Grazing can reduce greenhouse gas emissions

Apr 30, 2010

In a fascinating new twist in the debate over the environmental impact of livestock production, an international team of scientists has recently published research that suggests grazing under certain conditions can significantly reduce the amount of nitrous oxide, a greenhouse gas, released into the atmosphere. Published in the April edition of the peer-reviewed scientific journal Nature, the study documents the difference in nitrous oxide (N20) emissions from the soils in grazed and ungrazed rangeland in Inner Mongolia, China, a region characterized by cold winters and a temperate growing season.

The study found that the tall grasses on ungrazed rangelands trapped an insulating layer of snow that kept soil temperatures significantly warmer than on grazed rangelands, where shorter grazed grasses allowed for snow to be blown away, causing the ground to be exposed. The warmer soil temperatures characteristic of ungrazed range made it possible for the microbes which produce nitrous oxide to remain active through the winter, resulting in a significant burst of nitrous oxide release during spring thaw. Further, spring snow melt on ungrazed range provides water which is necessary for microbial activity. By contrast, active microbial levels in grazed rangeland soils are much lower due to lower winter temperatures and less available water, resulting in significantly less nitrous oxide emissions in the spring.

Nitrous oxide, commonly known as laughing gas, is the third most abundant greenhouse gas, with only carbon dioxide (CO2) and methane being more common. Yet N20 is the most potent of the three, having 300 times the heat-trapping potential of CO2.

Klaus Butterbach-Bahl of Karlsruhe Institute of Technology, one of the authors of the study, is careful to point out that livestock production has long been known to cause some N20 emissions through the decomposition of manure and urine.

"However," he adds, "what has not been considered is that grazing can reduce natural background N20 emissions. This means that steppe or prairie soils always emit some N20 (with the most important period being the spring-thaw period) and that these emissions—at least in semi-arid cool temperate climate regions—decrease if areas are grazed."

These findings challenge the commonly accepted notion that all livestock grazing will necessarily result in a net increase of N20 emissions. Explains Butterbach-Bahl, "It’s been generally assumed that if you increase livestock numbers, you get a rise in emissions of nitrous oxide. This is not the case."

What is also surprising is that cattle on range actually suppress far more N20 than they produce. Butterbach-Bahl writes that not only are the N20 emissions caused by livestock "more than offset" by the N20 emissions that grazing suppresses, but further, that "[t]he effect of grazing on N20 emissions from steppe/prairie soils in semi-arid cool temperate climate regions is huge ... [W]e estimated that the emission strength for N20 from livestock in these regions may be overestimated by 72 percent."

Butterbach-Bahl cautions that this research does not take into account livestock emissions of methane and carbon dioxide, but only shows that under certain conditions, livestock grazing results in a reduction of N20 emissions.

Given the recent negative press the livestock industry has weathered regarding its estimated contribution to greenhouse gas release, it will be interesting to see whether these findings will prompt a rethinking of the extent to which cattle production can be targeted as a major greenhouse gas producer. Dr. Frank Mitloehner, associate professor of animal science and air quality specialist at University of California-Davis, is hopeful that it will. Mitloehner himself is engaged in research which aims to rectify misconceptions and inaccuracies in the understanding of the environmental impact of raising livestock.

Mitloehner’s recent research takes issue with the highly damaging and controversial U.N. special report titled Livestock’s Long Shadow, which states that the livestock industry produces 18 percent of man-made greenhouse gases, more than the transportation industry. Among other criticisms, Mitloehner has pointed out that an accurate study must look at livestock production regionally, as industrialized countries like the U.S. are able to produce meat products with far less environmental impact than developing countries. Says Mitloehner, "Indeed, the United States and the developed countries are role models for the world with respect to environmental impact through unit of production. That means we produce a given amount of meat and milk and eggs with the fewest animals possible ... because our animals are very efficient."

According to the Environmental Protection Agency, only 3 percent of greenhouse gas emissions produced in the U.S. are caused by the whole livestock sector, with cattle alone representing some fraction of that number. Because so much of the American cattle industry depends upon grazing rangelands that replicate the conditions studied by Butterbach-Bahl’s team, and therefore could have a positive effect on N20 emissions, it is worth considering whether the impact of the U.S. cattle industry on greenhouse gas emissions might be significantly lower than previously thought.

Mitloehner believes that the study of N20 emissions represents only a first step in understanding ways in which agriculture can be used as a tool to address environmental issues. He hopes in the future to examine a similar relationship between grazing and CO2.

Remarks Mitloehner, "In my view, agriculture can provide very important solutions to environmental questions. Grazing has been known for hundreds of years to be a management tool to control weeds, and to improve the organic structure of the soil."

Yet it is difficult to obtain funding for research which does not support the standard global warming party line, and getting the attention of a public which is already heavily under the sway of negative media is an uphill battle. According to Mitloehner, however, this is a battle that must be fought and won.

"Agriculture keeps being in the defense, and trying to argue themselves out of claims that they are the worst thing for the environment, when in fact, they should educate the public about what it is they really, truly do. It’s one of the few really green industries, yet they are defending themselves as being one of the worst polluters in the world."