Ohio State University

Weed Specialist: Try to Apply Fall Herbicide Treatments Before December


Source: Ohio State University

COLUMBUS, Ohio -- Now is a good time for growers to apply herbicide treatments to their fields to control weeds and help ensure a good start for spring planting.

In fact, anytime between now and the week of Thanksgiving is a good time for fall herbicide applications, according to a researcher from Ohio State University’s College of Food, Agricultural, and Environmental Sciences.

Although growers may be busy with harvest, now is the time to start thinking about next spring and how to control the weeds that can have a negative impact on planting, said Mark Loux, an Ohio State University Extension weed specialist. OSU Extension is the statewide outreach arm of the college.

While some growers have applied herbicide treatments into late December, herbicide application is discouraged during periods of very cold weather or to snow-covered ground, Loux said.

“We have applied herbicides into late December and still eventually controlled the weeds present at time of application,” he said. “But once hard freezes start to occur, there is usually a substantial change in the condition of certain weeds, such as dandelion and thistle, which renders them less sensitive to herbicides. 

“The problem from a grower’s perspective is that sometimes the weather is too wet during October and November, so they can go into December before they’re able to do herbicide applications.”

While late application isn’t recommended, the rate of kill for some weeds, including marestail, slows down but the weeds eventually still die, Loux said.

“Marestail, which is also known as horseweed, is the No. 1 weed problem growers will have a problem controlling in the spring if they don’t do something to prevent the weeds in the fall,” he said.

Other weeds of particular concern for fall herbicide applications include:

  • Purple deadnettle, which is a broadleaf weed that is a host for soybean cyst nematode. Controlling it with herbicide application in the fall can help keep populations from increasing, Loux said.
  • Chickweed, which is a common weed of small grains and forage crops.
  • Dandelion. Fall is a prime time for control for this weed.

The primary goal for fall application is to take out the weeds that overwinter and can have a negative impact in the spring, Loux said.

“Fall herbicide application creates a better seedbed for those fields where you have a bunch of weeds that need control,” he said. “There are some fields that don’t need it, but where you have weeds, those areas will cause a problem in the spring if you don’t control them in the fall.”

Some tips to keep in mind when spraying:

  • Herbicides seem to work regardless of the crop residue that remains on the ground after harvest. Growers can, however, wait a while after harvest to let the residue settle down and the weeds to poke through, as dense crop residue usually prevents marestail from emerging.
  • Don’t make it too complicated or pricey. Keep in mind that the primary goal is control of weeds that have already emerged. 
  • Growers can start with 2,4-D, and then add another herbicide that results in more comprehensive control. Herbicides that make the most sense to add to 2, 4-D based on OSU Extension research include: glyphosate, dicamba, metribuzin, simazine, Basis (and generic equivalents), Express (and generic equivalents), Canopy/Cloak DF or EX, or Autumn Super. 
  • There is no advantage to including residual herbicides, as most peter out over the winter and fail to provide any control of spring-emerging weeds. 
  • Any effective fall herbicide treatment even without residual will result in a weed-free seedbed in spring, usually into April, so that the spring-applied burndown/residual treatment just has to control small weeds that emerge in the few weeks prior to planting. 
  • It doesn’t take a lot of herbicide to control weeds in fall, just the right types. Avoid most residual herbicides. Consider that fall treatments should comprise no more than about 25 percent of your total herbicide budget for a crop, and it can be accomplished for less.

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