Grazing in Prairies – Part 1

This is the first in a series of upcoming posts about grazing in prairies – something that is common and widely accepted in western tallgrass prairie and mixed-grass prairies but much less so in eastern tallgrass prairies.  Sorry for the length – there’s a lot to say in an introduction to this topic…

Cattle and prairies are like oil and water to some prairie biologists and enthusiasts – especially in the eastern tallgrass prairie region.  There’s no doubt that many prairies have suffered from chronic overgrazing during the last couple of centuries.  But to broadly categorize grazing as bad for prairies because some have been overgrazed is like saying that food is bad because some people are obese.

It's easy to see how people can be sceptical about the value of grazing if this is their mental image of cows in prairies.

To be clear, I’m not arguing that every prairie needs grazing.  Some prairies are too small for grazing to make any sense.  In other cases, the owner and/or manager of a prairie may be completely satisfied with the plant composition and habitat availability within a prairie – in which case the current management is probably perfectly adequate.

On the other hand, I think there is room for much more discussion about some potential benefits of grazing in prairies.  However, in order for that discussion to proceed, everyone involved needs to come into the discussion with an open mind.  Despite arguments about the extent to which bison and other large grazers were abundant in eastern tallgrass prairies – and for how long – there is no question that those prairies have experienced some pretty extensive and intensive grazing at times over the last several thousand years.  And they’ve survived.  That doesn’t mean we can just dump a load of cattle into a prairie and everything will be peachy.  But I think it’s important not to summarily dismiss grazing as irrelevant or automatically dangerous to prairies.

One of the obstacles to open discussion about grazing in prairies is the assumption that if you’ve seen one grazed prairie you’ve seen them all.  In reality, grazing may be the most flexible management tool available – you can control the timing, duration, intensity, and frequency of application.   Stocking rate (grazing intensity) is probably the most important of those.  The number of cattle (or bison or whatever) per unit area makes a tremendous difference in the way the site is grazed.  Light stocking rates allow the grazer to be selective – eating only what it wants.  In my experience, and that of many other managers, cattle strongly prefer grass over forbs when given the choice.  When stocking rates are higher, or when cattle are kept in a prairie long enough that they run short of grass to eat, they eat whatever is available.  Many species of plants that have long been considered “ice cream” plants for cattle – plants that cattle will repeatedly graze to the ground until they eventually disappear – are treated much differently by cattle under a light stocking rate.  Because the variability in the way grazing can be applied is not often recognized by researchers or casual observers, the world is full of reports and articles that document differences between “grazed” and “ungrazed” prairies.  In most cases, the “grazed” prairies are chronically overgrazed – not a fair comparison.

Cattle can be very selective grazers, and under light stocking rates show strong preferences for grass over forbs - even prairie legumes like purple prairie clover that are usually considered to be "ice cream plants".

Another erroneous assumption is that a grazed plant is a dead plant.  Being defoliated (clipped off) now and then doesn’t kill prairie plants.  If it did, mid-season haying and growing season burns would be devastating to prairies – and they’re not. If a perennial plant is allowed time to recover from being nipped or mowed off, it’ll come back just fine – especially if the competing grasses surrounding it are also weakened by the same defoliation event.  Defoliation is only fatal for perennial plants when it occurs continuously, season after season, with no time for recovery.  Because of that, it’s easy – but incorrect – to look at a prairie undergoing short-term intensive grazing and assume that many of the prairie plants are gone because they’ve been grazed off.  A prairie that looks short and ragged while cattle are present can be tall and flowery again after just a season or two of rest.  It’s true that when grazing treatments (or any other prairie management treatment) are applied repetitively, some plant species can gain a repeated advantage over others, limiting plant diversity.  In contrast, however, a dynamic (non-repetitive) management regime allows all plant species to complete their life cycle now and then and ensure their continued position in the plant community.

