The critical period for weed control for many crops has been well investigated and the results have been used to develop better weed management recommendations. However, research is limited on the critical period for weed control for alfalfa, a perennial crop with multiple harvests. With the advent of new, more effective POST herbicides for alfalfa, an understanding of the critical period may further enhance forage yield. The objective of this study was to determine the critical period for weed control in spring-seeded alfalfa. Alfalfa was seeded conventionally at two locations in Pennsylvania in the spring of 2004 and 2005; glyphosate and glyphosate-resistant alfalfa were used for weed management. A surrogate weed, Japanese millet, was seeded at three planting rates to simulate varied levels of weed density. In the analysis, the critical period for weed control was identified for the 2004 seedings, but not for 2005. In 2004, the critical period varied by location, but began at the 0.5 trifoliate alfalfa growth stage and ended at the 7 trifoliate growth stage. When identified, the critical period was similar for both first-harvest yield and first-year cumulative yield. Higher weed density caused the critical period to begin earlier than under lower weed densities, but weed density had no effect on when the critical period ended. Under moderate to heavy weed populations, the critical period for weed control can increase herbicide use efficiency and maximize alfalfa returns, although under low weed severity, the critical period may not exist.

Nomenclature: Glyphosate; alfalfa, Medicago sativa L.; Japanese millet, Echinochloa frumentacea Link.