Photo: Goats eating noxious weeds in Central Texas. Copyright 2018 Delena Norris-Tull
Livestock grazing to reduce noxious weeds
If used appropriately, livestock grazing can be used to reduce weeds. In fact, while recently re-introduced as an “innovative solution” to weed management, it is a practice that has been used for decades. A number of Western States use sheep or goats as part of the management of invasive plants, including Washington, Wyoming, and Texas.
And some recent management innovations include training livestock (sheep, goats, and cattle) to graze on invasive plant species that they do not naturally eat.
Sheep and goat grazing
There has not been a lot of research on the use of grazing with goats and sheep to manage invasive plants, and the studies I reviewed were typically just two to five years long, so little is known about long-term effectiveness and impacts.
The Colorado State University Extension Service provides information on how to use sheep and goats to control leafy spurge. The milky, latex sap in the stems and leaves of leafy spurge are injurious to the mouths of cattle and horses and can also cause photosensitivity in cattle and horses, but in small to moderate quantities does not harm sheep and goats. https://sam.extension.colostate.edu/wp-content/uploads/sites/2/2016/07/09-spring.pdf
While goats can cause damage to native plant species if allowed to graze unmanaged, in a number of locations goats are now being used to help control the spread of some invasive plant species. The use of goats for removal of invasive plants (especially woody plants) can greatly reduce the amount of herbicides used in an area.
The NRCS has sponsored some projects using goats as part of management plans to control noxious weeds. The Reeves family in Nebraska, which has relied on herbicides and prescribed burns for years, is carrying out an intensive intervention using goats to remove leafy spurge. An NRCS article, Grazing goats help control invasive weed species (McCrane, 2007), describes the combined strategies this family is testing. One advantage of using prescribed grazing with goats is that the weed seeds are destroyed by the goats’ chewing and digestive system.
In a five-year experiment on the effectiveness of grazing goats to reduce multiflora rose in a hardwood forest in Indiana, Ron Rathfon, at the Southern Indiana Purdue Agricultural Center, found that goats were effective at reducing invasive shrub cover and height within the forest (Mayer, 2021; Rathfon, et al., 2021). And while native understory woody species cover was also greatly reduced, herbaceous native species diversity and species richness remained steady or improved over time. Rathfon plans to continue monitoring the effectiveness and impacts of prescribed grazing with goats as part of his management plan, which also includes cutting and the use of herbicides.
In a three-year study in southern France (Moinardeau, et al., 2020), prescribed grazing with goats was compared with mechanical brush clearing and removal (with no grazing), and the combination of brush clearing and removal plus grazing, and a control (no management), within an embankment populated with trees, shrubs, and herbaceous plants. Mechanical brush clearing followed by grazing with goats was found to be the most effective strategy for both reducing the cover and height of invasive bramble (Rubus ulmifolius), and for improving understory herbaceous native plant species diversity and species richness.
Filbert (2016) also reported positive results in a two-year study by Penny Perkins, an ecologist specializing in land rehabilitation. She compared the use of goats with a control (an area with no management) in a timber stand in a native savanna in Ogden, Iowa. She found that goats were effective in reducing non-desirable woody species (including honeysuckle and multiflora rose). She found that intense grazing is needed, and that grazing did not negatively impact desirable herbaceous and woody species. She plans to continue the study. Sandy Vorce (2019) also described positive results in the use of prescribed goat grazing to reduce a variety of invasive plants, including multiflora rose in a meadow at Massachusetts Audubon Habitat Education Center and Wildlife Sanctuary in Belmont, MA.
