Spring 2023

Natural Selections

A wooded test site in Sun Prairie, Wis., before and after goats grazed on invasive shrubs. Photo courtesy of Stefania Cartoni Casamitjana


On a walk through the woods in Wisconsin, you might expect to see a few squirrels, a variety of birds, and maybe even a whitetail deer. Goats, on the other hand, don’t make that list. But goats might be spotted among the trees more often if ongoing CALS research shows they’re useful for controlling invasive species.

Invasive shrubs pose problems for wooded areas, whether they’re used for agricultural production or managed as natural areas. In cases of silvopasture, where trees and grazing livestock are integrated, the shrubs create a dense canopy that prevents forage growth. The shrubs also outcompete native species, reduce wildlife habitat, and change the soil’s ability to absorb water, all of which can hinder the restoration of woodlands as natural areas.

For these reasons, Stefania Cartoni Casamitjana MS’21 and Mark Renz are looking at the grazing prowess of goats as one possible method for controlling invasive shrubs in oak woodlands. They’re conducting research at two southern Wisconsin locations. One is a degraded oak woodland in need of restoration at the American Family Insurance headquarters in Sun Prairie. The other, situated 30 miles west in Prairie du Sac, is a plot slated for conversion to silvopasture.

“Woody shrubs likely have a greater impact in Wisconsin than all other invasive species combined because they completely dominate wooded systems, transform the plant and animal communities, and change ecosystem processes that Wisconsinites rely on,” says Renz, a professor and extension specialist in the Department of Agronomy.

Goats graze on invasive shrubs at a wooded test site in Sun Prairie, Wis. Photo courtesy of Stefania Cartoni Casamitjana

“A healthy oak savannah should be pretty open and allow a lot of light through,” explains Cartoni Casamitjana, a doctoral student in agronomy. “But there’s no open space in our sites anymore, and they’re invaded by mostly buckthorn and honeysuckle.”

To control these nonnative shrubs, the team is exploring the relative effectiveness of several techniques. In addition to goat grazing, they’re studying two widely used methods: herbicides and large mowers that can remove the aboveground portion of the shrubs. Goats, however, may bring special abilities.

“A unique thing about goats is they will preferentially feed on woody species, so their impact is more on the shrubs,” says Renz.

“Also, goats are useful in areas where you can’t go in with machinery or you don’t want to go in with herbicides,” adds Cartoni Casamitjana. “If you have steep terrain, for example, you can’t go in with machinery, but goats are very able to take that on.”

The researchers are in the early stages of a multiyear effort in which they will apply various treatments. In the first year, the team mowed, introduced goats, or left the woodlands untouched. In the second year, each of the first-year treatments will be split in two, with half treated with herbicide and the other half grazed by goats. To determine the effectiveness of each treatment, the researchers are measuring understory plants and the amount of light that can reach the forest floor.

In the first year of the study, the team saw immediate effects from both goat grazing and mowing, but the shrubs regrew as the season went on. By summer, the team saw no difference between the goat grazing areas and untreated woodland in terms of light interception, but the mowing treatment allowed more light to hit the forest floor. When surveying the plants to see how much vegetation remained and regrew, there was no difference in shrubs between the goat and mowing treatments and the area that was untreated.

“It’s not a surprise,” says Renz. “These are shrubs that have lived five to 50 years, probably. We knew we were going to need to do multiple years of management.”

Renz and Cartoni Casamitjana are also considering the expense associated with each treatment and what that means for farmers and landowners. “Cost of implementation of each of these techniques is very important,” explains Cartoni Casamitjana. “Yes, one method might be more effective, but if it’s many times more expensive, will anyone use it? We want to be able to give useful recommendations. That’s the goal.”

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