Competition from annual grasses and yellow starthistle can severely reduce perennial grass forage production and quality in pasture and rangeland. The purpose of this study was to determine yellow starthistle and weedy annual grass control by imazapic applied alone or in combination with picloram, and smooth brome tolerance to these treatments. Sixty days after treatment (DAT) imazapic applied at 70 and 140 g ae/ha reduced annual grass (downy brome, medusahead, and annual bluegrass) plant density and biomass by 66 to 76% compared with the untreated control. Picloram applied at 280 and 420 g ae/ha reduced yellow starthistle plant density and biomass by over 93%. Imazapic applied at 70 and 140 g/ha reduced smooth brome biomass by 79 to 95% at 60 DAT. Picloram did not affect the density or the biomass of annual grasses or smooth brome, whereas imazapic did not markedly affect the density or the biomass of yellow starthistle. Downy brome control increased to a maximum of 97% with increasing imazapic dose (maximum of 280 g/ha) at 30, 60, and 90 DAT.
Nomenclature: Ammonium salt of imazapic; potassium salt of picloram; annual bluegrass, Poa annua L. #3 POAAN; downy brome, Bromus tectorum L. # BROTE; medusahead, Taeniatherum caput-medusae (L.) Nevski. # ELYCM; smooth brome, Bromus inermis Leyss. # BROIN; yellow starthistle, Centaurea solstitialis L. # CENSO.
Additional index words: Herbicide tolerance.
Abbreviations: DAT, days after treatment; OM, organic matter; POST, postemergence.