Methane and grazing: A broader view

In the ongoing debate over sources of greenhouse gas emissions, cattle and other ruminants often take a hit. Critics point to ruminant livestock as being a major contributor to emissions of methane, one of the three main greenhouse gases – along with carbon and nitrous oxide – with global-warming potential. Ruminants emit methane in the process of digesting cellulose in the rumen.

Some critics go so far as to suggest reduced populations of ruminant livestock globally as a means of reducing methane emissions. This stance takes a narrow view, one failing to look at livestock's place in a regenerative ecosystem and their potential to contribute reductions to global warming.

"The research that looks solely at the methane emitted by livestock serves to simply put cows in a little box. It says this is what's going up into the atmosphere, but it doesn't take a look at the big picture – the natural processes in play to diminish the methane," says Richard Teague, research ecologist at Texas A&M AgriLife Research.

Neither does such a narrow view take stock of livestock's place in the ecosystem as a whole – and their potential contributions to reduced levels of greenhouse gases. Teague and other researchers set out to do just that by examining a plethora of researchers' data.

Analyzing the Research

Their resultant research editorial, titled "The Role of Ruminants in Reducing Agriculture's Carbon Footprint in North America," was published in the March/April 2016 issue of the Journal of Soil and Water Conservation. The title sums up the premise of the review – examining the carbon footprint of livestock.

"In beginning the research, we proposed that with appropriate regenerative crop and grazing management, ruminants not only reduce overall greenhouse gas emissions but also facilitate provision of essential ecosystem services, increase soil carbon sequestration, and reduce environmental damage," Teague says. "We tested our hypothesis by examining biophysical impacts and the magnitude of all greenhouse gas emissions from key agricultural production activities, including comparisons of arable- and pastoral-based agroecosystems."

Their examination took stock – most notably – of emission measurements for carbon, methane, and nitrous oxide. While the latter two are significantly more potent globa warming gases than is carbon dioxide (CO2), CO2 is by far the most prevalent greenhouse gas (GHG). Thus, all measurements were converted to CO2 equivalents to compensate for the differences in the potency of each GHG.

"Our assessment shows that, globally, GHG emissions from domestic ruminants represent 11.6% of total anthropogenic emissions, while cropping and soil-associated emissions contribute 13.7%," Teague says. "The primary source is soil erosion, which in the U.S. alone is estimated at 1.72 gigatons per year."

By contrast, well-managed permanent covers of perennial plants can be highly effective in reducing soil erosion. "Ruminants consuming only grazed forages under appropriate management results in more carbon sequestration than emissions," he says.

Adaptive Grazing System

Appropriate management of grazing is the key in order for grazing systems to sequester carbon from the atmosphere, thus, mitigating GHG emissions by serving as net carbon sinks. Through research and on-farm observations, Teague recommends regenerative grazing systems that are adaptive.

An adaptive multipaddock (AMP) grazing process uses large numbers of paddocks stocked at reasonably high rates for short grazing periods, followed by long rests. The process is adaptive in that stocking rates and grazing periods flex to fit forage biomass and growing conditions. The goal is to leave sufficient forage after the grazing period to maintain soil and plant health.

"Improved grazing management, such as AMP grazing, has been shown to reverse the causal mechanisms of the degradation of continuously grazed rangeland by decreasing bare ground, restoring productive plant communities, and increasing water-infiltration rates and soil water storage capacity," Teague says. "Improved grazing has also served to increase fungal-to-bacteria ratios that provide many benefits leading to increased soil carbon.

"In across-the-fence comparisons in southern tallgrass prairie in Texas, where AMP grazing was applied to areas previously degraded through prolonged continuous grazing, we calculated an average of 3 metric tonnes per hectare per year of additional soil organic carbon in the top 90 centimeters of soil after more than a decade in AMP grazing. This was in comparison with sites undergoing heavy, continuous grazing."

Whole Systems Approach

Such contributions of grazing livestock to the drawdown of GHGs are a critical component of accurate calculations of their carbon footprint using life cycle assessments (LCAs) to evaluate the impact of cattle on GHGs. Indeed, Teague argues for a whole systems approach to calculate LCAs.

"It's important to include all elements in the food production chain influencing the net carbon footprint in the whole system under review," he says. "This includes accounting for the beneficial ecosystem services such as those from carbon sequestered in well-managed grazing ecosystems. It should also account for any GHG emissions associated with the production of grain-based feeds, inorganic fertilizer, and other elements adding to the carbon footprint, such as soil erosion."

Hindering accurate accounting of LCAs, Teague says, is the limited inclusion of sound experimental data on soil carbon and GHG dynamics in grazing and cropping agroecosystems. "This lack of data includes the background environmental consumption of methane by methanotrophs and the larger-scale atmospheric photooxidation of methane in the presence of water vapor, which we have known about since the 1970s."

"When whole system LCAs include the carbon sequestered by ruminants using appropriate grazing management and the larger-scale atmospheric photooxidation of methane in the presence of water vapor that is also facilitated by appropriate management, cattle managed in regenerative grazing systems are an environmental boon," he says.

"Ruminant dams and their offspring spend most of their lives on perennial grass during which the carbon sequestered by the grassland they graze exceeds their emissions," Teague adds. "The grazing of ruminants in a manner that enhances soil health will reduce the carbon footprint of agriculture much more than simply reducing domesticated ruminant numbers to reduce enteric GHG emissions. Ruminant livestock are an important tool for achieving sustainable agriculture."

Helpers in the soil

In grazing systems that are regenerative, populations of soil microorganisms increase. These soil helpers drive the processes of carbon sequestration and the oxidization of some methane.

"Soil health is fundamental for ecosystem function because 90% of soil function is mediated by microbes, with a mutual dependency among microbes, plants, and animals," says Richard Teague, Texas A&M research ecologist. "How we manage plants in grazing or cropping ecosystems is critical to maintaining or regenerating full ecosystem function and the productivity that drives net profits on farms and ranches."

That soil life increases with adaptive multipaddock grazing (AMP) was borne out by data collected by Mississippi-based livestock consultant Allen Williams. After five years of AMP grazing, soil microbial biomass was four to five times greater on five sites across five states.

"A diverse population of soil microbes leads to greater carbon sequestration, including methane oxidation," Teague says.

The oxidation of some of the methane occurs when it's broken down by bacteria called methanotrophs.

Learn More

Richard Teague

940/552-9941, Ext. 235

r-teague@tamu.edu