Why Would Bison Have Done That??

I have an honest and earnest question:  What do we actually know about the movement and grazing patterns of historic bison herds in North America?

I’ve heard many similar versions of a story about the way in which historic bison herds moved across the North American prairie in pre-European days, but I’ve never been able to find someone who can substantiate it.  I’m not sure where the story (legend?) came from, but it seems to have become canon among many people in the grazing world.  It is particularly used by advocates of intensively managed grazing rotations, who say their grazing strategies mimic what bison did historically.

The most common variation of the story goes something like this: Back in the old days, bison herds were constantly on the move across the plains, grazing and stomping down the prairie as they went.  A bison herd would move into an area, stay briefly, and then move on – leaving behind short-cropped and trampled vegetation, which would get plenty of time to recover before another herd passed by.  The constant movement of bison was driven by humans, wolves, and other predators, who were constantly nipping around the edges of herds, picking off weak animals and keeping the herd moving.

I’m not saying the story is wrong, but I’m fairly skeptical.  Based on both observations and an impressive array of data from around the world, large grazing animals show a strong preference for recently burned and/or recently grazed vegetation.  The lush regrowth of grasses that have been recently burned or grazed is usually the easiest, tastiest, and most nutritious available, especially relative to undisturbed plants that are more mature.

This small area (a few hundred acres within a 12,000 acre pasture) of regrowth from a 2015 hay cutting became a predictable spot to find grazing bison during the remainder of that year and through 2016.  The animals visited the patch often, re-grazing many of the same plants over and over.  The Nature Conservancy’s Niobrara Valley Preserve.

In studies of patch-burn grazing, where cattle or bison are given access to both burned and unburned patches of vegetation, they graze spend something like 70% of their time grazing recently burned areas (and they stay on those areas most of the growing season), even when those burns make up a small percentage of the overall pasture.  The same pattern occurs in both small (a few hundred acres or less) and large pastures (10,000 acres or more).  These results are not coming from a few studies here and there; this is a large and comprehensive body of scientific research.  Bison and cattle are very good at optimizing their own diets, and when they are given the opportunity to regulate their own movements, without cross fences in the way, their optimal diet comes largely from recently burned and/or recently-grazed vegetation.

Given what we know about how bison and cattle (and many other large grazers) select their diets and grazing locations, how can that fit with the story I shared at the beginning of this post?  Why would bison have moved away from burned prairie into unburned/ungrazed grass that was more mature and less nutritious?  If they were forced to move from a particular place, why wouldn’t they have later circled back to graze the regrowth?  Many historic prairie burns, whether set by people or lightning, must have been of considerable size – probably plenty big to keep a herd of bison content for a full season, even if they were constantly bouncing around that burned area to stay ahead of predators.  No?  I just have a hard time imagining large areas of attractive, nutritious vegetation being sampled briefly by large herbivores and then abandoned and allowed to grow uninhibited for the remainder of the growing season.

I understand that much of our research on grazing patterns today is lacking the kind of predator pressure bison might have been under a few hundred years ago, and even a 10,000 acre pasture restricts the ability of bison to freely move across the landscape.  Given that, I’m open to the idea that movement patterns were different long ago than they are today.  Even so, I have a hard time imagining that bison wouldn’t have done everything they could to return to areas they knew contained high quality forage, even if they were being constantly pursued.  Can anyone provide some evidence to support that story?

In historic prairies, many patches of high-quality forage would have been transient (e.g., burned areas that were grazed for a season or so and then allowed to recover as bison switched to newer burns), but prairie dog towns might have represented a consistent supply of high-quality regrowth, and likely hosted fairly frequent visits from bison.

Even assuming (and I’m not) that bison herds were constantly on the move, never circling back to where they’d been earlier, it still seems unlikely that recently burned prairie would have been grazed for more than a few days and then just allowed to grow uninhibited for the remainder of the season.  Surely those burned areas (and later in the season, burned and previously grazed areas) would have been magnets for any other herd wandering nearby, and would have been grazed repeatedly (by a succession of nomadic herds) throughout the growing season?

