Weed Management

Jed Colquhoun

Wheat and grass seed research field tour

Please see the enclosed announcement for the upcoming weed science field tour on March 8, 2002. Note that registration is required.

Herbicide antagonism

There are many reasons for unexpected herbicide failure: environmental variables, weed resistance to herbicides, poor spray coverage, etc. Antagonism between two or more tank-mixed pesticides is relatively uncommon, but can also lead to poor weed control. Antagonism between herbicides has been suspected in several weed control failures in Oregon recently.

Three plant responses are possible when two or more pesticides are mixed: an additive response, a synergistic response, and an antagonistic response. An additive response occurs when the effect of two tank-mixed chemicals is equal to the sum of that expected when the two chemicals are used individually. Most chemical interactions result in an additive response. A synergistic response occurs when the effect of two tank-mixed chemicals is greater than when the two chemicals are used individually. Synergistic effects have been reported when trifluralin is applied prior to corn planting, followed by a post-planting application of alachlor. Although not tank-mixed, the combination of these herbicides results in improved broadleaf weed control. Herbicide antagonism occurs when the effect of two chemicals is reduced when they are combined as compared to when the two chemicals are used individually. It has been widely documented that the combination of ALS inhibitor herbicides with the grass herbicides results in antagonism and reduced weed control compared to when the herbicide classes are applied individually. For example, wild oat control is reduced when diclofop is tank-mixed with tribenuron. Weed control is not reduced when the two herbicides are applied separately, even when the application timings are separated by as little as twelve seconds. These results suggest that the antagonism occurs on the leaf surface, possibly decreasing herbicide uptake. In preliminary studies, we have also observed an antagonistic response when pyridate is tank-mixed with sethoxydim, resulting in poor grass control.

There are several mechanisms within a plant that can result in an antagonistic response. Herbicide antagonism can be a result of reduced uptake or translocation, increased metabolism, or an alteration in the sensitivity of the target site. In the case of the ALS inhibitor-grass herbicide antagonism, reduced uptake and translocation within the target plant results in poor weed control.

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