Grazing, especially by goats and/or sheep, is often promoted as a control method for weeds or shrubs. Depending upon the life strategy of the weeds being targeted, grazing can be effective, but it’s important to set realistic objectives. As you might expect, many perennial grasses, forbs, and shrubs have evolved strategies for surviving repeated defoliation. In those cases, grazing may appear to effectively control plants while grazers are present, but the plants bounce back right after grazers are removed.
One of my all-time favorite research projects showcases this exact phenomenon at a site in South Dakota owned by The Nature Conservancy. Back in the early 1990’s, an estimated 75% of the Conservancy’s Altamont Prairie Preserve was covered by leafy spurge (Euphorbia esula). In 1994, goats and sheep were installed in separate pastures and spurge was treated by using periodic high-intensity grazing sessions during both early summer and early fall. Both the goats and sheep were very effective at eating the spurge plants, and after five years, managers conducting walk-through inspections the site felt like excellent long-term control of spurge had been achieved. Inside small exclosures, spurge was still abundant and vigorous, but outside the exclosures, almost no plants could be seen. As a result, the goats and sheep were removed and everyone was happy.
…Until the next season when spurge plants popped right back out of the ground and the pasture looked essentially as it had before the grazing treatment had started. In dismay, the managers looked for another option and decided upon flea beetles (Apthona spp.), which ended up being a much more successful choice, greatly reducing the footprint of leafy spurge over the next several years.
You’d be excused for thinking the use of sheep and goats was a total waste of effort, but additional data collected at Altamont Prairie adds some interesting nuance. As it happens, mean Floristic Quality (a kind of qualified plant diversity metric) stayed relatively stable within the grazed area during the five years sheep and goats were present. During the same time period, mean Floristic Quality decreased significantly in exclosures. In other words, while grazing didn’t eliminate the spurge problem, it may have stabilized some of its negative impacts for a while.
This, to me, is one of the best attributes of many grazing-for-weed-control efforts. Even if grazing can’t eradicate many weeds/shrubs from a prairie, it might be a strategy that prevents further spread (eliminating flowers and reducing vigor for belowground reproduction) and/or reduces the weed’s ability to compete with desirable plants. In a large site where more effective long-term strategies (such as selective herbicide application or biocontrol releases) aren’t feasible across the whole area, using grazing as a suppression tactic in some areas of the site while you kill it in others can make a lot of sense. In other words, grazing might buy you time to work on a problem that would otherwise seem overwhelming in scope. (However, it’s also important to remember that grazers will also be eating and suppressing the vigor and reproduction of desirable species with similar growth strategies to the invader you’re targeting. If you do succeed in reducing populations of invaders, you might also reduce populations of those desirable plants.)
Grazing can sometimes provide effective control of short-lived plants if it prevents flowering and seed production and forces plants to die without reproducing. Just remember that more seeds are likely waiting in the soil, so it will likely take repeated grazing treatments to reach your goal. Here in Nebraska, we often use short-term intensive grazing as a tool to knock back the competitive ability of perennial cool-season grasses such as smooth brome (Bromus inermis) or Kentucky bluegrass (Poa pratensis). We don’t expect the grazing to kill those grass plants (and it doesn’t) but we can allow other plants a chance to flourish for a few years until the invasive grasses regain their vigor. By repeating the treatment periodically, we can maintain a more diverse plant community.
Personally, I’ve never used goats or sheep to help with a management challenge. In contrast with cattle, goats and sheep, feed preferentially on forbs, and I’m usually trying to suppress grasses and encourage forb growth. However, I do think goats, sheep, and cattle can all play important roles in controlling invasives as long as you don’t expect them to do more than they can. I worry that landowners and land managers can sometimes end up paying an exorbitant price to someone that brings animals in with the promise of weed control. It’s important to remember that if you do that, you’re providing food for that contractor’s animals, and that should be factored into whatever price one of you pays the other. When we use cattle for prairie management, the cattle owner always pays us. That seems not to be the case with many goat grazing operations. I’m not saying it’s wrong to pay someone to graze their goats on your land, I’m just saying it’s important to fully process what each party is getting from the transaction. That includes the forage provided to the animals from your land, the time and expenses incurred by the owner of the animals, and – importantly – the actual effectiveness of the treatment.
