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By the numbers on a hot topic: The role of grazing in reducing greenhouse gas

Kelli Boylen for Progressive Dairy Published on 11 March 2020

Are cow farts destroying the planet? Does eating meat matter? Can grazing reverse climate change?

Jasmine A. Dillon, Ph.D., assistant professor of beef and dairy agro-ecosystems at Colorado State University, tried to break down some of the facts and fallacies relating to agriculture and climate change.

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Agriculture represents about 8% of greenhouse gas (GHG) emissions in the U.S. and, of that, beef and dairy cattle GHG emissions represent about 4% of the total. Increasing concentrations of greenhouse gases trap more heat in the Earth’s atmosphere, resulting in climate change; carbon makes up more than 80% of GHG in Earth’s atmosphere.

The beef industry is the biggest contributor toward greenhouse gases in the agricultural sector. GHG emissions from the average American’s annual beef consumption are about four times greater than emissions from consuming about 18 gallons of milk per year, or about eight to nine times greater than emissions from consuming about 50 pounds of pork or 90 pounds of chicken per year.

Dillon said there are many things people do in their daily lives that have a bigger impact than eating beef – or even agriculture as a whole. For example, the GHG emissions associated with driving a passenger vehicle about 14,000 miles per year are 12 times greater than emissions from consuming about 53 pounds of beef per year, and the GHG emissions associated with a single commuter flight per year are about six times greater.

A study of Midwestern beef systems reported that if carbon sequestration is not included in the calculation, grass-finished beef cattle have a 30% greater carbon footprint (per kilogram of live-weight produced) than grain-finished animals. She explained, “Grass-finished animals tend to take a longer time to finish than grain-finished animals, and they spend a longer time on highly fibrous diets. The more fibrous the diet, the more methane produced. This results in greater carbon footprints for grass-finished animals than grain-finished animals when carbon sequestration is not included.”

But when carbon sequestration was included in the calculation for the Midwestern systems, the grass-finished animals had a 15% smaller carbon footprint (per kilogram of live-weight produced) than the grain-finished animals. Another study shows that adaptive, multi-paddock grazing may be a net carbon sink.

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“If carbon is indeed sequestered in the beef system, it should be accounted for,” Dillon said. “Historically, it hasn’t been accounted for because it is assumed that soils will eventually reach saturation (which is true in some cases), and a system that was sequestering carbon can become a net emitter again.”

About 72% of the dairy supply chain’s GHG emissions are incurred at the farm. About 19% from all processes involved in feed production, 23% from manure and 25% from enteric methane. The remaining amounts come from additional fuel, refrigerants and electricity used during processing, packaging, transportation and distribution, retail and consumption.

About 59% of the total GHG emissions from the U.S. beef supply chain is allocated to the cow-calf sector. Most of the cow-calf sector’s emissions are from enteric methane produced by the animal and nitrous oxide produced by the soil. Dillon said nitrous oxide is produced as a byproduct of denitrification and nitrification processes in soil. Cattle contribute nitrogen to soil through urine and feces. Nitrogen from fertilizer application also contributes to nitrous oxide losses from soil.

“Cattle are recyclers of carbon already in the atmosphere, not contributors of additional carbon from long-term storage,” she said. “Interestingly, in a recent comparison of the actual long-term climate impact of multiple beef systems, the systems with the greatest carbon dioxide – not methane – emissions had the greatest long-term impact on climate (beyond 100 or 500 years).”

Carbon dioxide is removed from the atmosphere (or “sequestered”) when it is absorbed by plants as part of the biological carbon cycle. “Well-managed grazing can contribute to the development of extensive root systems in pastures, which contributes to increased soil organic matter and carbon storage,” she said. “The amount of carbon stored varies according to soil texture, local climate (temperature, rainfall patterns) and historical management.”

Can grazing save the world? “Grassland soils can and do sequester carbon,” she said. “There is potential, but its exact value is currently uncertain.”

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Many scientists agree that although we need to reduce carbon emissions related to fossil fuels and other causes, sequestering carbon in the soil to reduce the amount in the atmosphere is very important as well.

Carbon is the main component of soil organic matter (SOM), so increasing the amount of SOM helps with the reduction of greenhouse gases as well as giving water its soil retention capacity, fertility and structure. The amounts of carbon sequestered over long periods of time depend upon the form the carbon is stored in.

Particulate organic matter (POM) is made up of partially decomposed, mostly plant materials, is found in large aggregates and lasts for about 10 years. It has a high carbon-to-nitrogen ratio.

Mineral-associated organic matter (MAOM) is composed of mostly microbial products of decomposition and leached chemical compounds that attach to soil minerals and is found in small aggregates. It has a low carbon-to-nitrogen ratio, but the particles can last for decades, if not centuries.

“We need to learn more about the role that grazing plays in POM and MAOM storage in different environments and management systems,” she said.

The form the carbon is stored in depends upon management practices, climate, soil texture, pH, and soil fauna and microbiota, she said.

What is the role of grazing in sustainable food systems? Dillon said grazing maximizes the ruminant’s superpower: the ability to convert human-inedible feedstuffs and marginal land into high-quality, human-edible protein. Grazing can contribute to the restoration of degraded land and innumerous ecosystem services through increased SOM storage.

The other benefits of increasing SOM, she said, include enhancing the soil aggregate stability, improved nutrient retention and availability for uptake by plants, reduction of potential for runoff and erosion, enhancing soil microbial communities and improved soil conditions (like temperature and pH).

And when someone asks you if cow farts are indeed killing the planet? Dillon said to tell them that cows belch methane, and carbon dioxide is the primary greenhouse gas and it stays in the atmosphere for longer periods of time. According to the EPA, methane makes up less than 10% of U.S. greenhouse gases (16% of global greenhouse gas emissions). According to www.epa.gov/ghgemissions/overview-greenhouse-gases, the primary contributors of methane to the atmosphere are the production and transportation of fossil fuels, followed closely by agriculture.  end mark

Dillon presented “Carbon, Climate, and the Role of Grazing in an Insecure Future” at the 2020 Grassworks Grazing Conference held annually in Wisconsin.

Kelli Boylen is a freelancer based in northeast Iowa.

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