Western ragweed is problematic in Oklahoma pasture and rangeland. There are no labeled PRE herbicides for western ragweed control in pastures to allow ranchers the flexibility of extending the spray season or the ability to spray before emergence of sensitive crops in bordering fields. Field studies were conducted in four locations during a 2-yr period to determine the effects of applying aminopyralid PRE for control of western ragweed. Good to excellent control of western ragweed was obtained when aminopyralid rates of 89 g ae ha−1 or greater were applied PRE. Control was season long and equal to that obtained with a formulated mixture of 2,4-D and aminopyralid applied POST at a rate of 457 + 57 g ae ha−1. Sward height at time of PRE application had little to no effect on the efficacy of the treatment, but time after application before a significant rainfall event did influence the aminopyralid rate needed to achieve acceptable PRE control.