As long as you have clear objectives and a good understanding of the plant(s) you’re targeting, grazing may be a great tool for invasive species control. Just remember one of the biggest lessons from the South Dakota spurge experiment: just because you can’t see the invasive plant anymore doesn’t mean it’s gone!
Note to my son (and others who mainly follow this blog to see if there are cool pictures or pictures of them): This is a pretty long and involved post – sorry. The first picture is probably the best one, though there are a couple other decent prairie photos further down (though none with you in them). The other pictures are more instructional than aesthetically pleasing. Don’t worry, I’ll put up some better photos later this week.
Great Plains grasslands are more resilient than most of us give them credit for. Just recently, for example, they survived the drought of 2012, the worst single year drought on record in many places (including at our Platte River Prairies). In a post I wrote during 2012, I compared that drought to similar conditions described by famous prairie ecologist J.E. Weaver back in the 1934. In both cases, prairies recovered nicely. In fact, in a 1944 paper, Weaver and collaborator Frederick Alberts provided a detailed summary of how prairies in Iowa, Nebraska, and east-central Kansas persevered and recovered from repeated drought conditions between 1933 and 1943.
Nebraska prairies have also shown resilience to intensive fire and grazing – historically by bison and currently by cattle. Chronic overgrazing, of course, can decrease plant diversity and cause cascading negative impacts on prairie animals and critical ecological processes. However, most prairies can easily withstand periodic bouts of intensive grazing followed by comparable rest periods. We’ve been tracking plant community responses to this kind of fire/cattle grazing management in our Platte River Prairies since 2002 and have seen plant diversity remain stable. Even grazing-sensitive plants such as rosinweed (Silphium integrifolium), Canada milkvetch (Astragalus canadensis), prairie clovers (Dalea sp.) and others have maintained their populations. Likewise, at our Niobrara Valley Preserve in the Nebraska Sandhills, prairies managed with more than 25 years of periodic fire and intensive bison grazing are still healthy, diverse, and full of wildlife.
Since coming to dominance in the Great Plains after the last ice age, prairies have survived and thrived through long droughts (some lasting multiple decades), severe fires, and intensive grazing episodes. Yes, we’ve managed to destroy and degrade many prairies through tillage, herbicide use, and chronic overgrazing, but those are departures from the kinds of stresses prairies have evolved with over time. The more time I spend in Nebraska prairies, the more important I think it is to run prairies through their paces now and then. That includes beating them up with periodic intensive grazing.
Not only can prairies rebound from droughts and intensive grazing, some aspects of prairies seem to depend upon those patterns of disturbance and recovery. As we’ve experimented with variations of patch-burn grazing over the years, I’ve observed a consistent response pattern from plant communities. We burn a patch of prairie, let cattle or bison graze it hard all season and then provide a different burn patch for the grazers the following year. In the year following fire and grazing, prairie patches get a weedy look to them because dominant grasses are weakened by the previous year’s grazing and there is open space for new plants to establish. Often, I see an increase of roughly 25-30% in the number of plant species found at the 1 m2 scale in the year after grazing.
Many of the plants that fill spaces left by weakened grasses are opportunistic (‘weedy’) species that grow, bloom, and produce copious amounts of seed within a year or two – and most of those plants disappear within a year or two as grasses reassert their dominance. In addition to those short-lived plants, however, I also see expansion of long-lived perennial plant populations that are taking advantage of weakened competition. Prairie clovers, leadplant, perennial sunflowers, and many others spread via both rhizome and seed during those periods when grasses are weakened. What I don’t see is the death or disappearance of plants following intensive grazing bouts that last a year or two. Even plants – both grasses and forbs – that are cropped close to the ground and kept that way by repeated grazing regain their vigor within a few years of rest.
Prairie animals also seem to benefit when prairies go through periods of severe stress and recovery. For example, many of the plant species that flourish following intensive grazing (and/or severe drought) are very attractive plants for pollinators. Annual sunflowers, hoary vervain (Verbena stricta), black-eyed Susans (Rudbeckia hirta), and biennial primroses (Oenothera sp.) are just a few examples. The prolific seed production by those opportunistic plants also provides a bonanza of food for insects and wildlife species.
Animals also benefit from habitat structure created by patterns of intensive grazing and recovery. Because every animal has its own individual habitat preferences, the highest diversity of animals are found in prairies with patches of intensively grazed vegetation, patches of recovering vegetation, and patches of tall/thatchy vegetation. Mobile animals can move to their favorite habitat structure and less mobile animals need only wait a few years for conditions that allow them to prosper.
