Using Strategic Sheep Grazing Can Reduce an Exotic and Invasive Legume

Exotic plants can be invasive and take over rangeland and pasture land. In Wyoming, yellow sweetclover (Melilotus officinalis) is an exotic legume introduced from Europe and Asia to the U.S. as a forage crop.  Yellow sweetclover has been documented to invade and dominate plant communities by outcompeting other herbaceous plants for resources.  Currently, yellow sweetclover is listed as “invasive” in 26 states, including Wyoming, by the Alien Plant Working Group. The use of strategic grazing with sheep has been suggested as a strategy to reduce invasion and dominance of invasive plants. However, no data exists documenting the preferential selectivity of sheep on sweetclover and the changes of sweetclover dominance and structure relative to other plant functional groups. 

In 2015 and 2016, we used two groups of sheep – Rambouillet and Hampshire/Suffolk commercial ewes – to strategically graze yellow sweetclover-invaded pastures at the University of Wyoming Agricultural Experiment Station Sheep Unit in Laramie. In 2015, there were 24 ewes plus lambs in each breed group, and in 2016 there were 21 ewes plus lambs in each breed group.  Pastures were eights acres in size, and sheep were rotated from June through August for approximately three-week intervals. 

As sheep moved into a new pasture, they were repeatedly observed to immediately select the yellow sweetclover plants, first by eating the flower blooms and then by eating the youngest leaves. We also conducted vegetation sampling before sheep entered a pasture and after sheep left a pasture by measuring abundance of grasses, yellow sweetclover, forbs or flowering plants and cactus and measuring the structure of the yellow sweetclover.  We were able to compare these measurements to pastures that had no sheep or cattle grazing but did have pronghorn present regularly.

We found no differences between breeds of sheep in how they changed the plant community composition. 

Before sheep grazing, relative abundance of yellow sweetclover was 33 percent. After sheep grazing, yellow sweetclover was reduced to seven percent, and in the control pastures with no sheep grazing, yellow sweetclover was 29 percent. 

In contrast, before sheep grazing, relative abundance of grasses was 61 percent. After sheep grazing, grasses were increased to 91 percent, and in the control pastures with no sheep grazing, grasses were 65 percent. 

Our sheep grazing treatments reduced elevated leaves on yellow sweetclover from 100 percent to less than 20 percent, reduced the height of yellow sweetclover plants by 50 percent and reduced the number of flowering stems per plant from 14 per plant to less than one per plant. 

The reduction in flowers is important because these are the reproductive plant parts that produce seeds.

So, as we deal with invasion of exotic plant species, sometimes we can employ sheep to use their flexible dietary preferences.  We will continue to investigate this project to see if the results in the growing season lead to long-term reductions. 

For more information contact me at jscasta@uwyo.edu and check out my blog ‘Rangelands4u’ at wyoextension.org/rangelands4u.