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Kniss: Weed management extends beyond use of herbicides

by Wyoming Livestock Roundup

“When we talk about herbicide resistance, we recognize that it is a problem, but there are a lot of other things that go on when we talk about weed management,” says University of Wyoming’s Andrew Kniss. 

Kniss, an assistant professor in the Department of Plant Sciences, notes that there are a number of factors to consider when looking at weed management.

Communities and populations

First, when thinking about resistance, Kniss says that the difference between community-level and population-level changes should be considered. 

“A community-level change is individuals of a species that are well-adapted to a management practice,” he explains. “If we apply the same herbicide, over time that herbicide is not going to be effective. We are going to see a shift in that weed community.”

The shift is often termed a species shift, and Kniss notes that community-level changes are not evidence of herbicide resistance. 

“Population-level changes are when we have certain biotypes that are resistant to changes,” he explains. “For example, with Common Lambsquarters, we have one to two plants that can survive herbicide or are well-adapted to tillage, those are population-level changes.”


“What we are really interested in here is adaptation,” Kniss says. “Resistance is one way weeds can adapt to herbicides, but there are other ways.”

For example, in looking at Red Root Pigweed in a long-term study in Torrington, Kniss notes that Roundup was used each year for eight years. In a separate treatment, Roundup was alternated every other year with another herbicide. A third treatment never utilized Roundup. 

“We look at Red Root Pigweed and the densities in each treatment,” Kniss says. “Where we did not use any Roundup, we had the lowest density, and after eight years, we had four to six times as much pigweed where we were using Roundup every year.”

He continues, “The first thing we think is that the pigweed is resistant to Roundup – but it isn’t. It just wasn’t very susceptible to glyphosate.”

In the trial, Kniss explains that while the pigweed wasn’t susceptible to Roundup, other plants were eliminated by the herbicide, enabling pigweed to spread more easily without competition.


Another aspect to consider in weed management is the timing of herbicide applications. 

“Pigweed comes up all the time – particularly later in the season after we have killed all the other weeds,” Kniss says. 

In analyzing weed emergence patterns, each weed species has adapted to an optimal germination temperature, Kniss explains. 

Many weeds germinate when the soil reaches 50 degrees Fahrenheit, but pigweed prefers warmer soil temperatures – near 90 degrees Fahrenheit.

“Pigweed germinates a lot later than other weed species and often when we are applying that last shot of Roundup,” he continues. “In a good year, particularly in corn, the only weed to emerge after June is Red Root Pigweed.”

This, he says, is an example of a community-level shift. 

“We select for species that are most able to deal with the repeated selection pressure,” Kniss says. “We have no residual activity, and we are shifting the weed so that Red Root Pigweed becomes dominant. This is an adaptation to repeated herbicide use.”

There are also other examples of weed management plans that result in adaptations, such as ability to germinate after pre-emergent herbicides are applied and are no longer acting.

Weed management plans

“When we talk about developing a weed management plan, a lot of the things we talk about focus around trying to delay or prevent herbicide resistance,” Kniss says. “A lot of it is also trying to get good weed management, so we don’t deal with reduced yields or allowing species to build up and become problematic.”

“Our goal is to incorporate as much diversity as possible,” he adds, mentioning that utilizing different tools and techniques can help prevent resistance. “It is not just about using different tools or herbicide modes of action. It is about using different tools at different times.”

Herbicide varieties

When considering use of herbicides, Kniss notes that just using a different product isn’t important. The important element to consider is the herbicide group being utilized. 

“We could use 18 different products and only use Group Two herbicides,” he says, noting that each group defines the mode of action of the herbicide, or how it works. 

At the same time, just using a different group isn’t necessarily going to work. Herbicides must also be effective for the target species.

Farming practices

“Another thing we can do is crop rotation because it alters the planting date,” says Kniss. “Once we plant the crop, we lose a huge number of weed control options because we can no longer till or apply certain broad-spectrum herbicides.”

When considering crop rotation, he encourages producers to consider the timing of planting for each crop. Rotating sugarbeets and barley, which are both planted from the end of March to the middle of April, is less effective because they are planted within several weeks of one another. 

“A crop like dry beans makes sense because we wouldn’t plant that until the last week of May,” Kniss says. “We have bought ourselves two months to allow weeds to germinate and kill them.”

“There are lots of options,” he adds.

Another important tool can be tillage, Kniss adds. 

Though conservation tillage provides other benefits, he says that for weed management, “As soon as we take tillage out, we are becoming more reliant on herbicides.”

By simply tilling and effectively placing weed seeds three to four inches below the soil surface, Kniss notes that weed populations can be significantly reduced. 

“Resistance management doesn’t come in a jug,” Kniss comments. “We need to start talking about some of the other things we can be doing.”

Kniss presented at the 2015 WESTI Ag Days, held at the beginning of February in Worland. 

Saige Albert is managing editor of the Wyoming Livestock Roundup and can be reached at


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