Incorporating glyphosate-resistant alfalfa into field crop integrated pest management programs of Pennsylvania

by Dillehay, Bryan L.

Alfalfa forage is vital to livestock production in the United States. Weeds can reduce alfalfa yield and changes in forage composition from weeds can have significant effects on feed quality. Although several herbicides are currently labeled for weed control in alfalfa, many have limitations. Glyphosate-resistant alfalfa permits the use of glyphosate for broad-spectrum weed control. Four experiments were initiated at two locations in Pennsylvania to address several management questions concerning glyphosate-resistant alfalfa. For the first objective, the critical period for weed control in alfalfa was quantified. At high weed density, the critical period began at 0.5 and ended at 7 alfalfa trifoliate, but varied by weed density, location, and year. For the second objective, glyphosate and other herbicides were evaluated for weed control in glyphosateresistant alfalfa and their effects on forage yield and quality were determined. In this study, glyphosate provided similar or better weed control than other herbicides, while yield response depended on weed severity. Forage quality was highest where weed content was lowest. Most differences from weed control occurred during the first harvest of the establishment year and dissipated in subsequent harvests and years. The third objective was to evaluate the potential use of glyphosate in glyphosate-resistant alfalfa mixed with orchardgrass. Orchardgrass was seeded with alfalfa or delayed 4 or 5 weeks; glyphosate was applied 4 weeks after alfalfa planting. At first harvest, glyphosate increased alfalfa, reduced weeds, and reduced or did not affect orchardgrass yield. By the third harvest, orchardgrass in glyphosate treated plots remained higher than in those without glyphosate and similar to simultaneous seeding without glyphosate. Longer-term iv results suggest that compositional differences at establishment dissipate during the second year. The final objective was to evaluate control of glyphosate-resistant alfalfa prior to no-till corn planting. Glyphosate-resistant and conventional alfalfa responded similarly and were controlled by 2,4-D and dicamba, either alone or in combination. Control after alfalfa harvest prior to no-till corn with 2,4-D and dicamba was also possible, although higher rates were necessary, especially when alfalfa regrowth was limited. These results will better inform Pennsylvania farmers about glyphosate-resistant alfalfa and general alfalfa weed management. v