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Weed management specialist provides considerations for post-harvest weed control

by Wyoming Livestock Roundup

According to Kansas State University (KSU) Weed Management Specialist Dr. Sarah Lancaster, late October and early November are ideal times to conduct fall-applied herbicide applications. 

“A few things growers might want to think about this year are the lack of soil moisture and rain in the forecast,” Lancaster says during the Oct. 16 episode of KSU’s Agriculture Today podcast. “They need to remember current weather conditions might require some adjustments.” 

She notes the other thing producers should keep in mind is accomplishing some sort of residual activity. However, with the lack of rain in the forecast, the ability to activate residual herbicides might be limited.

Weeds to target

Lancaster explains there are several annual winter weeds growers should target with their fall herbicide applications. However, she also notes in order for any herbicide to work, the weeds need to be actively growing. Therefore, drought-stressed weeds are not good targets.

“I was out checking soybeans to see if they were ready to harvest, and I saw a lot of henbit,” she says. “Henbit and other winter annuals can cause problems in the field because they take moisture away from crops, they harbor disease, and they create a mess in the spring when we are getting ready to plant.”

“Controlling them now will pay dividends later,” she adds. 

In addition to henbit, Lancaster says producers might see dandelion and some mustard species coming up this time of year. She also encourages them to pay special attention to two weeds in particular – marestail and kochia.

“Marestail is a growing problem in various summer crops. However, the weed is actually a winter annual species, so it is coming up now and will overwinter as a rosette,” she explains. 

She further notes marestail is fairly easy to control in the rosette stage. However, once the stem begins to elongate and the plant gets bigger, control is far more difficult.

“Growers need to take care of the rosettes in the fall and put down some residuals to try to reduce the emergence throughout the spring and summer,” she says.

“Kochia is also a special case because it comes up so early in the spring,” Lancaster continues. “There is a good possibility fall herbicide application might give growers a buffer for the late winter emerging kochia that comes up in February and March. So, although kochia is a summer annual weed, it may benefit from fall herbicide applications.”

Herbicide selection

As far as herbicide selection goes, Lancaster says it should be decided on a case-by-case basis. She says growers need to consider the crop they will be planting in the spring, plant-back intervals for the fall applied herbicide used and the species of weeds in their field. 

“With fall applied herbicides, there are really only two objectives – burn down existing weeds or residual activity. A grower’s objective depends on their unique situation,” Lancaster says. 

She notes if producers want to get rid of existing weeds, they should use glyphosate 2,4-D or dicamba. Some common residual herbicides she recommends are sulfentrazone and metribuzin.

“In terms of herbicide application management, fall herbicide application follows the same rules of thumb in-season applications do,” Lancaster states. “Growers need to stay within labeled guidelines, and they shouldn’t exceed maximum labeled rates.”

Lancaster also reminds growers not to apply herbicides to frozen ground.

“As we approach late October, we will likely see some freezing temperatures,” she says. “One thing I suggest, if time runs out for a true fall herbicide application program, is to split the shots for a well-adapted early spring herbicide application. Apply the first shot of herbicide 30 to 45 days before planting, and follow it with a second shot of herbicide at planting.” 

Hannah Bugas is the managing editor of the Wyoming Livestock Roundup. Send comments on this article to

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