Seedling Response of Downy Brome (Bromus tectorum), Russian Thistle (Salsola iberica), and 27 Desired Species to Aminocyclopyrachlor.

Aminocyclopyrachlor, a synthetic auxin, has recently been registered for non-crop applications.  One potential future use of aminocyclopyrachlor is invasive weed management in reclamation and restoration situations.  A greenhouse study was conducted in 2010 at the University of Wyoming to investigate the seedling response of 27 species accessions or cultivars and 2 exotic weeds to aminocyclopyrachlor.  Aminocyclopyrachlor was applied at rates of 10, 20, 40, 80, 160, and 320 g/ha 30 days after planting when grasses reached the 3 to 5 leaf stage and forbs and shrubs were less than 5 cm in height.  There were 7 replicates and all treatments included a nonionic surfactant at 0.25% v/v. Herbicide treatments were applied in a spray chamber delivering 187 l/ha at 276 kPa.  A four parameter log-logistic model was used to estimate the dry weight reduction in response to aminocyclopyrachlor rate.  Russian thistle biomass was reduced 99% at 90 g/ha.  At the same rate, reduction in grass biomass ranged from 11 to 49%. Variation in growth reduction by aminocyclopyrachlor was observed among genera and species, and even among germplasm within a species.  At 90 g/ha, growth of all flax and sagebrush species were reduced ≥81%.  If aminocyclopyrachlor were used in a reclamation or restoration situation for postemergence control of Russian thistle, most of the grasses in this experiment appear to be fairly tolerant; whereas the selected sagebrush and flax species were highly susceptible at this early growth stage even at low rates.