Sheep grazing
Johnston & Peake, 1960, examined the impact of sheep grazing an area near Pearce, Alberta, an area planted with crested wheatgrass that had become severely infested with leafy spurge. The study extended for five years. They found that the basal area of leafy spurge was significantly reduced after four years of grazing, which they concluded was the minimal length of time needed for effective control. The wheatgrass cover was not negatively impacted by the grazing. They recommend that grazing begin early in the season. Mature leafy spurge plants should be mowed before grazing begins. Sheep selectively grazed small leafy spurge plants, more than mature plants. And they tended to congregate on the areas with most abundant leafy spurge. A few sheep were poisoned by mature leafy spurge plants, but not by early growth plants. During summer months, crested wheatgrass becomes unpalatable to sheep. The sheep avoided those plants. However, Kentucky bluegrass remains palatable in summer, and was over-grazed by the sheep.
In a study in Bozeman, Montana, Olson, et al., 1997, found that both the density of spotted knapweed plants and the weed seed bank were reduced with three years of sheep grazing, leading to decreased weed density. In contrast, both the native Idaho fescue and Kentucky bluegrass (non-native) populations increased in density. They proposed the long-term use of sheep grazing to help control spotted knapweed populations.
Three years of intensive grazing by sheep reduced both spotted knapweed and leafy spurge seed banks (Olson, et al, 1997; Olson, et al, 1998). However, three years of sheep grazing did not reduce the density of leafy spurge stems. “Grazing increased density of Idaho fescue but reduced density of bluebunch wheatgrass.” Also, grazing increased the following species: Kentucky bluegrass (an introduced perennial that can become weedy), Sandberg bluegrass (a native perennial valued as forage and erosion control, and historically used as food by some Native American tribes), annual bromegrasses (non-natives, which includes the weedy cheatgrass), and sedges.
Olson, et al., 2001, found that three years of grazing by sheep reduced the height of the native Idaho fescue more than the height of leafy spurge plants. “Based on grazed plant frequency, sheep often grazed Idaho fescue more than spotted knapweed.” But because the sheep tended to select “younger…, smaller, presumably more palatable spotted knapweed,” they proposed that a longer-term commitment to grazing could reduce the amount of spotted knapweed compared to Idaho fescue.
Sheep Grazing & Herbicides
Sheley, Jacobs, & Martin, 2004, decided to compare the effectiveness of using sheep to reduce spotted knapweed infestations, when combined with 2,4-D treatments. They conducted field studies for four years in two abandoned hay fields sites in western Montana. They carried out three treatments at each site: “the effect of a single spring application of 2,4-D [applied only in year one], repeated annual sheep grazing, and combining a single spring application of 2,4-D with repeated sheep grazing on spotted knapweed-infested rangeland,” compared with control plots with no treatment.
In each year, “The first grazing began when spotted knapweed was in the rosette to early bolting stage, usually in late May. Subsequent grazing was determined by the spotted knapweed regrowth and targeted bolted stems and buds to prevent seed production, usually in late June and July.” In the first two years, subsequent grazing was carried out two times. Due to drought in the final two years, knapweed did not regrow after the second grazing, and thus a third grazing was not carried out. Each grazing event lasted 1-4 days, depending on the amount of knapweed present.
Sheley, Jacobs, & Martin, 2004, analyzed treatment effects on density, cover, and biomass of spotted knapweed. In the site near Missoula, spotted knapweed “rosette density was lower in the 2,4-D treatment and sheep grazing combined with 2,4-D treatment compared to the control and the sheep grazing treatment. In the site near Drummond, “In 1998 and 1999 there were no treatment effect on rosettes... In 2000, sheep grazing combined with 2,4-D reduced rosette density compared to the control and where 2,4-D was applied [alone]. Sheep grazing [alone] reduced rosettes compared to the control. By 2001, sheep grazing combined with 2,4-D reduced rosettes compared to all other treatments.”
