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Grass Antagonism with Dicamba + Clethodim (07/06/17)

Question: With the approved labeling of clethodim with dicamba on dicamba-tolerant soybeans, I was wondering your thoughts on this tank-mix. I have some deep reservations about mixing a Group 1 grass product with up to 0.5 lb ai/A of dicamba. Couple that concern with a TTI 11005 tip that produces very large droplets with some of the thickener type drift retardant adjuvants and clumps of volunteer corn. I go back to days of Tiller, Cheyenne, Puma and Discover NG. Dicamba was allowed with some of these small grain grass control products up to 2 fl oz/A but you lost wild oat control and control of other tougher grasses. That 2 fl oz/A used back then is a much lower rate than the rate of 22 fl oz/A of Xtendimax or the 12.8 fl oz/A of Engenia used now. If we saw a significant reduction in grass control back then from 2 fl oz/A of dicamba what will happen with 0.5 lb ai/A mixed with clethodim?

Grass Antagonism with Dicamba + Clethodim

Question: With the approved labeling of clethodim with dicamba on dicamba-tolerant soybeans, I was wondering your thoughts on this tank-mix. I have some deep reservations about mixing a Group 1 grass product with up to 0.5 lb ai/A of dicamba. Couple that concern with a TTI 11005 tip that produces very large droplets with some of the thickener type drift retardant adjuvants and clumps of volunteer corn. I go back to days of Tiller, Cheyenne, Puma and Discover NG. Dicamba was allowed with some of these small grain grass control products up to 2 fl oz/A but you lost wild oat control and control of other tougher grasses. That 2 fl oz/A used back then is a much lower rate than the rate of 22 fl oz/A of Xtendimax or the 12.8 fl oz/A of Engenia used now. If we saw a significant reduction in grass control back then from 2 fl oz/A of dicamba what will happen with 0.5 lb ai/A mixed with clethodim?

Answer: Your question couldn’t be more anticipatory! A couple of years ago my academic colleagues and I considered this scenario. We investigate this when Monsanto was considering dipotassium phosphate (MON 10) as a water conditioner for dicamba-resistant soybean. We used clethodim at 4 fl oz/A which would be a representative field use rate and dicamba at only 6 oz ai/A which less than the 8 oz ai/A that is labeled in DT soybean.

zollinger

Summary: The data shows severe antagonism of clethodim that even the most effective and aggressive oil adjuvants did not overcome. Several non-ams commercial water conditioning (WC) adjuvants were tested which did not overcome the antagonism (data not presented). The only water conditioner that overcame the dicamba antagonism was AMS which cannot be used in DT soybean. Your concerns are valid about poor volunteer corn control when dicamba and clethodim are mixed and applied together. These results validate the information on page 27 in the ND Weed Guide that states:  “Grass control is reduced by tank mixtures or close interval application of POST broadleaf control herbicides. Antagonism generally can be avoided by applying a higher rate of grass herbicide or apply the grass control herbicide 1 or more days before or 7 days after the broadleaf control herbicide.”

Rich Zollinger

Extension Weed Specialist

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