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Weeds in alfalfa Kniss suggests asking big questions when killing weeds

by Wyoming Livestock Roundup

Lingle – Andrew Kniss, University of Wyoming associate professor in weed science, said his job is to find weeds and the best way to kill them, but during the 2018 Forage Field Day, held June 12 at the James C. Hageman Sustainable Agriculture Research and Extension Center in Lingle, Kniss asked producers to consider why they control weeds and what the impact of not controlling weeds may be. 

“If we don’t control weeds, will we see quality impacts or lower yields?” he asked.

Organic production

Kniss assessed the question by first compared a recent study that looked at organic and conventional production. 

“Almost every crop shows a penalty to organic production because these producers don’t have the same access to fertilizers or synthetic pesticides, which is a big deal,” he said. “We find organic crops will produce 70 to 80 percent of conventional crops.” 

“Most interestingly, in alfalfa production, however, organic hay producers are producing yields almost exactly the same as conventional management,” Kniss commented. “Other hay producers see almost 20 percent higher yields than conventionally managed fields. All hay is right in between, about 10 percent more than conventional hay.”

“This is very different than almost every other crop, so we asked ourselves, what’s going on?” he continued. “My thought is, they harvest weeds. Organic producers see a benefit because they’re harvesting the weeds, as well as the alfalfa and hay crop.” 

Kniss continues the impact is further demonstrated by looking at the impacts of weed management on alfalfa yields. 

“Studies show, out of total forage yield, a herbicide treatment in the fall or springs shows impacts yield,” Kniss explained. “In two out of three sites, yield was reduced when herbicide was applied. The study did not see a response of increased yield to herbicide application.” 

Managing weeds

Depending on the goals of an operation, Kniss further commented perhaps the right answer would be to let weeds grow to increase forage production.

“We should think through what our tolerance should be for weeds in our forage, because weeds can actually increase the amount of production we harvest,” Kniss said. 

The reason weeds are controlled in alfalfa and hay fields is for quality, he continued. 

“The same study showed us, when we have two different herbicide treatments for weed control, we didn’t see a change in overall yield, but the yield of alfalfa changes,” Kniss said. 

Kniss noted the amount of alfalfa per acre was 0.6 tons, but when treated with herbicides, alfalfa production doubled. 

“We’re producing two tons per acre, and it’s either alfalfa or something else,” he explained. “We manage and get rid of weeds to maintain quality.”


A 1987 study looked at one aspect of alfalfa quality, showing dramatic declines in quality when more than 15 percent of an alfalfa stand is weeds. 

Data from a Steve Miller study in 2002 looked at the protein and relative feed value (RFV) of alfalfa stands ranging from less than five percent weeds to greater than 40 percent weeds. 

A broadleaf weed infestation of less than five, 10 to 15 and 20 to 30 percent weeds resulted in protein percentages of 23, 22 and 21 percent, respectively, and RFV of 157, 148 and 141, respectively. When the weed composition increases to 40 percent, protein drops to 19 percent and RFV hits 136. 

Green foxtail in forage crops has nearly identical impact, dropping the protein to 14 percent and RFV to 110 at greater than 40 percent infestation. 

“Quality is why we’re interested in managing weeds,” Kniss said, “but we have to consider if we’re getting paid on quality.” 


“What we do with our alfalfa dictates how much tolerance we might have to manage weeds,” Kniss said.

For example, Kniss said if the alfalfa crop is going to be used to feed cattle, a weed-free crop might not be necessary.

“Some weeds have high nutritive content, and some weeds have less impact on quality than others,” he explained. “Unfortunately, our most common weeds give us the big hit on quality.”

Common lambs quarter and green foxtail negatively impact forage quality, but kochia is relatively good quality forage. 

“We have to watch kochia during drought years, however, because it accumulates nitrates, so it’s dangerous to feed,” Kniss said. “Weeds also decline in quality as they get more mature, so we must also consider timing of harvest when we have weeds that get stemmy and decrease quality.” 

Kniss encouraged producers to answer these questions for themselves and determine their tolerance for weeds.

“When we’re decided whether to control weeds, we need to keep in mind things like toxic and noxious weeds or weeds that can get out of control quickly,” Kniss said. “We also needs to keep in mind our marketing strategy, which may change our tolerance for weeds.”

Saige Albert is managing editor of the Wyoming Livestock Roundup. Send comments on this article to

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