Alfalfa Yield Response to a Between‐Cutting Contact Herbicide 1

Alfalfa ( Medicago sativa L.) fields sometimes develop weed infestation problems when conditions at harvest favor germination of weed seeds or when the stand begins to thin and lose vigor. A recently approved label for use of the contact herbicide paraquat (1,1′,dimethyl‐4, 4′‐bipyridinium ion) allows application between cuttings for weed control. Label directions require application of no more than 0.28 kg ha −1 within 5 days after cutting. The net result of this practice is increased yields of alfalfa if effective weed control is achieved. Our studies with weed‐free alfalfa stands indicated a 7.8% yield reduction caused by paraquat applied within 5 days after hay harvest. A greater reduction might have occurred if treatment was delayed beyond 5 days. This slight harmful effect was expressed only in the first cutting after treatment of weed‐free stands and always greatly offset by control of weeds in weedy alfalfa stands. The benefit from between‐cutting paraquat treatment was positively related to the degree of weed infestation. Alfalfa yields in two harvests following the between‐cutting paraquat treatment were increased by 2.40 Mg ha −1 as compared with nontreated weedy plots. In an old weedy alfalfa field, yields of alfalfa were increased three‐fold with a 70% decrease in weed composition for the first harvest following treatment. The weed control benefit was also observed in the second harvest after treatment. Stand longevity and sustained high yields should prove to be additional advantages to the practice of between‐cutting weed control using paraquat.