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Title: Interaction of Glyphosate and Diquat in Ready-To-Use Weed Control Products

item Altland, James

Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/12/2008
Publication Date: 7/28/2008
Citation: Altland, J.E., Glenn, W., Gilliam, C. 2008. Interaction of Glyphosate and Diquat in Ready-To-Use Weed Control Products. Weed Technology. 22(3):472-476.

Interpretive Summary: Glyphosate is the most commonly used herbicide for postemergence weed control in home landscapes. Glyphosate is commonly pre-mixed with other herbicides, such as diquat, to provide 'faster control'. The objective of this research was to determine if additional herbicides added to glyphosate offer a synergistic or antagonistic effect on weed control. Results show that while weeds appeared to die more quickly with diquat added to the spray solution, the same weeds regrew at a faster rate. Adding diquat to the glyphosate spray caused weed foliage to rapidly dessicate, preventing movement of the glyphosate herbicide throughout the rest of the plant (parts not contacted by the spray, such as the roots). This research lends clarity to the decision of adding additional herbicides to glyphosate for weed control. When rapid weed burndown is desirable, adding diquat to glyphosate is justified; however, when long-term weed control is desired glyphosate should be applied alone. Better decisions on herbicide selection will ultimately lead consumers to use less herbicide more effectively.

Technical Abstract: Glyphosate-based, ready-to-use weed control products frequently contain diquat (typically 0.04 by weight relative to glyphosate) under the supposition that the diquat, “makes glyphosate work faster“. However, in light of the known modes of actions of glyphosate and diquat, we hypothesize that diquat may be antagonistic to glyphosate activity. Greenhouse experiments using longstalked phyllanthus were conducted to test this hypothesis. Glyphosate was applied at a series of rates, ranging from 0.11 to 1.12 kg ae/ha, either alone or tank mixed with either 0, 0.02, 0.04 and 0.06 diquat. Onset of visual injury was more pronounced with the glyphosate +diquat tank mixtures compared to glyphosate alone. However long term control, as expressed by regrowth suppression, was greater with glyphosate alone. Regression analysis indicated that at marginally effective glyphosate rates, the amount of glyphosate must be increased by approximately 60% to compensate for the diquat-based antagonism. Absorption and translocation studies using 14C glyphosate revealed that the antagonism of diquat toward glyphosate can be attributed to reduced translocation of absorbed glyphosate.