Analysis of flowering plant density: “At Missoula in 1998, flowering plant [knapweed] density was lower in the 2,4-D treatment compared to the control but not the sheep grazing treatment... Combining sheep grazing and 2,4-D reduced flowering plants compared to the control and the sheep grazing treatment. In 1999, the 2,4-D treatment and the sheep grazing combined with 2,4-D treatment reduced flowering plant density compared to the control and the sheep grazing treatment. In 2000, all treatments reduced flowering plant density compared to the control, and combining sheep grazing with 2,4-D reduced flowering plant density compared to the 2,4-D and sheep grazing treatments. By 2001, flowering plant density in the 2,4-D treatment was no different than that of the control. Sheep grazing and sheep grazing combined with 2,4-D reduced flowering plant density compared to the control but was not [signifcantly] different than the 2,4-D treatment.”
In the site near Drummond, “in 1998, flowering plant density was not different in any of the treatments... In 1999, combining sheep grazing with 2,4-D reduced flowering plant density compared to the control and the sheep grazing treatment, but not the 2,4-D treatment. In 2000, 2,4-D and combining sheep grazing with 2,4-D reduced flowering plants compared to the control and the sheep grazing treatment. By 2001, flowering plant density in the 2,4-D treatment was no different than the control, sheep grazing reduced flowering plants compared to the control, but not the 2,4-D treatment, and combining sheep grazing with 2,4-D reduced flowering plants compared to the 2,4-D treatment and the control.”
At Missoula, “All treatments reduced cover compared to the control.” The combined treatment of grazing and 2,4-D was the most effective at reducing knapweed cover. At Drummond, “In 1999, only sheep grazing combined with 2,4-D reduced cover compared to the control... In 2000 and 2001, cover was not different in the 2,4-D treatment compared to the control. Sheep grazing and sheep grazing combined with 2,4-D reduced cover compared to the control, but not compared to the 2,4-D treatment.”
At Missoula, “All treatments reduced [spotted knapweed] biomass compared to the control in 1998, and sheep grazing combined with 2,4-D reduced biomass compared to the sheep grazing and 2,4-D treatments... In 1999, all treatments reduced biomass compared to the control, but there were no differences between treatments. In 2000, biomass was the same in the 2,4-D treatment as the control, sheep grazing reduced biomass compared to the control and 2,4-D [alone] and sheep grazing combined with 2,4-D reduced biomass compared to all other treatments. By 2001, biomass was the same in the 2,4-D treatment as the control, biomass was the same in the sheep grazing treatment as sheep grazing combined with 2,4-D, and the sheep grazing treatment and sheep grazing combined with 2,4-D treatment were lower than the 2,4-D treatment and control.
At Drummond, “Spotted knapweed biomass was affected by treatment… Sheep grazing combined with 2,4-D reduced biomass compared to the 2,4-D treatment and the control... There were no other treatment differences.”
But reducing invasive plant abundance alone does not improve the site. Researchers need to also pay attention to whether or not the abundance of desired plants, in this case perennial grasses, improves over time as well. The competition that the perennial grasses provide, as they increase within the site, is the long-term solution to keeping invasive species at bay.
In this study, Sheley, Jacobs, & Martin, 2004, analyzed the changes to the abundance of perennial grasses (in this case, non-native but desired grasses that were already present in both sites). They found that by 2001, at Missoula, the combination of 2,4-D and grazing was the most effective at improving desired grass cover. Grass cover was also improved by the single herbicide treatment. It was not improved by grazing alone. At Drummond, both grazing alone and 2,4-D alone improved grass cover, but the combination of 2,4-D and grazing was the most effective at improving grass cover over time.
Delena’s thoughts: It is interesting to note that by year four, the single 2,4-D treatment, with no grazing, had not improved spotted knapweed flowering plant density, cover (except at Missoula), or biomass, when compared to the control. But nevertheless, perennial grass cover and biomass improved under all treatments (except that grass cover did not improve at Missoula by grazing alone). The authors point out that “the value of herbicides in sustainable weed management is the short-term control of weeds, and the long-term increase in perennial grass production and competition with spotted knapweed.” In other sites where perennial grass is not already present in sufficient abundance, land managers may need to seed with perennial grasses to attain the long-term effects wherein competition from perennial grasses suppresses the re-emergence of invasive forbs.
