Wet Weather Spurs Weed Growth

MU Extension specialist offers suggestions for marestail control

Published on: Jul 9, 2013

"Tell me how to control marestail. The herbicides aren't working." That request was phoned in all spring, says Kevin Bradley, University of Missouri weed specialist.

He puts that as his No. 1 topic at the MU Pest Day, July 11 at MU Bradford Farm, Columbia. Wagon tours run 9 a.m. to noon.

"We will have three tour stops on marestail, also known as horseweed," says the MU Extension specialist.

He will be joined by Reid Smeda, MU weed scientist with the MU College of Agriculture, Food and Natural Resources. He has studies of marestail underway.

But Bradley says his answers may get some pushback.

This year, weather was a big factor

Herbicide-resistant weeds like marestail will be a hot topic at MU Pest Day, July 11 at MU Bradford Farm, Columbia, where weed specialist Kevin Bradley and weed scientist Reid Smeda will cover the issue and disease and insect-related problems.
Herbicide-resistant weeds like marestail will be a hot topic at MU Pest Day, July 11 at MU Bradford Farm, Columbia, where weed specialist Kevin Bradley and weed scientist Reid Smeda will cover the issue and disease and insect-related problems.

"Everyone seems to think we have new herbicide resistance, but I think it was the year, not failure of herbicides," he says. "We do have, and have had for a long time, resistance to glyphosate. But I don't see resistance to other herbicides."

It was the weather, Bradley says. Rain-delayed plantings allowed weeds to get too big before they were sprayed.

"We have to go back to basics. The herbicides work if they are sprayed when weeds are 4 inches tall," Bradley says.

Too many weeds were sprayed when they were more than a foot tall. "Delayed spraying just doesn't work."

There are also concerns about new resistance in waterhemp to PPO-type herbicides, besides glyphosate, Bradley says. However, last fall he collected seed from 170 populations across the state. Those were grown in his greenhouse and tested last winter. PPO resistance was found in only 11% of the populations.

Growers in Illinois and Iowa have found higher rates of PPO-herbicide resistance. "They grow continuous corn. We grow rotations of corn and soybean. That helps us," he adds.

"But, again, you must spray waterhemp when it is 4 inches tall, or less."

More herbicide was applied this year, with fewer good results, Bradley says. "We just couldn't get them applied at the right time. It was that kind of year."

Those who put on a fall application and then could follow with an early spring application had good results.

Other topics

The Pest Day will cover more than just weeds. Tour stops will include both disease and insect problems as well.

Bradford Farm is located 7 miles east of Columbia on Rangeline Road, off of Route WW. Details will be given at registration.

Registration opens at 8:30 a.m. After lunch, visitors can tour the hundreds of labeled plots. These will include herbicides that may be released in 2014.

Source: University of Missouri Extension