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LAWRENCE, Kan. (DTN) -- Checking your burndown will be more important than ever this spring, weed scientists are warning growers.
The cool, wet spring that has delayed field work in some parts of the country may also reduce herbicides' effectiveness and complicate timing of application. At the same time, herbicide-resistant weeds are creeping into fields throughout the Midwest, and growers should be aware that their burndown may not kill winter annuals as effectively as in the past.
Extended periods with below-40 degree temperatures, such as much of the Midwest saw this week, is enough to affect weed control, University of Missouri weed scientist Kevin Bradley explained in a university newsletter.
"This is especially the case with any burndown application that includes glyphosate (Roundup, Touchdown, etc.), which is a systemic herbicide and needs time to penetrate the leaf cuticles and move throughout the plant in order to have optimum activity," Bradley wrote. "Weed control will likely be even poorer if you have made a burndown application and there is an extended period of cool, cloudy conditions following that initial drop below 40 F."
In past years, growers have experienced widespread burndown failures across the state of Missouri, Bradley noted. He attributes these failures to cool air temperatures experienced before, during or after burndown herbicide applications were made in those years.
To add to the confusion, both herbicides and weeds can vary in their responses to different air temperatures at the time of application. Bradley points to research from the University of Illinois, which found lower temperatures (below 60 degrees F) at application had a significant impact on glyphosate activity on henbit, but had very little influence on common chickweed control with glyphosate. In this same study, glyphosate seemed to be more sensitive to low air temperatures at application than paraquat (Gramoxone).
Bradley recommends watching weather forecasts closely and waiting for favorable temperatures to arrive before making your burndown herbicide applications. The decision must also be balanced by the size of the weeds at the time of the application, because weeds like marestail and giant ragweed can grow quickly at this time of year.
"So if there is no other alternative other than to spray and you know cool conditions are going to persist after application, you may want to increase the rate of glyphosate or whatever burndown herbicide you are using and consider at least one other tank-mix partner to ensure the best chance of burndown success," Bradley added.
A secondary concern for cool-weather herbicide applications is the fate of the crops you'll plant in that field, University of Illinois weed scientist Aaron Hager noted. "From a residual herbicide standpoint, if a crop is trying to emerge at a time of very cool and wet conditions, if it encounters a high dose of herbicides in the soil environment, we can see a certain type of injury symptoms develop," he explained.
RESISTANCE ON THE RISE
In addition to weather concerns, the spread of herbicide-resistant weeds demands vigilant scouting, Hager said. He urged growers to keep an eye out for herbicide-resistant horseweed (marestail) seedlings that survived spraying or tillage. Even weed species without widespread resistance can bite into yields, so don't dismiss any weed escapes from winter annuals, he added.
An easy rule of thumb is to make sure all vegetation is controlled before you put seed in the ground, Hager said. However, different herbicides do their work within a range of timelines, and spotting initial injury symptoms doesn't mean your job is done.