Watch what you tankmix with dicamba, particularly if spraying for grasses

weedy cotton field
preliminary research results have found that tankmixinig dicamba and glyphosate can be antagonistic — photo courtesy Alabama Cooperative Extension

In 2020 we began seeing several cases where summer grass weeds like goosegrass and junglerice were poorly controlled by postemergence treatments in many Xtend crops. This led to a regional project partially funded by the Alabama Soybean Producers to determine factors affecting reduced grass control.

Preliminary findings determined that tankmixes of dicamba and glyphosate resulted in antagonism and reduced grass control by up to 29%. Our studies found that grass control was improved by applying glyphosate alone and spraying grass weeds when they’re less than 4 inches tall.

We realize that convenience, timeliness and weather patterns often dictate what gets put in the tank especially regarding weed control. But if you find that the majority of the weed species in your fields consists of summer annual grasses, it is best to make a separate dicamba application for broadleaves apart from glyphosate or clethodim applications for grass control.

On a separate but similar note, wheat harvest has increased the last two weeks across much of the state. I have seen fields and heard from many growers that cool season annual grasses like Italian ryegrass, brome and fescue species have infested many acres this year.

Although I can’t predict the future, these mature weedy grass seed will inevitably be spread across fields during harvest, setting us up for another challenge when they germinate this fall. Please make a note of these locations now so you can be proactive in weed control for your next crop. If you suspect any herbicide resistant cases on your farms please let us know as we are attempting to monitor these populations statewide.

Contact Assistant Extension Professor David Russell for more information.

Alabama Cooperative Extension contributed this article.

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