Weed Control in Alfalfa (Medicago sativa) Grown for Seed

Applications of 1.6 kg ai/ha of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] to established alfalfa (Medicago sativaL. ‘Rambler’) at the start of each growing season for 4 yr increased seed yield by 68% and controlled established dandelion (Taraxacum officinaleWeber. in Wiggers ♯ TAROF) and smooth brome (Bromus inermisLeyss. ♯ BROIN). A lesser amount of metribuzin was not sufficient for smooth brome control, and a greater amount damaged the alfalfa. Hexazinone [3-cyclohexyl-6-(dimethylamino)-methyl-1,3,5-triazine-2,4(1H,3H)-dione] applied at 1.0 kg ai/ha also controlled dandelion and smooth brome and produced increases in alfalfa seed yield. Mixtures of terbacil (3-tert-butyl-5-chloro-6-methyluracil) with reduced rates of metribuzin or hexazinone controlled dandelion well, but gave only fair control of smooth brome. Alfalfa seed yield was negatively correlated with the smooth brome population.