Why does all of this matter?  In a way, it really doesn’t, other than for the sake of curiosity.  As I’ve discussed before, the way we design prairie management strategies should be based on today’s world, challenges, and objectives.  For example, it doesn’t make any sense to burn a prairie every three years just because we know that was the average historic fire frequency in that landscape.  The same is true with historic grazing patterns.  With increased levels of nitrogen deposition, woody plant encroachment, and habitat fragmentation, not to mention invasive species and climate change, today’s world is not the same as it was at whatever mystical point in history we might choose to try to replicate.  We have to manage today’s prairies in ways that make sense today.

Having said that, history does matter when it helps us understand how plant and animal species used to respond to the world around them, and what current adaptations and traits they have as a result.  That understanding can help us design and evaluate contemporary management strategies that fits with those adaptations.  It doesn’t mean we try to replicate history, it just means that we incorporate our understanding of it as we move forward.

Cattle seem to exhibit the same preferences for grass regrowth as bison.  These cattle are grazing in a burned patch in our Platte River Prairies this year.  That burned patch was grazed repeatedly and intensively all season while adjacent unburned areas were only lightly grazed.  The cattle were very selective, grazing mostly on their favorite grasses because of a moderate stocking rate.

If we’re going to incorporate a historic context into today’s strategies, however, we should make sure we’re as accurate as we can be about that history.  As I mentioned earlier, advocates of management-intensive grazing (and/or mob grazing, small cell grazing, and other similar strategies) mention the historic bison story often as a reason for managing cattle as they do today.  Because they contend that bison moved quickly across the historic landscape, never staying long in any one place, they propose that we manage cattle herds in that same way today.  I’m not saying those rapid rotation systems are right or wrong, I’m just wondering whether the historic context used to justify them is accurate.  I can’t understand why bison would have done what they’re supposed to have done, and would love to see evidence either way.

Even if bison were pushed off of areas they preferred, why wouldn’t they have returned later to take advantage of the nutritious forage?

I personally prefer the way prairies and wildlife respond to the kind of shifting mosaic approach we (mostly) employ on the sites I’m involved with, but that’s because I have a particular set of objectives, and I measure success based on those.  My personal guesses and extrapolations about how historic bison herds might have interacted with the landscape play a role in why I like our current approach, but they’re not really a major factor.  If I find out that historic bison acted very differently than I think they might have, I’ll appreciate that knowledge, but I’ll still evaluate our current strategies based on the objectives we have for today.  I will, though, pay closer attention to hints that species or natural communities might be showing stress due to exposure to conditions they’re not well adapted to.

Mostly, I’m just curious, and tired of listening to the same old story without knowing if it’s true.  Can anyone help me?

ADDENDUM:  Since writing this post a couple people directed me to this excellent article by Richard Hart that addresses my question fairly well.  Thanks to those of you who shared it!

A Skeptical Look at Mob Grazing

Mob grazing is attracting a lot of attention lately, especially among people who are fans of other intensive rotational grazing systems.  Usually, mob grazing is an extreme form of rotational grazing, in which high numbers of cattle are grazed in very small areas – for very short periods of time.  Often, cattle are given a new area to graze daily (or more frequently) and formerly grazed areas are allowed to rest for several months or more before being grazed again.  The intensity of grazing in individual paddocks varies by the rancher or grazier running the system.  In some cases, that intensity can be very high.  Proponents list off multiple benefits to the land from mob grazing, including increased soil organic matter, weed control, and “grass health”.

Mob grazing in central Nebraska. Cattle are just being moved from the paddock on the left to the one on the right. These cattle were being moved multiple times per day.

My purpose for this post is not to make any kind of final judgement on mob grazing, but to point out a few things that concern me from a prairie conservation perspective – and perhaps slow down the mob grazing bandwagon just a bit.  Those of you who have followed my blog for any length of time are aware that I’m generally a fan of using grazing as a tool for prairie management.  There is abundant data showing benefits of grazing to wildlife habitat and plant species diversity, both on my own sites and others.  I don’t advocate grazing for all prairies, but I do think prairie managers should look at grazing with an open mind, and consider how it might help them achieve specific objectives.