This study pointed out the importance of carrying out such research, and re-applying grazing treatments, over several years. Unfortunately, many studies on the effectiveness of herbicides or grazing are short-term, either one or two years. In this study, the impact of both 2,4-D and of sheep grazing were more apparent over the four years, but differed between sites.
Grazing with Chickens
Farmers in Alaska have sought alternatives to grazing with sheep and goats. With a short three-month growing season, the Homer Soil & Water Conservation District is researching the effectiveness of chickens in controlling invasive plants. As of 2023, the project is already showing some benefits for the management of orange hawkweed. The Western IPM Center is providing funding for this project.
References:
Links to additional Innovative Solutions:
Livestock grazing to reduce noxious weeds
If used appropriately, livestock grazing can be used to reduce weeds. In fact, while recently re-introduced as an “innovative solution” to weed management, it is a practice that has been used for decades. A number of Western States use sheep or goats as part of the management of invasive plants, including Washington, Wyoming, and Texas.
And some recent management innovations include training livestock (sheep, goats, and cattle) to graze on invasive plant species that they do not naturally eat.
Sheep and goat grazing
There has not been a lot of research on the use of grazing with goats and sheep to manage invasive plants, and the studies I reviewed were typically just two to five years long, so little is known about long-term effectiveness and impacts.
The Colorado State University Extension Service provides information on how to use sheep and goats to control leafy spurge. The milky, latex sap in the stems and leaves of leafy spurge are injurious to the mouths of cattle and horses and can also cause photosensitivity in cattle and horses, but in small to moderate quantities does not harm sheep and goats. https://sam.extension.colostate.edu/wp-content/uploads/sites/2/2016/07/09-spring.pdf
While goats can cause damage to native plant species if allowed to graze unmanaged, in a number of locations goats are now being used to help control the spread of some invasive plant species. The use of goats for removal of invasive plants (especially woody plants) can greatly reduce the amount of herbicides used in an area.
The NRCS has sponsored some projects using goats as part of management plans to control noxious weeds. The Reeves family in Nebraska, which has relied on herbicides and prescribed burns for years, is carrying out an intensive intervention using goats to remove leafy spurge. An NRCS article, Grazing goats help control invasive weed species (McCrane, 2007), describes the combined strategies this family is testing. One advantage of using prescribed grazing with goats is that the weed seeds are destroyed by the goats’ chewing and digestive system.
In a five-year experiment on the effectiveness of grazing goats to reduce multiflora rose in a hardwood forest in Indiana, Ron Rathfon, at the Southern Indiana Purdue Agricultural Center, found that goats were effective at reducing invasive shrub cover and height within the forest (Mayer, 2021; Rathfon, et al., 2021). And while native understory woody species cover was also greatly reduced, herbaceous native species diversity and species richness remained steady or improved over time. Rathfon plans to continue monitoring the effectiveness and impacts of prescribed grazing with goats as part of his management plan, which also includes cutting and the use of herbicides.
In a three-year study in southern France (Moinardeau, et al., 2020), prescribed grazing with goats was compared with mechanical brush clearing and removal (with no grazing), and the combination of brush clearing and removal plus grazing, and a control (no management), within an embankment populated with trees, shrubs, and herbaceous plants. Mechanical brush clearing followed by grazing with goats was found to be the most effective strategy for both reducing the cover and height of invasive bramble (Rubus ulmifolius), and for improving understory herbaceous native plant species diversity and species richness.
