Herbicide Varieties Effectively Control Winter Annual Weeds

October 30, 2001

COLUMBUS, Ohio - The struggle to effectively manage winter annual weeds may get easier for corn and soybean farmers.

Ohio State University weed scientists have identified several herbicides that help control common chickweed and purple deadnettle with at least 90 percent effectiveness in no-till corn and soybean fields.

Mark Loux, an Ohio State weed specialist, said the purpose of the research was to provide management options for controlling problematic winter annual weeds.

"The activity of glyphosate (more commonly known as Roundup) in the spring before planting has been slow under cool conditions. It just wasn't killing the weeds fast enough," said Loux. "Over the past two or three years, the winter annual population has just soared and I suspect it's due in part to shifts in herbicide use. The issue here is how can we better manage these weeds?"

Common chickweed and purple deadnettle, which primarily emerge in late summer or fall, tend to cause headaches for farmers. They prevent the soil from drying out and warming up in the spring; they interfere with tillage because they grow so thick they snag equipment; and they harbor insects or serve as insect hosts. Research has shown that purple deadnettle may serve as an alternative host of soybean cyst nematode, a pathogen that damages soybeans by feeding on plant roots, robbing the plants of nutrients, and providing wound sites for root-rotting fungi to enter.

Researchers found that fall treatments provided more consistent control than spring applications. Among soybean herbicides studied, for example, only three provided 80 percent control of weeds when applied in March, but 10 provided 90 percent control when applied in November. Researchers analyzed herbicides with at least 20 different active ingredients.

Loux theorizes that smaller plant size and warmer soils in the fall may attribute to more effective fall herbicide applications. "I think the plants are smaller in the fall. Their growth rate hasn't geared up yet in early spring. Plus colder weather in the spring may prevent the rapid herbicide activity and plant death needed so a farmer can plant his field," said Loux. "The issue here is crop establishment. If a farmer is having difficulty in getting his crop established in the spring due to winter annual weeds, then he may need to consider a fall application.

"We are not saying that fall herbicide applications are always better than spring applications. Both can be effective," said Loux. "But it just may be a better strategy for the same money to use a fall herbicide that has good residual that can carry through spring."

Loux said by applying herbicides in the fall, even under cool conditions, gives a farmer a better chance of establishing his field for planting. "If a farmer is planting in April and applies the herbicide in March, that only gives him few weeks to get that weed out of his field before planting," said Loux. "That's really not that much time."

Farmers are recommended to apply fall herbicides now through mid-November. Research has found that herbicides perform well even when applied under cold conditions, so applying herbicides through early December may also be an option.

Researchers also studied the effectiveness of the herbicides on marestail and dandelion with positive results.

For more information on the project or for a list of effective herbicides, log onto Ohio State Extension's C.O.R.N. newsletter at http://corn.osu.edu/archive/index.html and select issue 01-35.

Author(s): 
Candace Pollock
Source(s): 
Mark Loux