Fall is best time to mount attack on weeds

If the good weather continues to hold through harvest, this fall will be the best time to get aggressive about weed control, especially if you’re dealing with dandelions, glyphosate-resistant weeds or plan to rotate out of an old alfalfa stand.

“This past spring, we got lots of calls about controlling alfalfa [going out of rotation] and dandelions, due to the fact that last year was horrible for getting the crop in and for weed management,” says Jeff Gunsolus, University of Minnesota Extension weed specialist. “The way this growing season is going, and if harvest goes at a good clip, weed control this fall could be well-timed.”

Key Points

• Fall weed control may be an option if weather cooperates.

• Good fall control means the spring seedbed is ready to plant.

• Scout fields now for any herbicide-resistant weeds.


In alfalfa fields rotating to corn or beans, Gunsolus suggests 1 quart of 2,4-D ester per acre, and if some grass is in there, add a quart to 1½ quarts Roundup.

“With alfalfa in northern Minnesota, you’ll want to apply it early, by mid-September,” he says. “In central Minnesota, apply it from mid-September to early October.”

You can apply the weed control until the first hard frost.

For dandelions, the best weed control also is a quart of 2,4-D ester per acre and/or Roundup, he says.

With successful fall weed control, growers will have a warmer, cleaner seedbed for the spring, says Jeff Carpenter, DuPont corn portfolio manager. For broad-spectrum control in corn, he recommends the company’s herbicide Basis with 2,4-D and a little glyphosate.

“As soon as your crop is off, spray. You’ll get fall moisture to push it through the ground,” he says. Plus, when you apply herbicide right after harvest, the chore won’t conflict with fall application of anhydrous ammonia.

If you can’t get to the field right after taking off the crop, you can apply herbicide as long as the weather allows you to spray. “Stop when the ground is frozen,” he says.

In broad-spectrum control in soybeans, DuPont’s Susan Macy, soybean portfolio manager, recommends the company’s Synchrony XP mixed with Express.

“It can go on worked ground but the tankmix benefits no-till better,” she says. “Growers tell us that when they apply this in the fall, they don’t have to worry about weed control until after they plant soybeans. They can start with clean ground and plant when they are ready.”

No-till ground usually is cooler and wetter, too. “If you have clean ground and the weeds aren’t sitting there, the soil can warm and dry out quicker,” she adds.

Scout for resistant weeds

“Now is a good time of year to assess for herbicide-resistance issues,” Gunsolus says. Minnesota growers are dealing with three herbicide-resistant weeds: common ragweed, giant ragweed and common waterhemp. West-central and southwest Minnesota growers have been dealing mostly with the latter two.

“Growers should scout fields now, and if they see only one species, take note of that and pull the plant before it sets seed. And they should note if there are any heavy weedy patches along fencelines and ditches. If they see weed problems there, I suggest they do more intensive management,” he says. That means coming back with a preemergence program and changing mode of action to Liberty rather than glyphosate.

“Those weeds will just continue to set seed and cause problems,” Gunsolus says. “It’s a good time to look for them and target those fields.”

Your other options in dealing with resistance? Crop rotation and good old cultivation.

“I’d never talk anyone out of between-the-row cultivation,” Gunsolus adds. “It is an effective way to control weeds.”

This article published in the September, 2010 edition of THE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.