Filbert (2016) also reported positive results in a two-year study by Penny Perkins, an ecologist specializing in land rehabilitation. She compared the use of goats with a control (an area with no management) in a timber stand in a native savanna in Ogden, Iowa. She found that goats were effective in reducing non-desirable woody species (including honeysuckle and multiflora rose). She found that intense grazing is needed, and that grazing did not negatively impact desirable herbaceous and woody species. She plans to continue the study. Sandy Vorce (2019) also described positive results in the use of prescribed goat grazing to reduce a variety of invasive plants, including multiflora rose in a meadow at Massachusetts Audubon Habitat Education Center and Wildlife Sanctuary in Belmont, MA.
Sheep grazing
Johnston & Peake, 1960, examined the impact of sheep grazing an area near Pearce, Alberta, an area planted with crested wheatgrass that had become severely infested with leafy spurge. The study extended for five years. They found that the basal area of leafy spurge was significantly reduced after four years of grazing, which they concluded was the minimal length of time needed for effective control. The wheatgrass cover was not negatively impacted by the grazing. They recommend that grazing begin early in the season. Mature leafy spurge plants should be mowed before grazing begins. Sheep selectively grazed small leafy spurge plants, more than mature plants. And they tended to congregate on the areas with most abundant leafy spurge. A few sheep were poisoned by mature leafy spurge plants, but not by early growth plants. During summer months, crested wheatgrass becomes unpalatable to sheep. The sheep avoided those plants. However, Kentucky bluegrass remains palatable in summer, and was over-grazed by the sheep.
In a study in Bozeman, Montana, Olson, et al., 1997, found that both the density of spotted knapweed plants and the weed seed bank were reduced with three years of sheep grazing, leading to decreased weed density. In contrast, both the native Idaho fescue and Kentucky bluegrass (non-native) populations increased in density. They proposed the long-term use of sheep grazing to help control spotted knapweed populations.
Three years of intensive grazing by sheep reduced both spotted knapweed and leafy spurge seed banks (Olson, et al, 1997; Olson, et al, 1998). However, three years of sheep grazing did not reduce the density of leafy spurge stems. “Grazing increased density of Idaho fescue but reduced density of bluebunch wheatgrass.” Also, grazing increased the following species: Kentucky bluegrass (an introduced perennial that can become weedy), Sandberg bluegrass (a native perennial valued as forage and erosion control, and historically used as food by some Native American tribes), annual bromegrasses (non-natives, which includes the weedy cheatgrass), and sedges.
Olson, et al., 2001, found that three years of grazing by sheep reduced the height of the native Idaho fescue more than the height of leafy spurge plants. “Based on grazed plant frequency, sheep often grazed Idaho fescue more than spotted knapweed.” But because the sheep tended to select “younger…, smaller, presumably more palatable spotted knapweed,” they proposed that a longer-term commitment to grazing could reduce the amount of spotted knapweed compared to Idaho fescue.
Sheep Grazing & Herbicides
Sheley, Jacobs, & Martin, 2004, decided to compare the effectiveness of using sheep to reduce spotted knapweed infestations, when combined with 2,4-D treatments. They conducted field studies for four years in two abandoned hay fields sites in western Montana. They carried out three treatments at each site: “the effect of a single spring application of 2,4-D [applied only in year one], repeated annual sheep grazing, and combining a single spring application of 2,4-D with repeated sheep grazing on spotted knapweed-infested rangeland,” compared with control plots with no treatment.
In each year, “The first grazing began when spotted knapweed was in the rosette to early bolting stage, usually in late May. Subsequent grazing was determined by the spotted knapweed regrowth and targeted bolted stems and buds to prevent seed production, usually in late June and July.” In the first two years, subsequent grazing was carried out two times. Due to drought in the final two years, knapweed did not regrow after the second grazing, and thus a third grazing was not carried out. Each grazing event lasted 1-4 days, depending on the amount of knapweed present.
