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Grazing pinpointed as possible option for larkspur management

by Wyoming Livestock Roundup

Larkspur has big impacts for many folks grazing across Wyoming, as the plant is toxic to cattle. In an attempt to mitigate the impacts of the native plant, University of Wyoming Graduate Student Julia Workman initiated a study in 2014 to determine if sheep grazing is a viable control option for the plant. 

“Larkspur is a native, but we worry about it because it is highly toxic to cattle,” Workman says. “We see a lot of death loss in cattle when larkspur is abundant.”

Economic consequences

Workman continues that the impacts of larkspur on livestock production extend beyond death loss impacts.

“Death isn’t the only economic impact of larkspur,” she describes. “In some situations, producers will choose not to graze pastures until later in the year, meaning they lose the spring forage, as well.”

While herbicides are effective for larkspur control, they can be expensive, and Workman notes, “Herbicides may make the problem worse by making the plant more toxic and also more likely to be eaten.”

As a result, use of herbicides means that pasture may have to be left un-grazed until the plant is totally desiccated. 

“Historically, cattle growers sent sheep into these pastures first,” she explains. “The sheep would eat the larkspur and leave the grass. There have been mixed reports on the efficacy of this practice.”

Grazing impacts

Current research has shown mixed impact from larkspur grazing on different varieties, but not on the Wyoming species Geyer larkspur. 

“We want to look at targeted grazing and how it impacts larkspur,” Workman notes.

For the study, Workman used 20 and 40 sheep grazing for four, six and 12 hours in the spring. Sheep were turned out into cells that measured 30 feet by 60 feet, for a stocking rate of approximately 0.5 animal unit months per acre in the four-hour grazing treatment.

Before sheep were turned into each of four cells, the number of larkspur plants were counted for the cell. Following the treatment, all plants still present were counted again. 


“Most of the larkspur plants had at least one bite taken out of them, and some were just stems,” Workman notes. “We counted to see if we could find the plants, but it was a treasure hunt.”

Workman mentioned that most plants were trampled, partially eaten, pulled up or a combination of the above. 

“The sheep really liked the larkspur,” she says. “In our plots with the highest grazing pressure, we saw over 90 percent reduction in larkspur numbers in the spring. By mid-summer, we were seeing less than five percent of the larkspur remaining in all of our grazing treatments, so that was exciting.”

She also mentioned that regrowth was fairly minimal, as well. 

“Four hours of grazing by 20 sheep was enough to reduce larkspur density and allow some grass recovery,” Workman says. “That might be a positive note. For future research, we can see if even lower grazing intensity can give us really good larkspur control without impacting the grasses.”

Next steps

With positive results in her first year, Workman also mentions that the study was designed to be conducted over two years and will continue into 2015. 

“The big difference in 2015 will be that we did some summer and fall grazing treatments in 2014 after we collected midsummer data,” she explains. “These grazing events weren’t reflected in the initial data we collected. We may detect some differences in the coming field season.”

She also notes potential for future research, saying, “I think future research will really want to focus on finding that level of grazing that impacts the larkspur plants without damaging the grasses too much, since we saw pretty high impact with these treatments.”

Because the larkspur was so palatable for sheep, Workman believes that lowering grazing intensity would be beneficial for grasses while also providing for good control of larkspur. 

“Of course, if we could find that optimal level, the next step would be to see if management would be doable in a pasture setting with a group of sheep and a good herder,” Workman says.

For those producers interested in her research, Workman notes that the USDA Agricultural Research Service station in Cheyenne will be hosting a toadflax field day on June 16 to tour her research plots, as well as many others.

Look for more information on the field day in an upcoming edition of the Roundup

Workman presented her research during the 70th Annual Wyoming Weed and Pest Conference held in Rock Springs at the beginning of November 2014. 

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

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