Regenerative grazing

In an impoverished savanna with lots of bare soil and less apt at nourishing wild and domestic herbivores the conclusion has always been that there must be too many animals (otherwise, why would there be less and less grass?) and the decision is then made to reduce those numbers, sometimes by culling in the case of wild animals. Unfortunately, this conclusion is based on a poor understanding of the processes at hand and usually results in an acceleration of the degradation. What is really happening? It is first important to understand that wild and domestic herbivores do not graze haphazardly. Except for when they are very young they bite off vegetation thousands of times a day for their entire life and doing so they accumulate an extraordinary amount of experience: they may not know the scientific names of the plants they eat but they know precisely the plants they like and the ones they don’t, those that are good for them and those that are not, those they should seek and those they should avoid and each bite they take is therefore the consequence of a choice. Let’s illustrate how this choice can be detrimental to the health of the savanna by considering the following scenario: it is the rainy season, grasses and other plants are in full growth but because savanna degradation was apparent the numbers of cows grazing in a given area have been significantly reduced. Two perennial grasses, plant A and B, are growing side by side. They are totally identical, same species, same age, same succulence. A cow takes a bite of plant A as she walks by, but as she is following the herd she does not have the time to take a bite of plant B. Plant A is thus diminished and will have to use up some of the energy stored in its roots to resume growth while plant B continues to grow without interruption. A week later another cow passes by again. It would be better for plant A if that cow now took a bite of plant B as it would give plant A more time to recover from the first grazing event and would not impact plant B that much since it had been spared the first time. Unfortunately, that is not what happens and the second cow is going to take a bite of plant A again because the new growth generated by the first bite is fresher, more succulent and more nutritious. As a result plant A must again use stored energy from its roots system before it had even had time to replenish them. In the mean time, the leaves on plant B have gotten a little bit older and less succulent and the cow ignores them. This scenario plays out again and again so long as their is enough moisture to allow the grasses to keep growing and plant A becomes overgrazed. Little by little it becomes exhausted and eventually, maybe during a bad drought year, it dies, leaving bare ground behind it.

In the mean time, what happens to plant B? In fact, it also suffers. Its leaves become less and less palatable to grazers as they become older and nothing eats it. It eventually flowers, goes to seed and its above ground parts dry out. The lack of moisture during the dry season that follows ensures that this dried out grass material does not decompose, it just stays in place and will hamper any new growth that will have to grow through it to reach the sunlight when the rains come again. This new growth may then be attractive to grazers, but because it is intermingled with the dead growth from the previous growing season it will be hard to reach and so will mostly be avoided. Plant B becomes undergrazed and year after year dead plant material will accumulate as new growth becomes old, leaving the plant unhealthy.

Now let’s push this reasoning a little bit further: if this scenario plays out with two plants that are identical at the start, how would it play out if plant A and plant B are of different species, with plant A being a favoured one?  It is easy to see that all other things being equal, overgrazed plant A will tend to disappear from the landscape over time, leaving more space for plant B which will tend to take over.  Overgrazing will thus not only eliminate the species that are the most desirable to the grazers, it will also change an area’s species composition.

The conclusion is that, even after having significantly reduced the number of herbivores in the area, and because herbivores select the plants that they eat, some plants will still be overgrazed while others will be undergrazed. If maintained over the years such a grazing system will lead to a degradation of the savanna, more plants will disappear with resulting increases in bare soil, reduced biodiversity and water infiltration and retention. These are well known processes of desertification.

What lesson can we learn from this example? It is not because there were too many cows that plants A and B were respectively over and undergrazed. Rather, it is because the few remaining cows were kept in the same area for too long, giving them the possibility to graze the new growth of the same plants again and again. Over and undergrazing are therefore not a function of the number of herbivores in a given area but a function of how long they are allowed to stay there.

Lets imagine two more scenarios to illustrate this further:

One cow on 1 hectare for 365 days:

As seen above this cow will consistently choose the new growth of grasses she has grazed previously as those are the most palatable, succulent, nutritious and digestible.  All the while, she will ignore plants that have not yet been grazed and have become older and less desirable. Just one cow, even on a large surface area, will always result in over- and undergrazing.

Now lets consider 365 cows on the same hectare for just 1 day followed by 364 days of rest:

The pressure on the pasture is the same as above, 365 cow-days spread over one year, but the effects on the grassland are very different. In such a scenario there can be no overgrazing because the grasses do not have time to grow and be grazed again in just one day. With so many cows concentrated on a relatively small area undergrazing will probably be non-existant as the competing cows will want to eat anything in front of them to get their fill, and even older plants from the previous season will tend to be either eaten or trampled down. This trampling, which is spread over the entire area, furthermore will help break down any crust that may have formed on top of bare soil patches, enhancing water infiltration and the germination of any seed that may lie dormant in the soil while waiting for the right conditions. To finish, the more homogeneous spreading out of the cows over the area also insures that their dung and urine are more evenly spread, fertilising the entire area. In short, while heavily impacted by this severe grazing event on the first day, the savanna is now ready to regenerate and will indeed look resplendent after the 364 days of rest.

So, what about regenerative grazing?

Neither over and undergrazing are thus function of the number of animals in a given area. Rather, they are a function of the time these animals stay there. Unfortunately, pastoralism as it is often practiced does not take this into consideration. Livestock is all too often allowed to graze at low densities over large areas for extended periods of time without any efforts to concentrate them. Under such continuous grazing systems livestock has the time to choose the plants they prefer and thereby exhaust them while leaving others untouched and choking.

It may be worthwhile to point out that if it is obvious that grazers need grasslands to survive, those grasslands also need those grazers to thrive. This is in fact a symbiosis that has been refined over millions of years of evolution: herds of wild herbivores have stayed concentrated to fight off predators and moved on as soon as the local grass resource was depleted and the accumulation of their dejections made an area unbearable to them. The savannas are rejuvenated by these short term concentrations followed by movements and, in fact, the scenario above where 365 cows are concentrated for just one day before being moved mimics this age old relationship. Another important implication is that total rest, like can occur when managers decide to remove all livestock for the creation of a national parc, may not benefit the savanna as expected.