Effects of Repeated Herbicide Applications on Alfalfa(Medicago sativa)

Six herbicides, simazine [2-chloro-4,6-bis(ethylamino)-s-triazine], propham (isopropyl carbanilate), terbacil (3-tert-butyl-5-chloro-6-methyluracil), carbetamide [D-N-ethyllactamide carbanilate (ester)], pronamide [3,5-dichloro(N-1,1-dimethyl-2-propynyl)benzamide], and secbumeton [N-ethyl-6-methoxy-N′(1-methylpropyl)-1,3,5-triazine-2,4-diamine] were applied annually for four years to establish forage alfalfa(Medicago sativaL. ‘Washoe’). Downy brome(Bromus tectorumL.) control ranged from 80 to 100%. The average control of broadleaf weeds was 90 to 100% with simazine, terbacil, and secbumeton, compared to 30 to 40% with propham, carbetamide, and pronamide. Only simazine and terbacil were phytotoxic to the alfalfa. Based on weed control and crop tolerance, secbumeton performed best on the coarsetextured soil in this experiment.