As part of a broader management plan, there are a number of ways periodic grazing can potentially be a valuable tool.  For example, intensive grazing in the spring followed by summer rest can repeatedly defoliate and stress cool-season exotic grasses like smooth brome while providing opportunities for many native plants to gain an advantage.  Grazing cattle under a light stocking rate can be used to create unique and valuable habitat structure because the cattle clip off some plants but not others.  That selective grazing creates patchy vegetation that can help invertebrates and reptiles more easily thermoregulate and provide many animal species the ability to forage in open areas while remaining close to protective cover.  Under a higher stocking rate, cattle can create and maintain large patches of short vegetation that benefit bird species like upland sandpipers.  After that kind of intensive grazing, many plant species that were previously suppressed by dominant grasses can be temporarily released from that competition.

Selective grazing by cattle can help forbs compete against dominant grasses, but also creates valuble habitat structure that is difficult to obtain with other management tools.

One grazing strategy that has been getting a lot of recent attention is called patch-burn grazing.  In its most basic form, patch-burn grazing uses prescribed fire to concentrate grazing in some portions of a prairie while other portions get little to no grazing.  The system works because recently burned prairie patches are extremely attractive to cattle and those cattle will spend the majority of their time there – to the exclusion of less recently burned patches.  The intensity of grazing in burned patches and the extent to which unburned patches are grazed, both increase with stocking rate.  Whenever a new patch of prairie is burned, the grazing shifts to the new burn patch and the previous patch begins its recovery phase.  At any one time, a portion of the prairie is being intensively grazed, a portion is recovering from recent grazing, and other portions are largely ungrazed – all without any interior fences.

Researchers from Oklahoma State University and The Nature Conservancy have been testing patch-burn grazing as a way to improve wildlife habitat in ranch country by providing more  without  heterogeneous vegetation structure without sacrificing income for ranchers.  They and others have documented benefits to wildlife species such as prairie chickens and Henslow’s sparrows that require habitat structure not often found in more traditionally-grazed grassland.  They’ve also found that prairies under patch-burn grazing support a larger total diversity of wildlife species than more homogenously managed grasslands because of the wider range of available habitat types.

I began experimenting with patch-burn grazing in 2001 because I was looking for a way to maintain plant diversity in restored (reconstructed) prairies along the Platte River in Nebraska.  We had invested a lot of time and energy seeding 150-230 plant species into former crop fields and I needed a way to manage them that would prevent grasses from becoming too dominant and/or plant diversity declining over time.  I reported some preliminary results of our work in an article for the journal Ecological Restoration in 2005, but now have 9 years of data from our sites.  Overall, I’ve been very encouraged by the results I’ve seen.  Our restorations are maintaining their plant diversity and species composition – while exclosures that get prescribed fire but not grazing have lower plant diversity and are largely dominated by grasses. I’m also tracking individual plant species and have seen plant species traditionally thought to decrease in abundance under grazing maintain their abundance over 9 years of annual patch-burn grazing.

Cattle grazing in the burned patch of a patch-burn grazed restored prairie (light stocking rate) - note the ungrazed Canada milkvetch (bottom left) and compass plant (tall).

I continue to experiment with ways to modify the basic patch-burn grazing method to favor plant diversity, including the use of varying stocking rates and the length of the grazing season.  In addition, most of my sites are relatively small (between 100 and 200 acres in size) so plants that are especially tasty to cattle sometimes get grazed even in unburned areas.  It appears that a year of complete rest from grazing now and then is probably important (and sufficient) for maintaining strong populations of those species.

There is still much to learn about the potential usefulness of patch-burn grazing and other methods of cattle grazing for prairie management.  For example, most research projects have not been set up in ways that have provided long-term or intensive data on the impacts of grazing on individual plant species over time.  Konza prairie researchers have a long history research on grazing – largely on its effects on ecological processes, however (but with insights on plant species responses as well), and some people are concerned about how well their data on shallow-soil prairies applies to other prairie types.  While my own data show promising results regarding plant species responses, I’m looking primarily at restored prairies and my sites are on sandy soils with annual rainfall amounts between 25 and 28 inches per year.  I’m seeing cattle graze select very strongly for grasses, with very little grazing of forbs – especially with light stocking rates.  However, it’s difficult to extrapolate those results to prairies with different soil types and average precipitation without considerably more testing.

