Diflufenzopyr is a synergist that improves broadleaf weed control when mixed with certain auxinic herbicides. In nonagricultural settings, it is only available in a premix with dicamba, which is labeled for noncrop sites, pasture, hay, and rangeland. Our objectives were to determine the influence of diflufenzopyr dicamba when applied with auxinic herbicides for Russian knapweed control. Studies were conducted near Ethete, WY, from 2005 to 2008 in a pasture heavily infested with Russian knapweed. Treatments were applied in the fall (September) and included aminopyralid, clopyralid, clopyralid 2,4-D, clopyralid triclopyr, and picloram at standard and reduced rates, with and without diflufenzopyr dicamba. At 12 and at 24 mo after treatment (MAT), diflufenzopyr dicamba did not influence Russian knapweed control when applied with standard rates of aminopyralid, clopyralid, clopyralid 2,4-D, clopyralid triclopyr, or picloram. All of these treatments except clopyralid 2,4-D consistently provided ≥ 80% control 24 MAT. Reduced-rate herbicide interactions with diflufenzopyr dicamba were also not significant at 12 MAT. However, at 24 MAT, aminopyralid applied with diflufenzopyr dicamba controlled Russian knapweed 83% compared with 59% when aminopyralid was applied alone. Russian knapweed control with all other reduced-rate treatments, except picloram, fell below 80%. These results indicate that diflufenzopyr dicamba does not generally improve Russian knapweed control at 12 or 24 MAT with either standard or reduced rates of typical fall, auxinic herbicide treatments.

Nomenclature: 2,4-D amine; aminopyralid; clopyralid; dicamba; diflufenzopyr; picloram; triclopyr; Russian knapweed, Acroptilon repens (L.) DC. ACRRE.