ORIGINAL ABSTRACT

Premix or tank mix of glyphosate and 2,4-D are a good alternative to control glyphosate-resistant and -tolerant weeds; however, the combination of herbicides may increase the environmental impacts, since mixtures often have higher toxicity than a single herbicide. In addition, antagonism between these herbicides has also been reported. We compared the efficacy of a premix glyphosate+2,4-D formulation with respect to the tank mix of both herbicides on glyphosate-resistant Conyza canadensis and -tolerant Epilobium ciliatum populations in laboratory and field experiments. 2,4-D suppressed the glyphosate-resistance/tolerance of both species, whose populations presented similar responses to their susceptible counterparts (LD50 ≥ 480+320 g ha-1 glyphosate + 2,4-D, respectively). Plants of both species treated with the premix formulations retained ∼100-μL more herbicide solution, accumulated 20-25% and 28-38% more shikimate and ethylene, respectively, and presented greater 14C-glyphosate absorption and translocation, depending on the species, compared to plants treated with the tank mix treatment. Although doubling the field dose (720 + 480 g ha-1) improved (5-22%) the control of these weeds in the field, split applications of both premix and tank mix provided the best control levels (≤70%), but premix treatments maintained control levels above 85% for longer (120-d). No antagonism between glyphosate and 2,4-D was found. The addition of 2,4-D controlled both broadleaf species. For all parameters evaluated on the C. canadensis and E. ciliatum populations in the laboratory and in the field, the premix treatments showed better performance than the tank mix treatments. Premix formulations could reduce the environmental impact of herbicides used to control glyphosate resistant/tolerant weeds by decreasing the herbicide amount needed to achieve an acceptable weed control level.