However, the questions about how or whether a particular grazing system or another impacts prairies miss a larger point.  The bigger question is whether or not grazing itself has potential as a tool for prairie managers to use to address specific challenges on prairies, including encroaching invasive species, over-dominance of native grasses or other plants, homogenous wildlife habitat structure, etc.  The answers to those questions will have to come from experimentation by many people at many sites.  I hope that prairie biologists, managers, and enthusiasts will continue their legacy of innovation and take an open-minded view toward prescribed grazing.  We have too few prairies left, and too many threats to them, not to explore every tool available to us.

(There are a number of other issues related to grazing that I don’t have room to discuss here – but will in later posts.  Those include differences between cattle and bison grazing, more details about my research on individual plant reponses to grazing, issues relating to cattle behaviors like trailing and concentrating around water and shade, the infrastructure needed for cattle grazing, and others.  Stay tuned – or ask specific questions by commenting below or sending me an email at chelzer@tnc.org.)

Measuring Success in Prairie Conservation – Species Composition vs. Structure and Process

Stick with me – this isn’t as complicated as the title might lead you to believe…

I was involved in an interesting discussion a couple weeks ago among some fellow prairie ecologists about what makes a “good prairie”.  The discussion brought into sharper focus something I had thought a lot about but only in general terms.  Here is the discussion – with some over-simplification of the respective positions:

Position A – Species Composition:

A high quality prairie (tallgrass prairie, in this case) can be judged largely by its plant species composition.  A “good” prairie might have 20 or 30 plant species per square meter, for example, and more than 100 species per acre – including a mixture of both common and rare species.   Besides the value for the direct conservation of species, that kind of plant diversity provides multiple ecological benefits.  More plant species means more choices for animal species that rely on them as food and/or habitat resources, as well as a more consistent supply of those resources through the season – because some plants will always be emerging/blooming as others go dormant.  Because of that, prairies with lots of plant species tend to have lots of insect and other small animal species as well.  In addition, prairies gain resilience from plant species diversity, because if multiple species fill similar ecological roles the prairie community can better withstand the temporary decline of some species due to drought or pest outbreak.   A prairie that is missing many of its species, or that is dominated by a few species with only scattered small populations of others, can’t be considered to be of high quality or to be “conserved.”

A diverse prairie at the Madison Arboretum – Madison, Wisconsin.

Position B – Structure and Process:

What really defines successful grassland conservation is the presence of large-scale and intense disturbances (e.g. fire and grazing).  The combination of fire and grazing shaped historic prairies and that combination is needed today to maintain them.  Without fire, prairies lose integrity in several ways – the most obvious being the encroachment of trees that fire otherwise suppresses.   Fire also helps drive the cycling of nutrients and regulates the amount of standing dead vegetation and thatch in prairies.  Furthermore, the high-quality of the fresh vegetative regrowth following a fire attracts intense grazing by herbivores large and small.  Historically, there would have been few cases where a prairie would burn and not be intensively grazed right afterwards.  That intense grazing suppresses dominant prairie grasses, opening up space for the abundant growth of weedy vegetation once the grazers move on.  As the prairie recovers, the dominant grasses reassert themselves and the vegetation becomes tall and dense enough to carry fire once more.  A landscape consisting of a heterogeneous mixture of recently burned patches and patches that haven’t burned for several years provides the full range of habitat structure – from very short to very tall – and thus supports the full range of prairie wildlife species.  Perhaps most valuable in that range of habitat structure is the post-fire/grazing recovery phase that provides simultaneously provides a wealth of stemmy vegetation cover and abundant seeds for wildlife food.

An expansive prairie landscape in the Nebraska sandhills.

So is Position A or Position  B correct?  Well, yes.  It’s like owning a sports car.  You need to keep all the sparkplugs, tires, and other parts – and keep them in good condition.  On the other hand, a sports car is no good if you can’t drive a standard transmission and/or don’t have good roads to drive on.  Aldo Leopold said, “To keep every cog and wheel is the first rule of intelligent tinkering” – but those important components include both species and the processes that maintain them.

