Introduction to Managed Grazing
Grazing to control woody invasive plants involves introducing domesticated animals to damage the plants by browsing the leaves, twigs and bark (e.g., goats) or physically damaging plants (e.g., cattle) . A one-time application of grazing is unlikely to be effective at controlling woody invasive species, but can be useful for improving access to implement other control strategies. Alternately, repeated intensive grazing can reduce woody invasive populations if conducted properly.
How it works
Goats consume the leaves, twigs and bark of woody species, preventing photosynthesis and reproduction. Cattle generally prefer herbaceous fodder, and their main form of damage to woodies is from rubbing against and knocking down stems. In either case, grazing does not damage the root system, and invasive plants regrow from grazed stumps. Further treatment is required to prevent regrowth.
Methodology in Detail
Grazing to control woody plants is most effective with goats because other domesticated livestock species do not browse most woody species. Goats show a preference for woody vegetation, and leaves from woody plants can constitute a significant portion of the goat herd diet when supply is plentiful (Hart 2001). Recent research on goat browsing to control woody undergrowth in Southwestern Wisconsin found that woody plant material made up 84% of the herd diet (Nolden 2020). While rubbing and browsing by cattle can reduce woody species cover in some instances, the response is very species specific and rarely effective without integrating other management techniques.
The first step in grazing for woody invasive plant control is the development of a grazing management plan. Grazing plans should include a detailed map of the site. The plan should also include a professionally developed prescription for grazing that includes the stocking rate (number of animals per unit area), grazing frequency and animal movement (rotation) that will obtain the desired vegetation management goals (NRCS MN 2016). Inclusion of information about grazing units (paddocks), supplemental feed, and access to water are also important factors to consider.
If woody plants are grazed/damaged repeatedly for long enough timeframe, individuals will die. However, little data on grazing frequency and duration necessary to successfully control most woody species is available. The Natural Resources Conservation Service in both Iowa and Minnesota recommends establishing several grazing units or paddocks and rotating animals through, starting in the unit with the highest concentration of target vegetation until 20% of foliage remains, and then returning animals to that paddock as soon as the target vegetation sprouts new leaves. The NRCS unit in Iowa estimates that most brush species can be killed with 2-3 seasons of intensive grazing using this technique (NRCS IA 2013). NRCS in Minnesota is more conservative, estimating mortality between 2-8 seasons (NRCS MN 2016). Data collected in Wisconsin also indicate that intensive grazing (90% defoliation) is more effective than light grazing (50% defoliation) for reducing woody species cover and height (Nolden 2020).
It should also be noted that the grazing and housing of livestock is usually regulated through municipal ordinances (Salter et al. 2015). Any grazing plan for invasive plant management should be checked against local ordinances prior to implementation.
Grazing can be conducted from the point at which target plants are fully leafed out through late summer. The impact of continued browsing on woody species diminishes in early-to-mid August (NRCS IA 2013, NRCS MN 2016).
Grazing by goats can be effective on species that are either under 7’ in height or that have flexible stems that can be pulled down by the animals (i.e., sapling trees, most shrubs, low-growing vines) (NRCS MN 2016).
The Natural Resources Conservation Service reports that goats show a strong preference for buckthorn species, autumn olive, multiflora rose, locust species, mulberry, poplars, honeysuckles, and raspberry species. After preferred species are eaten, they may also browse barberry, winged burning bush, and Siberian elm (NRCS IA 2013, NRCS MN 2016). There is very little information published on the impacts of goat grazing on desirable, non-target species, particularly in a natural areas context. This issue is currently being researched at the University of Minnesota (Silvola 2019).
Use of cattle has found reductions in limited invasive species when implemented at low stocking densities.
If invasive plant seeds are likely present on site (e.g., the plants being pulled are fruiting or have fruited previously), animals and clothing and footwear of handlers should be cleaned carefully before leaving the site to prevent spreading invasive plant seeds to new locations. There is also potential for goats to move invasive plant seed by eating and expelling it Preliminary research indicates that only a very small proportion of common buckthorn seed is able to germinate following digestion by goats (Silvola 2019). However, more research is needed to assess the risk with other species. Therefore it is recommended to maintain animals on site for at least 24 hours prior to moving, feeding non-contaminated feed. This will allow any digested seed to pass prior to transportation off site.
Any invasive species treatment needs to be monitored in the years following treatment. Regrowth should be anticipated following the defoliation of invasive woody plants by goats or other grazers. Soil disturbance may result in a strong seedbank response from invasive plants, but information on the strength of this response, especially relative to other control methods, is lacking.
- Hart, SP. 2001. Recent perspectives in using goats for vegetation management in the USA. Journal of Dairy Science. 84 (E. Supp.): E170-176. Online:
- Natural Resources Conservation Service, Iowa. 2013. Iowa conservation practice 314: brush management with goats.
- Natural Resources Conservation Service, Minnesota. 2016. Minnesota practice code 528: biological brush management implementation guide.
- Nolden, C. 2020. Goat dietary selections, performance and browsing effects on a brush invaded oak savanna in Southwest Wisconsin. Master’s thesis, University of Wisconsin-Madison.
- Salter, M, MacDonald, E and Z Richardson. 2015. Prescribed goat grazing in urban settings: a pilot study of the legal framework in nine U.S. cities.
- Silvola, C. 2019. Ahead of the herd: how goat grazing research is helping with buckthorn control. Minnesota Invasive Terrestrial Plants and Pests Center, University of Minnesota.
Photo Credit: Scott Bauer, USDA Agricultural Research Service, via bugwood.org