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Why science doesn’t support rotational grazing

Ben Norton Published on 24 June 2015
Grazing for a two-day period

One of the more fascinating aspects of grazing management in recent decades has been the contrast between what scientists say about rotational grazing and what ranchers say.

The vast majority of research studies to measure the effects of rotational grazing have concluded there is no significant benefit to either livestock production or the pasture or rangeland.

Yet all over the world, more and more ranchers are implementing various rotational grazing strategies and claiming that they are able to carry more stock, make more money and maintain or improve ecological condition.

How can we explain these different points of view? The source of much of the disagreement lies in the background to modern grazing studies. They fail to reflect the reality of livestock grazing on a commercial ranch.

Read more insights from the author in this Q&A.

Clipping studies do not represent livestock grazing

Nearly 100 years ago, scientists in England did research that found an increase in forage production if livestock were rotated around a series of paddocks. However, the “grazing” was done not by animals but by a lawnmower, and the “paddocks” were small plots.

Subsequent research followed a similar approach. Grazing trials relied on clipping plots to simulate the way livestock harvest forage under different grazing schedules.

Invariably, continuous grazing was represented by clipping more frequently and more severely than in the treatments representing rotational grazing.

The Frenchman Andre Voisin is one of the leading lights in the history of grazing management. His 1959 book on grass productivity had a major influence on the thinking of rotational grazing gurus such as Allan Savory.

Voisin’s book is full of scientific information about rotational grazing, but he dismisses continuous grazing in three lines of text near the beginning of the book: “One can say without fear of contradiction,” he says, that under continuous grazing grass tillers are defoliated once every six days in spring and once every 12 days in August. That’s it – no argument.

Voisin supplies no data to support this characterization of continuous grazing; he just assumed that it was a bad thing because of the intensity of defoliation, just like the scientists doing clipping studies.

They did grazing research without involving a grazing animal, and their assumptions turned out to be false.

Frequency of defoliation is disconnected from grazing treatment

When scientists got around to looking at individual grass plants and tillers, beginning in the 1970s, they found that the impact from continuous grazing was not so bad after all, at least over one or two years. The damage arose from a patchy pattern of grazing.

The preferred patches experienced a grazing pressure that was much higher than the stocking rate calculated for the paddock as a whole.

And the patchy pattern was repeated with utilization concentrated in the same places from year to year. The problem is uneven grazing distribution being reinforced over time.

shortgrass prairie with 10-12 steers

The scientists who studied defoliation of individual grass plants and tillers also discovered, to everyone’s surprise, that the pattern of tiller defoliation didn’t match the pattern of grazing and rest periods in rotational grazing experiments.

In fact, if you looked only at the data on defoliation, it was impossible to deduce what the grazing schedule was. The treatment intended did not become a treatment in reality.

No wonder the scientists could not detect a difference between continuous grazing and rotational grazing.

This was true for rotational grazing using a relatively small number of paddocks, usually from four to 12, that most grazing research trials employed.

Patterns of defoliation will be closer to the rotation schedule when more than 20 paddocks are involved and livestock graze at high density for short periods with corresponding long rest-period intervals.

That’s what many ranchers do, and now scientists are beginning to use research designs that copy modern ranching practice. But the consensus from the scientific literature is biased by the weight of past reports of grazing research that employed a small number of paddocks.

It’s hard to shake that massive collection of results and its overwhelming conclusion that rotational grazing has little if any value.

Not that the conclusions are wrong; they are correct within the limited terms of reference of the studies themselves, namely, controlling stocking rate and attempting to control the timing of the grazing and rest periods.

The missing factor in research: Livestock distribution

But those terms of reference are missing a key factor: the distribution of grazing livestock. If scientists do not include livestock distribution as a parameter in their grazing research, it has little relevance to a commercial ranch.

There is a clue to the importance of animal distribution when you compare the stocking rates on research stations to the stocking rates recommended for ranches in the neighborhood.

There is a big difference. Grazing research publications often fail to mention the stocking rates recommended for nearby commercial ranches, but when they do, it appears that the research station can carry livestock at much higher stocking rates than the local ranches, up to twice as much, without any damage to the environment.

And this is true for the continuous grazing treatment as well as the rotationally grazed paddocks on the station.

Remember, the majority of research on rotational grazing has found no benefit to the rotation and no environmental damage over many years of the trial for either the continuous or rotation treatments.

This is a puzzle: How can the research station carry more animals than the ranch next door on a per-acre basis without damaging the pasture?

I believe the answer lies in the different sizes of paddocks. In the small paddocks on a research station, the livestock have no problem exploring the entire paddock. There is no animal distribution issue.

On the ranch, on the other hand, livestock cannot cover the area of a large paddock in one grazing day. They develop preferred areas within the paddock and track to those preferred patches instead of exploring new areas each day.

A pattern of patch grazing emerges that is driven by location of water, shade and fences, topography, preference for some vegetation types and behavior.

Livestock are creatures of habit, just like we are. In a supermarket, we forage in familiar places, in aisles that stock the food we like and are accustomed to.

Livestock return to those places in the paddock where they have grazed before and establish prominent trails in the process.

When rotational grazing is introduced to a commercial ranch, the existing paddocks are usually subdivided, creating smaller paddocks.

Some of these smaller paddocks will include areas that previously were neglected by livestock. Therefore, the forage is increased simply by exposure to feed that they used to ignore.

If the subdivisions are rotationally grazed, livestock will graze at a higher density than occurred in large paddocks, and they will tend to graze over the entire available area of the smaller paddock.

The change in livestock distribution due to paddock subdivision and rotation is the missing piece in the rotational grazing puzzle.

Stocking rate, timing and distribution

Scientists have studied stocking rate and the timing of rotations at length, but they fell off this two-legged stool with results that are not relevant to a commercial rancher.

When the third leg of the stool, livestock distribution, is incorporated into the design of rotational grazing, on a ranch as well as in research, the system is balanced in both theory and practice.

On this three-legged stool of stocking rate, timing of grazing in the rotation and livestock distribution, grazing research will reflect the reality of livestock management on a ranch.

Ranchers and scientists will then be on the same page asking and answering the same questions. This is something we can look forward to.  end mark

PHOTO: Portion of a mixed herd of 2,000 sheep and 300 head of cattle in a 32-acre paddock on a ranch in Australia. The livestock are nearing the end of a two-day grazing period and there is still plenty of forage remaining.

The rotation involves 60 paddocks, giving each paddock a rest period of 118 days and an average of six days of grazing per year. The stocking rate for the ranch as a whole is twice the level under the previous continuous grazing management, and the labor required to operate the ranch is one-third of what it was. Photo courtesy of Ben Norton.

Ben Norton
Emeritus Associate Professor
Utah State University

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