In the case of mob grazing, however, I’m very concerned about what I’ve seen in my (limited) personal experience, and even more concerned that I’ve been unable to find ANY published research on the topic.  I think there are good reasons to cautious before buying into anything supported only by testimonials, so I’m nervous about how strongly mob grazing is being promoted.  To be clear, I’m approaching this from a prairie conservation perspective, not a graziers perspective, so my thoughts should be taken in that context.

Impacts on Soil Organic Matter

Returning to the purported benefits of mob grazing, let’s look at soil organic matter first.  While there are various explanations of how mob grazing affects organic matter in the soil, the general idea seems to be that mob grazing cattle eat about 60 percent of the standing vegetation and stomp the remaining 40 percent into the soil.  Thus, soil organic matter increases and becomes more productive.  This has never jived with my understanding of soil organic matter (soil carbon) production, so I checked with four prominent scientists around the country who study soil nutrient cycling, including soil carbon.  When I asked them if the claims from mob grazing advocates made sense, their response was unanimous and strikingly blunt.  To quote one of them, “That’s totally bogus”.

In reality, soil organic matter is formed mainly by belowground processes, including root decomposition, root exudates, and mycorrhizal carbon inputs.  In prairies, a substantial percentage of plant roots are abandoned to decompose each year and replaced with new roots.  Those old roots provide organic matter in abundance, and more importantly, that organic matter becomes a stable part of the soil profile – and is added to and enhanced by the other two processes listed above.  My panel of experts said that stomping vegetation into the soil might provide a slight and temporary increase in organic matter near the soil surface, but that it would be unstable and wouldn’t last long.  It’s the stable supply of organic matter deeper in the soil profile that actually drives plant productivity, and that supply comes from plant roots themselves.  In fact, the experts suggested that the kind of vegetation stomping I asked them about was likely to have fairly negative consequences.  They thought that soil compaction and disruption of soil structure as a result from heavy trampling would probably decrease -not increase – plant productivity.  None of this means soil organic matter can’t increase under mob grazing, but any increase would be due to the same belowground processes listed above.

As an aside, I’ve heard some rotational grazing proponents talk about why fire is a bad thing in grasslands because it burns up vegetation that would otherwise be incorporated into the soil – thus, fire decreases organic matter in soil.  This is clearly not the case, and has been thoroughly dismissed by multiple researchers who have shown stable or increasing levels of soil carbon under frequent fire.

Some proponents of mob grazing say that this kind of heavy impact adds organic matter to the soil. Scientists who study soil and organic matter disagree, and suggest it’s likely doing more harm than good.  To be fair, not all mob grazing is this intensive.

Impacts on Weeds

A second purported benefit of mob grazing is weed control.  First, of course, we need to define what a “weed” is.  As has been discussed in this blog before, it’s a very subjective term.  Generally, there are two categories of plants that people consider to be weeds; opportunistic plants that take advantage of weakened dominant plants (e.g. ragweeds, annual grasses, and other short-lived rapidly-reproducing plants), and truly invasive species that are non-native to a particular ecosystem and become dominant to the expense of other species.  Let’s look at each of those two in the context of mob grazing.

If opportunistic plants are the weeds of concern, it seems unlikely that mob grazing would help suppress them.  Mob grazing proponents say that the high grazing intensity makes cattle eat – or stomp – all plants in the paddock, thus removing the weeds that cattle wouldn’t normally eat.  Unfortunately, while that might be true in the short-term, it’s the recovery from that grazing that’s more important.  Opportunistic plants are successful because they can recover from intense disturbances faster than others.  Big strong grasses are the biggest competitors to those “weeds”, and those grasses are greatly weakened by severe defoliation.  Until those grasses and other major perennials recover their dominance of the plant community again, opportunistic plants run rampant.  If the time until the next grazing bout allows those grasses to fully recover their vigor, those opportunistic plants will eventually fade – but only until the next grazing bout.  In other words pulses of intensive grazing will result in flushes in opportunistic plant abundance as well.  I would argue that most opportunistic plant species are non-threatening in any regard, but if suppressing them is an objective, the smart strategy is to strengthen the surrounding plant community.

