Maximizing Soil Carbon Sequestration: Carbon Farming and Rotational Grazing

Amongst scientists, there is widespread, probably universal, agreement that agricultural soils can sequester carbon in much the same way that tree cover can, and that the potential for them to do so in some circumstances is not negligible. Beyond that there is uncertainty and dispute. For every scientific paper showing that a land use change such as converting pasture to woodland, using minimum tillage on cropland, or excluding livestock from pasture increases the amount of carbon sequestered in the soil, there is another showing that, in other circumstances, the effect is the opposite. Moreover an increase in soil carbon on one site, for example by converting it from arable to pasture, may result in a decrease on another where pasture is converted to arable. An increase in soil carbon may also result in the release of other greenhouse gases: pasture sequestrates carbon, but grazing animals release methane. Similarly, adding nitrogen, by planting legumes for example, increases vegetation and hence the amount of carbon which the soil can potentially assimilate; but it also releases nitrous oxide into the atmosphere.

Within the scientific mainstream there is little consensus as to whether soil carbon sequestration can only have a minor impact upon our overall greenhouse gas emissions, or whether it has the potential to solve all our problems. One scientific paper, authored by nine scientists, states that “the IPCC estimates for the global mitigation potential of carbon sequestration in agricultural soils are 0.4 to 0.6 billion tonnes per year (over 100 years) — which is less than 10 percent of our current annual carbon emissions from fossil fuels … From this perspective, soil carbon sequestration can make only modest contributions to the overall need for mitigation of atmospheric CO₂ build-up.” Yet a year later, one of the nine authors, Dr Rattan Lal — who is the world’s No. 1 guru on soil carbon, and frequently cited by the IPCC — stated that “the maximum potential rate of Soil Organic Carbon (SOC) sequestration of three billion tonnes of carbon per year is high enough to almost nullify the annual increase in atmospheric concentration of CO₂ at 3.4 billion tonnes per year.” 

Rotational Grazing

Graham Harvey speaks for a different breed of carbon farmers who see livestock as a proactive way of improving soil carbon sequestration. For these farmers, a herd of grazing animals is not a voracious destroyer of soil and landscape, but an indispensable element of an integrated semi-natural environment. Enhancement of the carbon carrying-capacity of the soil is inseparable from the enhancement of both the plant and the animal carrying capacity of the land. Insofar as these elements are all seen as symbiotic, livestock carbon farming represents one possible permaculture approach to carbon sequestration.

This movement can be traced back to a single book, written in the 1950s with the unprepossessing title Grass Productivity. Though ostensibly dry and scientific, this is no textbook. The author, André Voisin, was a French biochemist and small farmer, who, the blurb tells us, “was known to spend hours watching the cows graze on his farm in Normandy. It was then that he realized that simple observation of the cow at grass could teach more about ecological relationships than the most sophisticated research of the time.” One attraction of Voisin’s book is that he goes some way toward explaining how to “think like a cow” with less of the anthropomorphism that pervades some other attempts to unveil the secret thoughts of bovines. But the reason why the book has a cult following is that it is the first attempt to analyse in detail the principles behind what Voisin called “rational grazing,” and has since come to be known as “rotational grazing,” “pulse grazing,” “mob grazing,” “short duration grazing” or even “controlled overgrazing.”

Pulse grazing involves carefully rotating livestock around paddocks so that they eat grass at the optimum moment in its growth, and for the optimum period of time. If they are left for more than two or three days in the same paddock, they will return to the favoured plants that were eaten on day one and have now begun to grow back to take a second bite, while ignoring the less palatable grasses, resulting in excessive grazing pressure on the higher-quality grass, and inefficient use of the lesser. This is hardly a radical observation, but Voisin has a keen eye for detail and he goes on to propose some sophisticated methods of managing and fencing land to ensure “rational grazing”.

Voisin’s book was largely ignored until it was picked up in the 1970s by a biologist, Allan Savory, who was working with buffalo, elephant and other big game in what was then Rhodesia, and was dismayed that cattle were “overgrazing my beloved Africa to death.” Savory was approached for help by some cattle ranchers who themselves were becoming worried by the deterioration in their rangelands:

Although my antagonism to cattle was well known, what I was saying publicly at the time made sense to them … In tackling this new challenge to manage cattle and wildlife together while improving the land I dusted off my copy of Grass Productivity and was astounded to find that Voisin had already solved the riddle of time. He had proven that overgrazing had little relationship to the number of animals but rather to the time plants were exposed to the animals, the time of exposure being determined by the growth rate of plants. If animals remained in any place for too long, or returned to it too soon, they overgrazed certain plants. Suddenly I could see how trampling also could be either good or bad. Time determined that too. The disturbance needed for the health of the soil became an evil if prolonged too much or repeated too soon.

Savory surmised that the ecology of natural rangelands had evolved to favor rotational grazing. In wide open grassland, large herbivores herd together for safety from predators who escalate the numbers of game by hunting in packs. Wildebeest and zebra in Africa do not amble across the savannah in family groups sampling the vegetation, picking out the choicest morsels. They move like an army upon one location kicking up dust and spewing out methane, and graze close to the ground before moving on. It sounds like a scorched earth policy, but the volume of dung deposited on the areas they have grazed deters them from returning until the grass has recovered. Their migration patterns are as much a carefully timed rotation as Voisin’s tidy fences, while their hooves break up soil crusts and trample dead vegetation and seed into the aerated surface, like a rotary harrow. By contrast, a herd of sheep or cows left to their own devices in a fenced field with sufficient grass for a season and no fear of predators, will spread out, graze in a haphazard fashion, and scatter their dung here, there and everywhere.

Migratory mob-stocking works for wildebeest (as the rise in their numbers in Serengeti demonstrates); could it work for cattle? Savory began to put his theories into practice in Zimbabwe but his outspoken opposition to Ian Smith’s regime forced him to leave. He moved to the United States in 1978, set up a Centre for Holistic Management in Albuquerque, and began to spread the gospel that “western rangelands are understocked and overgrazed.”

Savory collected his observations into a lengthy volume now entitled Holistic Management, which, though it never mentions the word “permaculture” could justly be subtitled “a permaculture approach to rangeland management.” As his message has spread, it has raised a storm of controversy between various factions in the western ranges. Conservationists from the Bureau of Land Management and the Sierra Club have long argued that there are far too many cattle on the public and private lands of the West; and a good many agree with these sentiments.

Not any more, replies the new breed of holistic cowboy. Cattle have been mismanaged, not overstocked. Ever since the Dust Bowl years there has been a widespread view that extensive grazing lands have been overgrazed, and this perception has led the Bureau of Land Management in the USA to reduce grazing on 124 million acres of public lands from about 22 million authorized livestock units in 1941 to 12.5 million in 2008 (though not all these permits are used and in 2008 the actual stocking level was 8.6 million). There were once 60 million bison on the Great Plains, now there are an estimated 60 million cattle on the USA’s 336 million hectares of grazing land.

This excerpt has been reprinted with permission from Meat: A Benign Extravagance, published by Chelsea Green, 2010.