Soybean School: How to avoid antagonism in the spray tank

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Two herbicides are better than one, right? That’s the goal when products are tank-mixed together, but they don’t always deliver better weed control when antagonism occurs.

Antagonism takes place when two or more herbicides are combined and results in lower weed control than if they had been applied separately. This most often occurs when a broadleaf and grass herbicide is mixed together. On this episode of RealAgriculture Soybean School, we’re joined by Syngenta Canada sales representative, and former field biologist, Matt Underwood, for tips on how growers can better mitigate antagonism in the tank.

Underwood evaluated herbicide tank-mixes for potential antagonism on grass weed control as part of his Masters of Science studies at the University of Guelph. He says antagonism between Group 1 and Group 4 herbicides has been observed before, but it is likely to be a more common challenge to manage as dicamba can now be sprayed in-season on soybeans.

Underwood notes that there are tank-mix scenarios that carry a higher risk of antagonism. The most likely case is when a grower wants to apply a dicamba-based product in-season to control weeds such as fleabane or waterhemp, but also wants to use a graminicide to control volunteer corn. In this situation, antagonism increases when a higher rate of dicamba and/or a lower rate of the graminicide is used.

There are a couple options to mitigate this risk, says Underwood: “Growers can split out the products and apply separately one to two days apart, or increase the rate of the graminicide, which will help overcome the antagonistic effect.”

Similar antagonism occurs whether using aryloxyphenoxypropionates (FOP) or cyclohexanediones (DIM) group herbicides, adds Underwood, so a grower should pick their preferred graminicide and plan to increase the rate to achieve adequate control of volunteer corn.

Check out the full conversation between Underwood and RealAgriculture’s Bernard Tobin, below:

Click here for more Soybean School episodes.

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