Sheley, Jacobs, & Martin, 2004, analyzed treatment effects on density, cover, and biomass of spotted knapweed. In the site near Missoula, spotted knapweed “rosette density was lower in the 2,4-D treatment and sheep grazing combined with 2,4-D treatment compared to the control and the sheep grazing treatment. In the site near Drummond, “In 1998 and 1999 there were no treatment effect on rosettes... In 2000, sheep grazing combined with 2,4-D reduced rosette density compared to the control and where 2,4-D was applied [alone]. Sheep grazing [alone] reduced rosettes compared to the control. By 2001, sheep grazing combined with 2,4-D reduced rosettes compared to all other treatments.”
Analysis of flowering plant density: “At Missoula in 1998, flowering plant [knapweed] density was lower in the 2,4-D treatment compared to the control but not the sheep grazing treatment... Combining sheep grazing and 2,4-D reduced flowering plants compared to the control and the sheep grazing treatment. In 1999, the 2,4-D treatment and the sheep grazing combined with 2,4-D treatment reduced flowering plant density compared to the control and the sheep grazing treatment. In 2000, all treatments reduced flowering plant density compared to the control, and combining sheep grazing with 2,4-D reduced flowering plant density compared to the 2,4-D and sheep grazing treatments. By 2001, flowering plant density in the 2,4-D treatment was no different than that of the control. Sheep grazing and sheep grazing combined with 2,4-D reduced flowering plant density compared to the control but was not [signifcantly] different than the 2,4-D treatment.”
In the site near Drummond, “in 1998, flowering plant density was not different in any of the treatments... In 1999, combining sheep grazing with 2,4-D reduced flowering plant density compared to the control and the sheep grazing treatment, but not the 2,4-D treatment. In 2000, 2,4-D and combining sheep grazing with 2,4-D reduced flowering plants compared to the control and the sheep grazing treatment. By 2001, flowering plant density in the 2,4-D treatment was no different than the control, sheep grazing reduced flowering plants compared to the control, but not the 2,4-D treatment, and combining sheep grazing with 2,4-D reduced flowering plants compared to the 2,4-D treatment and the control.”
At Missoula, “All treatments reduced cover compared to the control.” The combined treatment of grazing and 2,4-D was the most effective at reducing knapweed cover. At Drummond, “In 1999, only sheep grazing combined with 2,4-D reduced cover compared to the control... In 2000 and 2001, cover was not different in the 2,4-D treatment compared to the control. Sheep grazing and sheep grazing combined with 2,4-D reduced cover compared to the control, but not compared to the 2,4-D treatment.”
At Missoula, “All treatments reduced [spotted knapweed] biomass compared to the control in 1998, and sheep grazing combined with 2,4-D reduced biomass compared to the sheep grazing and 2,4-D treatments... In 1999, all treatments reduced biomass compared to the control, but there were no differences between treatments. In 2000, biomass was the same in the 2,4-D treatment as the control, sheep grazing reduced biomass compared to the control and 2,4-D [alone] and sheep grazing combined with 2,4-D reduced biomass compared to all other treatments. By 2001, biomass was the same in the 2,4-D treatment as the control, biomass was the same in the sheep grazing treatment as sheep grazing combined with 2,4-D, and the sheep grazing treatment and sheep grazing combined with 2,4-D treatment were lower than the 2,4-D treatment and control.
At Drummond, “Spotted knapweed biomass was affected by treatment… Sheep grazing combined with 2,4-D reduced biomass compared to the 2,4-D treatment and the control... There were no other treatment differences.”
But reducing invasive plant abundance alone does not improve the site. Researchers need to also pay attention to whether or not the abundance of desired plants, in this case perennial grasses, improves over time as well. The competition that the perennial grasses provide, as they increase within the site, is the long-term solution to keeping invasive species at bay.
In this study, Sheley, Jacobs, & Martin, 2004, analyzed the changes to the abundance of perennial grasses (in this case, non-native but desired grasses that were already present in both sites). They found that by 2001, at Missoula, the combination of 2,4-D and grazing was the most effective at improving desired grass cover. Grass cover was also improved by the single herbicide treatment. It was not improved by grazing alone. At Drummond, both grazing alone and 2,4-D alone improved grass cover, but the combination of 2,4-D and grazing was the most effective at improving grass cover over time.
