Farmers may experience weed control failure this growing season if they put on burndown herbicides right before temperatures dropped below freezing.

Kevin Bradley, University of Missouri Extension weed specialist, says applying herbicides in the spring is typically not a problem. However, this year was full of extreme temperature swings right at the height of spray season.

Across some areas of Missouri, farmers saw nighttime highs in the upper 50s one week, but temperatures dropped into the low 20s the next. As a result, there may be a reduction in the effectiveness of herbicide treatments.

Herbicides fail

“What you want to look out for as far as cooler temps and herbicide efficacy is basically nighttime temperatures,” Bradley says. Most days, it is fine for farmers to spray even in the 40s, but Bradley would like to see higher temperatures, more in the 50-degree F range.

“But it's really what happens that night after the spray has occurred that can really cause a failure, and often does," Bradley says.

The problem comes with translocation or movement of a herbicide, which allows the chemical to be absorbed by the leaf and then moved throughout the plant to its stem and eventually roots. Cooler temperatures, Bradley says, mess that process up.

“There are certain herbicides that don't respond well to cool temperatures more than others,” he explains. “Gramoxone is one that is probably just as much to do with cloud cover and not having sun out as it is cold temperatures, but it doesn't respond well.”

Glyphosate, which is the go-to base burndown herbicide for most of the state, also does not like cold temperatures. “It's not really the issue of what's happened in that day when you're spraying, it's more about how it cannot translocate at night when those temperatures drop drastically,” Bradley adds. “And if they're dropping too far, you're not going to get additional translocation at all, and that's where we see a failure.”

There is hope for catching those weed escapes, but it will take using the right amount of chemical.

Treat tall weeds

When farmers face weeds larger than they would like, Bradley recommends boosting the level of control by simply moving to more gallons per acre.

He says herbicides such as Liberty kill weeds really well at 20 gallons per acre, and while a minimum of 15 gallons per acre will elicit a weed response, boosting it only 5 gallons more “works best.” He adds that Gromoxone and Sharpen, any contact herbicides, respond well to increased application volumes.

One weed control strategy that has been common for some time in the mid-South is spraying Roundup or Glyphosate with Liberty in burndown. Bradley says the practice is becoming more common across Missouri.

He conducted a herbicide mix treatment on glyphosate-resistant horseweed a few years ago at the University of Missouri.

“When you look at the Glyphosate and Liberty together, we can see how much the control increases by just going from 10 to 20 gallons per acre on this particular weed,” Bradley says. “So, it is something we encourage with certain mixes, certain herbicides and depending on how big the weeds are getting.”

He doesn’t know if farmers have reached the point yet in the growing season where they are having problems with bigger weeds. But as rains continue to be weekly and spotty across the state, farmers may not be able to get into the field to spray. Bradley says if weeds get out of hand, farmers may want to consider these control options.