/ha of glyphosate increased from 10% when treated 4 days before the first frost to a maximum of 91% when treated the day of the first frost. No significant effects of frost in decreasing glyphosate toxicity to alfalfa were found. These results minimize the possibility of using this herbicide to remove quackgrass from alfalfa.