In this pasture, mob grazing was being used as a tool for controlling musk thistle. The heavy grazing intensity did get the cattle to eat some (but not all) of the thistles. (Continued on next photo)


Although heavy grazing intensity can get cattle to eat at least some musk thistles, areas like this one that are a couple weeks into the recovery from that grazing tell the real story. The severe weakening of dominant grasses opened up space for opportunistic plants (like these numerous musk thistle rosettes) – and the grazing led to the exact opposite of the desired impact.  Fields of blooming thistles can be seen in the background of this photo where they’ve had sufficient time since grazing to reach maturity.

It is possible to reduce the abundance of opportunistic plants in pasture through periodic moderate grazing.  Mob grazing that moves animals through paddocks quickly enough that the lower leaves of grasses are left ungrazed, could actually stimulate the matrix of grass to thicken, due to increased growth of rhizomes and tillers (stems).  Higher density of grass that chokes out other plants might be seen as beneficial from a grazing standpoint if grass is the only thing the grazier wants – especially in tame grass pastures.  However, from a plant diversity and wildlife habitat standpoint in native praireis (the perspective I’m coming from) it’s certainly not a good thing.

In the case of truly invasive plant species, the story is a little more complicated because every invasive species has its own unique strategy for becoming dominant.  In most cases, the invasive plant has been released from pests and pathogens that suppressed it in its native habitat, and the plant species in the community being invaded have not had time to develop strategies to combat it.  In some cases, concentrating cattle grazing into a relatively small area can lead to the defoliation of an invasive species that would otherwise avoid being grazed.  If that ability to remain ungrazed while surrounding plants are weakened by grazing is the primary way that invasive plant gains dominance, that defoliation could reduce its spread.  However, in most cases, the story is much more complicated, and invasive plants use a more diverse mixture of advantages and strategies to force their way into plant communities.  Weakening the surrounding plant community through something like mob grazing is likely to increase the spread of invasive plants rather than decrease it.  I would use extreme caution when testing mob grazing as a tool for controlling invasive plants.


Impacts on “Grass Health”

When I first heard the claim that mob grazing increases grass health, my initial response was, “I didn’t know the grass was sick!”  It’s hard to glean from the various claims what the specific benefits to grass health are, or how that health is defined.  I also have a hard time understanding why mob grazing would provide any benefits to grass plants that other kinds of grazing systems don’t – as long as those other grazing systems include a mixture of grazing and rest periods.  As with all other plants, I think its important that grasses are allowed to flower and produce seed periodically, and mob grazing may do that (depending upon the length of the recovery period) – but many other grazing systems do the same, without some of the potential risks I see from mob grazing.  In some cases, I think grass health refers mostly to soil organic matter, which I addressed earlier.  Until I hear more specifics about how mob grazing affects grass health, I can’t really respond more.

Other Benefits – Livestock and Wildlife

I’m not sure how this system can be good for livestock performance – especially when paddocks are grazed very intensively.  Forcing a cow to eat plants it wouldn’t normally eat seems to override the cow’s effective inherent ability to optimize its own diet.  Why would it benefit a cow to eat plants – or plant parts – that are not the best available choices within a larger pasture?  I have the same concern with some other rotational systems, but this takes it to an extreme.  In order to gain weight, cattle test and refine their forage intake on a daily basis, constantly adjusting what they eat based on the phenology of the plants.  Under extreme mob grazing, cattle have to eat the least palatable plant species and plant parts along with the good stuff.  I don’t understand the logic of that strategy, and, in fact, even some proponents of mob grazing admit some “inconsistency” in livestock weight gains.  The only research project I know of that has started looking at weight gains and other aspects of mob grazing has found very poor livestock performance during its first season (2011).  Again, I’m not saying that cattle can’t gain weight in mob grazing systems, only that I think people should be cautious about accepting that claim.

An additional benefit promoted by mob grazing advocates is that the system increases the carrying capacity of pastures.  This is a tricky claim to evaluate, because it depends upon your definition of carrying capacity.  On the one hand, it’s surely possible to increase the number of cattle in a pasture, and claim that the carrying capacity of the pasture is now higher – though you can do the same with any grazing system.  On the other hand, a more formal range science definition of carrying capacity is “the maximum animal numbers which can graze each year on a given area of grassland for a specific number of days without inducing a downward trend in forage production, forage quality, or soil.”  In other words, carrying capacity isn’t just the number of cows you can put in a pasture, it’s the number of cows that doesn’t degrade that pasture over time.  This latter definition can only be evaluated by long-term data, which doesn’t currently exist for mob grazing systems.

From a wildlife perspective, it’s hard to say what the impacts of mob grazing would be.  Much depends upon the size of the grazing area, the intensity of grazing, and the length of recovery time.  Clearly, very intense grazing that stomps vegetation into the soil will have extremely negative impacts on any nesting birds or invertebrates in that immediate area.  On the other hand, the majority of the site is always in a recovery phase with no active grazing, so there should be a nice diverse mixture of habitat conditions available.  My guess is that mob grazing could be beneficial for many wildlife species – in terms of habitat structure – depending upon how it’s set up.

A bigger issue is that of plant diversity and overall ecological resilience.  While I think that many people overstate the potential negative impact of cattle grazing on “sensitive” prairie plants, including some rare wildflowers, the impacts from mob grazing on those plants could be a legitimate concern.  I think all prairie plants can put up with some degree of defoliation, even when it’s repeated multiple times over a season or two, but I think we would need some careful study of how intensive mob grazing impacts could affect prairie communities before introducing it as a potential management tool.  The potential soil impacts of more extreme versions of mob grazing are particularly concerning.  I’m sure historic prairies were exposed to high concentrations of bison grazing, but I have a very hard time believing that bison stuck around one place and grazed so intensively that they forced themselves to eat substandard forage.  Until I see some well-supported research on the recovery of plant communities, I’m not comfortable exposing native prairies to that kind of severe disturbance.

The Upshot

I’m not against grazing in prairies, and I’m not even against mob grazing per se.  There may be circumstances under which mob grazing, or some variation of it, could be used to achieve certain objectives.  In tame grass pastures, for example, where tilled land has been converted into forage grasses and the sole purpose of the site is to feed cattle, mob grazing might be worth a try.  In those kinds of pastures, the native plant and soil communies have already been severely altered, so out-of-the-box experiments have a relatively low risk of making things worse.  I still don’t buy most of the claims about the purported benefits to livestock, grasses, or soils, but as long as cattle producers test the system with eyes wide open, who am I to say they shouldn’t?

However, in native prairies and rangeland, I think the potential risks of the more extreme versions of mob grazing far outweigh any purported benefits, at least until there is some actual research that says otherwise.  We have abundant evidence that many aspects of native prairie plant and soil communities do not recover well from tillage, and mob grazing impacts can come uncomfortably close to those of tillage, in my opinion.  There are countless other options for using grazing – even intensive grazing – to suppress dominant grasses, control invasive species, create wildlife habitat structure, and achieve other objectives.  I strongly support active experimentation with grazing techniques that could help us with our numerous prairie conservation challenges, but with grazing, as with anything else, it IS possible to have too much of a good thing.


For those interested, here are two links to relevant research papers on soil carbon (organic matter) and fire/grazing, followed by three non-scientific reports on mob grazing.

Kitchen et al, 2009.  (Effects of fire on mowing on soil carbon and other factors.)

Johnson and Matchett, 2001.  (Effects of fire and grazing on belowground processes)

Glowing review of mob grazing

An even more glowing review of mob grazing

Mixed review of mob grazing