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Cool nighttime temperatures affect burndown herbicide efficacy.

Mindy Ward, Editor, Missouri Ruralist

May 6, 2020

3 Min Read
Weeds peeking through a field
TACKLE WEEDS: Whether your weed control program worked this year may come down to when you sprayed. You may see a few weed escapes or even a complete failure because of cold spring temperatures. Mindy Ward

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.

About the Author(s)

Mindy Ward

Editor, Missouri Ruralist

Mindy resides on a small farm just outside of Holstein, Mo, about 80 miles southwest of St. Louis.

After graduating from the University of Missouri-Columbia with a bachelor’s degree in agricultural journalism, she worked briefly at a public relations firm in Kansas City. Her husband’s career led the couple north to Minnesota.

There, she reported on large-scale production of corn, soybeans, sugar beets, and dairy, as well as, biofuels for The Land. After 10 years, the couple returned to Missouri and she began covering agriculture in the Show-Me State.

“In all my 15 years of writing about agriculture, I have found some of the most progressive thinkers are farmers,” she says. “They are constantly searching for ways to do more with less, improve their land and leave their legacy to the next generation.”

Mindy and her husband, Stacy, together with their daughters, Elisa and Cassidy, operate Showtime Farms in southern Warren County. The family spends a great deal of time caring for and showing Dorset, Oxford and crossbred sheep.

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