Delena’s thoughts: It is interesting to note that by year four, the single 2,4-D treatment, with no grazing, had not improved spotted knapweed flowering plant density, cover (except at Missoula), or biomass, when compared to the control. But nevertheless, perennial grass cover and biomass improved under all treatments (except that grass cover did not improve at Missoula by grazing alone). The authors point out that “the value of herbicides in sustainable weed management is the short-term control of weeds, and the long-term increase in perennial grass production and competition with spotted knapweed.” In other sites where perennial grass is not already present in sufficient abundance, land managers may need to seed with perennial grasses to attain the long-term effects wherein competition from perennial grasses suppresses the re-emergence of invasive forbs.
This study pointed out the importance of carrying out such research, and re-applying grazing treatments, over several years. Unfortunately, many studies on the effectiveness of herbicides or grazing are short-term, either one or two years. In this study, the impact of both 2,4-D and of sheep grazing were more apparent over the four years, but differed between sites.
Grazing with Chickens
Farmers in Alaska have sought alternatives to grazing with sheep and goats. With a short three-month growing season, the Homer Soil & Water Conservation District is researching the effectiveness of chickens in controlling invasive plants. As of 2023, the project is already showing some benefits for the management of orange hawkweed. The Western IPM Center is providing funding for this project.
References:
- Filbert, M. (Dec. 20, 2016). Research report: Using goats to control invasive species. Practical Farmers of Iowa: Ames, Iowa. https://practicalfarmers.org/2016/12/research-report-using-goats-to-control-invasive-species/
- Johnston, A., & Peake, R.W. (July, 1960). Effect of selective grazing by sheep on the control of leafy spurge (Euphorbia esula). Journal of Range Management, 13: 192-195.
- Mayer, W. (Sept. 7, 2021). Goat grazing could be an option for invasive species removal. Purdue University.
https://ag.purdue.edu/fnr/Pages/GoatGrazingInvasiveSpeciesRemoval.aspx - McCrane, P. (2007). Grazing goats help control invasive weed species. NRCS, Washington, D.C. https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_029102.pdf
- Moinardeau, C., Mesléard, F., Ramone, H., & Dutoit, T. (2020). Using mechanical clearing and goat grazing for restoring understorey plant diversity of embankments in the Rhone valley (Southern France). Plant Biosystems, 154(5): 746–756. https://doi.org/10.1080/11263504.2019.1686080
- Olson, B.E., Wallander, R.T., & Lacey, J.R. (July, 1997). Effects of sheep grazing on a spotted knapweed-infested Idaho fescue community. Journal of Range Management, 50: 386-390.
- Olson, B.E., & Wallander, R.T. (March, 1998). Effects of sheep grazing on a leafy spurge-infested Idaho fescue community. Journal of Range Management, 51 (2): 247-252.
- Olson, B.E., & Wallander, R.T. (Jan., 2001). Sheep grazing spotted knapweed and Idaho fescue. Journal of Range Management, 54 (1): 25-30.
- Rathfon, R.A., Greenler, S.M., & Jenkins, M.A. (May 2021). Effects of prescribed grazing by goats on non-native invasive shrubs and native plants in a mixed-hardwood forest. Restoration Ecology, 29(4): 1-11.
- Sheley, R.L., Jacobs, J.S., & Martin, J.M. (July, 2004). Integrating 2,4-D and sheep grazing to rehabilitate spotted knapweed infestations. Journal of Range Management, 57: 371-375.
- Vorce, S. (2019). Goats as an ecological management option for invasive plants. Ecological Landscape Alliance. https://www.ecolandscaping.org/09/landscape-challenges/invasive-plants/goats-as-an-ecological-management-option-for-invasive-plants/
Links to additional Innovative Solutions: