Tag Archives: cattle

A Skeptical Look at Mob Grazing

Posted on by
89

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

Patch-Burn Grazing in Missouri Prairies

Posted on by
10

If you read this blog frequently, you know I manage many of our prairies with combinations of prescribed fire and grazing.  I like the heterogeneous habitat structure I get from patch-burn grazing, and have documented benefits to plant diversity in our prairies.  (I’ve summarized the experiences I’ve had with multiple variations of patch-burn grazing here.)

Patch-burn grazing with cattle is still viewed with skepticism by many people – especially some in eastern tallgrass prairies.  I can understand why people would be concerned about the potential impacts of cattle grazing on some plant species and prairie communities, and I certainly don’t advocate cattle grazing for all prairies.  However, I also think that many common concerns stem from limited experience with cattle grazing.  If the only cattle grazing I’d ever seen was the kind that annually beat grasslands down to the ground and resulted in soil erosion and a gradual loss of native plant diversity I’d be skeptical too – to say the least! 

When you see a prairie like this, it's easy to see how prairie enthusiasts could be nervous about cattle grazing. This Nebraska prairie has never been plowed, but it's missing many prairie plant species - though that is likely due more to past herbicide practices than cattle grazing.

However, chronic overgrazing is one extreme in a broad spectrum of grazing regimes, and cattle can also be used in ways that produce very positive results for plant diversity and wildlife habitat.  The first time I saw a prairie being stomped and chomped by lots of cattle it was pretty unsettling.  However, watching that prairie recover the next year after cattle had been removed gave me a much greater respect for prairies than I’d had before.  Over the last 20 years, I’ve seen that process over and over in many tallgrass and mixed-grass prairies, and the resilience of prairie plants never ceases to amaze me.  Of course, I’ve also seen instances where repeated overgrazing has degraded prairie communities, but that degradation has usually come from not giving plants sufficient opportunity to rest and recover from grazing bouts – not from grazing per se.  (And often because of a history of broadcast herbicide use as well.)

Most of my personal experience with grazing (and patch-burn grazing in particular) has come from mixed-grass and lowland tallgrass prairies in east-central Nebraska.  I’ve also seen a lot of grazing on western tallgrass prairies in Kansas and Oklahoma.  However, my experience with cattle grazing in eastern tallgrass prairies is much more limited – mostly because it is such a rarity.  This summer has given me two chances to observe the impacts of patch-burn grazing on eastern prairies in Indiana and Missouri.  I wrote briefly about the Indiana experience in a previous post, but I want to spend more time on what I saw in Missouri last week.

The Missouri Department of Conservation (MDC) began a research project on the use of patch-burn grazing on public lands prairies back in 2005.  One of their hopes was to increase the habitat quality of those grasslands for prairie chickens and many other grassland species without hurting the diversity or quality of the plant communities.  Many of the prairies they grazed during the project were considered to be some of the higher-quality prairies in Missouri (botanically speaking) so protecting the diversity of those plant communities – and the rare and conservative plant species in them – was extremely important.  Several people, including me, with prior experience using patch-burn grazing provided input to MDC as they designed the project.  During the summer of 2007, I finally got the chance to see some of the grazed prairies during the third year of the research project.  One of those prairies was Taberville Prairie, north of Eldorado Springs.

The burned patch of Taberville Prairie in 2007 (part of the MDC's patch-burn grazing research project). The grazing was pretty intensive in the burned patch, with only a few plant species remaining ungrazed. Even I was a little unsettled by the way the prairie looked. (Sorry about the photo quality, the tour was during the middle of a bright sunny day...)

I remember being a little shocked as I walked around Taberville Prairie back in September 2007, because the cattle had grazed it much harder than I’d expected to see.  The most recently burned patches of the prairie were nearly universally cropped close to the ground, with only a few plant species remaining lightly grazed or ungrazed.  That was pretty different from my own sites, where our relatively light stocking rates lead cattle to graze pretty selectively in burned areas, leaving many forb species ungrazed – even many that are typically considered to be favorites of cattle.  At Taberville, even unburned portions of the prairie showed evidence of moderate grazing, and it was difficult to find conservative plant species such as compass plant, purple coneflower, blazing star, and leadplant.  What I was seeing at Taberville made me wonder whether MDC had pushed the prairie a little further than was prudent.  Of course, the plan was to rest the prairie for several years following the three years of patch-burn grazing, so logic and experience told me this was something the prairie could easily recover from – but even so, I’ll admit it was a little disturbing to see.

This photo shows the patch that was burned in 2006 (the year before the photo was taken). Though grazed less intensively than the 2007 burn patch, there are still few conservative plants visible.

Since my 2007 trip there has been considerable discussion (to put it mildly) among prairie enthusiasts and biologists in Missouri about the impacts of cattle grazing in those prairies where patch-burn grazing was tested, especially on conservative plant species.  I can easily understand why people were concerned – especially after my own experience at Taberville.  I was anxious to see for myself how the prairies had recovered, so I was glad to accept an invitation from MDC to participate in a grassland ecology workshop last week.  The day before the workshop started, I got a tour from Len Gilmore and Matt Hill of MDC, and made my return to Taberville prairie.

MDC's Len Gilmore manages Taberville Prairie. In this photo, he's showing me the kind of vegetation structure favored by nesting prairie chickens. This is a portion of the prairie currently open to cattle - but is not the most recently burned patch.

We started the tour in a portion of Taberville than had not been included in the grazing back in 2005-2007, but that was currently in year three of a patch-burn grazing rotation.  Len, who manages Taberville Prairie, showed me the kinds of habitat structure they’re trying to create with patch-burn grazing, including nesting habitat for prairie chickens.  We also discussed other aspects of patch-burn grazing MDC is concerned about (and testing) – including potential impacts to headwater streams, most of which are currently fenced out.  The overall look of the prairies under patch-burn grazing this year was similar to those I saw in 2007.  This time, however, I looked harder for conservative plants, and was able to find them in the patches that weren’t the most recently burned.  Most weren’t blooming, but they were certainly alive and well.

This is the most recently-burned patch in the current grazing area at Taberville Prairie. The grazing was obviously very intense (the cattle had been removed the week before my visit).

What I really wanted to see, however, were the portions of the prairie I’d seen in 2007 that had been rested (with one burn) since I’d last seen them.  When we arrived, I think I let out an audible sigh of relief.  The prairie looked great.  Even in what was a very dry summer, the prairie looked like my visual image of Missouri tallgrass prairie.  Lots of showy blazing star flowers and abundant conservative plants, including leadplant, compass plant, purple coneflower, rattlesnake master, and others.  Len took me to several locations where they had built grazing exclosures during the original patch-burn grazing research project.  The exclosures had allowed MDC researchers to compare the ungrazed plant community inside the exclosures to adjacent plots that were exposed to cattle grazing.  Even without seeing the data, being able to walk through and compare those areas that had never been grazed with those that had been exposed to three years of patch-burn grazing (the exclosures had been removed but their locations were still marked) was a powerful testament to prairie resilience.  I looked hard for differences, but the truth is, if Len hadn’t told me which areas had been the grazed areas and which had been the exclosures, I never would have known. 

This is the portion of Taberville prairie shown in my 2007 photo above when there was almost nothing taller than a couple inches. Several years of rest (and a fire) following the three years of patch-burn grazing changed the look of the prairie considerably. Even in a summer during which the prairie had almost no rain in June or July, the plant community looks tall and vibrant.

.

The same portion of Taberville Prairie. Compass plant, and many other conservative plant species were abundant.

.

Another photo from the same portion of Taberville Prairie (grazed 2005-2007, but rested since then.) Eastern gamagrass (the thick leafy grass with tall stems) was abundant and full of vigor across the prairie. In the burned/grazed patches of Taberville and other MDC prairies gamagrass was being grazed extremely hard - even to the point where I could see rhizomes the cattle had pulled out of the ground. Clearly, gamagrass recovers well from that kind of treatment...

.

A final photo from the area grazed 2005-2007 but rested (and burned once) since. This photo shows leadplant in the foreground. Rattlesnake master and purple coneflower (two other conservative plants) were also abundant, but not shown in these photos.

During the next several days at the grassland workshop, I listened to MDC biologists from wildlife and fisheries divisions talk about what they like and don’t like about their experiences so far with patch-burn grazing.  One of the interesting issues they (and I) are wrestling with has to do with the appropriate length of grazing and rest periods.  Figuring out how to mix grazing and rest periods in a way that allows all plant and animal species to “win” periodically is a major challenge.  There was also considerable discussion about how to better evaluate potential impacts to plant communities and streams – as well as exploration of ideas about how to modify current management to better address needs of pollinators, amphibians, and other species.  I think those who are worried about patch-burn grazing in Missouri would have been comforted to hear the thoughtful discussion and see the obvious dedication of MDC staff to the prairies in their charge. 

This bush katydid was one of many insects I saw at Taberville Prairie. I don't think this species is necessarily rare or conservative, but the regal fritillary and Henslow's sparrow I saw nearby are (not that seeing one of each necessarily determines success...)

There are still plenty of important questions about whether, where, and how cattle grazing should be used to manage eastern tallgrass prairies, but the Missouri Department of Conservation is leading the effort to answer some of those.  Early results show improvements in habitat structure for many species of insects and animals, including greater prairie chickens – where they occur.  MDC has asked faculty from two universities to help evaluate impacts on streams, and is fencing out the majority of headwater streams until that evaluation is complete.   The responses of plant species and communities to various fire, grazing, and rest treatments is still being evaluated, and probably will be for some time.  In the meantime, it was good to see confirmation of the ability of plants to bounce back from periodic grazing, even in prairies that are pretty different from the ones I know best.  I think the knowledge that plants (even conservative species) don’t immediately die from being grazed for a season or two gives us a little cushion as we forge ahead with our attempts to find appropriate tools and strategies for maintain the broad array of biological diversity in what remains of tallgrass prairie.  If you live and/or work in the tallgrass prairie region, I hope you’ll be a productive part of that effort.  We need all the help we can get.