Interestingly, the two positions seem to be strongly correlated with geography.  People who work with the fragmented tallgrass prairies in Midwestern States like Illinois, Wisconsin, and Indiana tend to fall more heavily in the first camp – emphasizing species composition.  In contrast, people who work in the large expanses of grassland in Oklahoma, Kansas, western Nebraska, and the Dakotas, tend to hold Position B – emphasizing process and structure.  In both cases, the positions are reasonable based on the local situation.  In the eastern tallgrass prairie, most of the prairie is gone, so any remaining grassland is precious and there is great concern about the loss of prairie plant and insect species.  Further west, where grasslands still dominate the landscape, there is much less concern about the loss of plant and insect species and more focus on larger wildlife like prairie chickens that require a range of habitat conditions not always found in those ranching-dominated landscapes.

As usually happens in a discussion among reasonable people, a partial consensus emerged in ours, and there was general agreement that neither Position A or B was sufficient by itself.  Proponents of species composition as the measure of prairie conservation success would surely not be fully satisfied with a flowery prairie that was missing species like prairie chickens, upland sandpipers, and bison.  They just don’t often have the opportunity to work with grasslands large enough to support all of those species.  Likewise, proponents of structure and process wouldn’t be happy with 20,000 acres of switchgrass just because it had a heterogeneous mix of fire and bison grazing and lots of prairie chickens.

The point here is not that we need to subscribe fully to either Position A or Position B, but that we can’t afford to ignore either one.  Those working in the fragmented eastern tallgrass prairie need to be sure to emphasize strategies like prairie restoration that can strategically convert crop fields back to prairie vegetation and enlarge remnant prairies to the point where they have a chance of supporting prairie chickens and upland sandpipers, if not bison.  And even at smaller scales, finding creative ways to reintroduce the combination of fire and grazing, where possible, may help provide better wildlife habitat – and might even pay dividends for plant species conservation (more discussion on this in posts to come.)

Meanwhile, ecologists with the luxury of large unplowed expanses of native grassland need not to forget the importance of restoring and/or maintaining both large-scale and small-scale plant diversity.  While adequate habitat structure for species like prairie chickens can be created in a landscape dominated by grasses and weedy forbs, pollinators and many other insects may have a much more difficult time surviving there.  In addition, whether the intervening landscape is dominated by grass or corn, small isolated populations of prairie forbs (and the insects that rely on them) aren’t likely to survive forever if they’re not able to cross pollinate or otherwise interact with each other.  Native bees that have to find consistent sources of nectar within a small radius from their nest rely on small-scale plant diversity to provide abundant blooms every day of the growing season – and pollination by those bees is critically important for the survival of many plant species.  Finally, proponents of process should recognize and appreciate the potential (but understudied) values associated with a diverse plant community – including a diverse and vigorous soil fauna, and the overall resilience offered by a mix of species that provides redundancy of ecological function.

The danger for all of us is that we tend to look at prairie conservation through a cultural lens – and we sometimes don’t see what our prairies, and our strategies, are missing.  It would be great if we could facilitate some cultural exchanges, in which we sent Texas cowboys to the Illinois black soil prairies and Wisconsin prairie restoration experts to the flint hills of Kansas.  Imagine the discussions that would ensue – not to mention the neighborhood coffee shop gossip.

Participants in the Grassland Restoration Network discussing prairie conservation at The Nature Conservancy’s Nachusa Grasslands, Illinois.

I feel fortunate to be involved in two groups that do a great job of stimulating interaction and discussion – the Grassland Restoration Network and the Patch-Burn Grazing Working Group.  The former is a loose affiliation of ecologists working to use prairie restoration as a tool for grassland conservation in fragmented landscapes.  The latter is a network of scientists, land managers, and ranchers trying to find better ways to combine fire and grazing and create heterogenous prairie landscapes.  (I’ll provide more information on both of those groups in future blog posts.)  Now if I can just figure out how to convince the two groups to hold a joint meeting…