Pennsylvania State University’s Dwight Lingenfelter and emeritus professor Bill Curran have published an article on spring weed control in grass hay and pastures. They break down best management practices for winter annuals like mustard weeds and common chickweed (emerge in the fall and flower/set seed in the spring), summer annuals like pigweeds, common lambsquarters, and common ragweed (emerge in the spring and flower/set seed in the late summer or fall), and biennial weeds like common burdock and bull and musk thistle (take two years to complete their life cycle). Check out their advice for the best timing of your spring weed management practices.
Agricultural Weed ID for New York State