Vegetation recovering from intensive grazing can provide particularly unique and valuable habitat. Habitat structure consisting of short, weakened grasses and tall ‘weedy’ forbs provides wonderful brood-rearing habitat for birds such as grouse, quail, and pheasants, for example. The low density of grass leaves and litter at ground level makes it easy for young birds (and other wildlife) to move around and feed beneath a canopy of protective cover. Coincidentally, insect abundance also skyrockets under those same conditions, which is good for insects and also for the wildlife that eats them.
What are we afraid of?
I’m more and more convinced of the importance of putting prairies through periods of stress and recovery, especially when those stresses are applied in a way that provides a shifting mosaic of stressed, recovering, and full strength vegetation patches across a prairie. No matter how hard we have grazed prairies, even during severe and extended drought periods, the plant communities have always bounced back during the recovery periods that follow. In addition, my own observations and data collected by many researchers have documented benefits to wildlife and invertebrates that come from the variety of habitats provided by this kind of management. (You can read more about patch-burn grazing here and the way I manage my family prairie here.)
While I’m confident that it’s valuable to beat prairies up now and then, I’m having a hard time convincing others to try it. Both ranchers and public land managers tend to have strong visceral reactions when I walk them through patches of really intensively grazed prairie. Ranchers are often convinced that I’ve killed the grasses and created weed and erosion problems that will never go away. Conservation area managers worry about potential weed invasions too, and some also wonder if sensitive wildflowers will survive the stress. Many of those land managers also flinch at the lack of habitat structure for wildlife species they care about.
Walking tour participants from an intensively grazed patch into a nearby area of recovering vegetation doesn’t usually help things. Ranchers tend to see the flush of opportunistic plants as validation of their fears about dead grass and weed problems, even when I next show them patches where the grasses have regained dominance after a few years of rest. Some wildlife managers recognize the value of the weedy vegetation as habitat, but worry about the perceptions of neighbors and the public who just see weeds, not habitat. Others see the weedy vegetation as a sign that the plant community has been degraded, despite the fact that the plant species they like better haven’t gone away.
To be clear, I’m not suggesting that all prairies would benefit from the kind of periodic intensive grazing I’m talking about here. Some prairies, for example, are simply too small and isolated. There are big logistical challenges associated with fencing and providing livestock water in small prairies. More importantly, there just isn’t space to graze some portions of small prairies while leaving other areas to rest and recover. (Small prairies have a number of other challenges that make any kind of management difficult. If you’re interested, I tried to address some of those in a blog post several years ago.)
I also have a number of lingering questions about how best to apply intensive grazing and recovery periods to prairies. For example, I strongly suspect that the best results come from intensive grazing bouts that last between a couple months and a full growing season. Grazing for only days or weeks doesn’t seem to stress the grasses as much as longer periods of repeated grazing on those plants. In addition, different grass species grow strongly at different times of year. Because of that, grazing for a short time can just shift dominance from the grasses growing strongly during the grazing bout to species that peak in their growth after grazers leave. Since my objective is to really weaken the entire grass community, I don’t think short grazing periods will do the job – but I’d like to test that more.
I’d also love to see more data on the responses of various vertebrate and invertebrate populations to intensive grazing, recovery periods, and the overall mix of habitat provided by the kind of management I think is important. There is strong research showing benefits to small mammal and bird communities, and some information on invertebrates, but only some of them. Learning more about the response of reptiles and a wider selection of invertebrates to this kind of grazing management would be really helpful.
In the meantime, however, I’m going to keep beating up my prairies. Not only do I think they can take it, I think the prairie communities I work with thrive best under that kind of management. (I will keep an eye on what happens, however, constantly vigilant for signs that I’ve gone a little too far.) I’ll also keep trying to convince others to stop babying their prairies so much. I’m hoping I can find a few ranchers willing to push pastures a little harder than they usually do – allowing longer rest periods to compensate, of course. That shouldn’t require changing stocking rates, but might provide some really nice benefits for pollinators and wildlife species. I know I’m going to continue to face skepticism from many corners, but I really think this concept needs to be explored further. Prairies have shown their resilience over thousands of years. I think we can trust that kind of track record.