Control of Bitterweed with Herbicides

Abstract

Herbicide 2,4-D was less effective for bitterweed control when applied at air temperatures below 14 degrees C compared to applications at 22 degrees C even when soil water was adequate for active plant growth. There was no difference in bitterweed control whether the ester or amine formulations of 2,4-D were used. At 22° C air temperature, 1.12 kg/ha of 2,4-D + dicamba (3:1), picloram, or 2,4,5-T + picloram (1:1) did not improve short-term control (36 to 182 days) of bitterweed compared to 2,4-D alone at the same rate. However, at 14 degrees C temperature or when bitterweed were in advanced phenological states (75% with inflorescences), these herbicides provided excellent short-term bitterweed control whereas 2,4-D was inconsistent. Mixtures of dicamba with 2,4-D slightly improved residual bitterweed control, compared to the same rate of 2,4-D alone. Picloram at 0.56 to 1.12 kg/ha controlled 60 to 100% of the bitterweed populations for a year or more following applications in winter or spring. Tebuthiuron at 0.56 to 1.12 kg/ha was not as effective as 2,4-D at 1.12 kg/ha relative to initial bitterweed control, but provided excellent residual control after 1 year following winter application.