Control of Threadleaf Snakeweed(Gutierrezia microcephala)and Common Goldenweed(Isocoma coronopifolia)with Herbicides

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Abstract
Percentage canopy reduction of threadleaf snakeweed [Gutierrezia microcephala (D.C.) Gray] and common goldenweed [Isocoma coronopifolia (Gray) Greene] was substantially greater when sprays were applied during the spring of a year characterized by above-average rainfall than in a drought year. Applied at 2 kg/ha or less, 2,4-D [(2,4-dichlorophenoxy)acetic acid] only partially controlled threadleaf snakeweed in either year. Picloram (4-amino-3,5,6-trichloropicolinic acid) or triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid} gave satisfactory control of threadleaf snakeweed at 0.5 or 1 kg/ha, depending upon year of application, whereas these treatments were effective against common goldenweed only in the “wet” year. Herbicide 3,6-dichloropicolinic acid tended to be less effective against either species than picloram or triclopyr. Approximately 1 and 2 yr after herbicides were applied, standing forage crop increased 37 and 25 kg/ha, respectively, with each percentage point reduction of threadleaf snakeweed canopy cover.