A Holistic Approach To Measuring Greenhouse Gas Emissions In Grazing Management Systems For Beef Production

Beef production is among the most contentious issues of the past few years, having one of the largest greenhouse gas (GHG) footprints among all food products. The United States is the leading beef producer in the world and among the highest beef consumers, where each person consumes an estimated 25.6 kg/capita annually- double the OECD average (1).

Another unique feature of beef cattle in the US is the method of production. While in most of the world cattle are grazed for their entire lives, often leading to deforestation and overgrazing, the US has adopted a specialized method of finishing cattle to maximize weight gain: feeding cattle grain in the feedlot. Feedlot finishing certainly increases efficiency, but also leads to environmental consequences of its own. This unique set of circumstances has led to diverging ideas on what exactly leads to “sustainable” beef production.

To make matters even more complicated, the 2% of cattle still finished on grass (commonly called “grass-fed”) in the US are not homogeneous (2). Most grass-fed cattle are continuously grazed, generally meaning that they are left to roam pastures with very little management-leading to overgrazing, high GHG emissions, and other negative impacts. However, new grazing management systems, such as adaptive multi-paddock (AMP) grazing, focus more on rotation, forage and soil quality, and prevention of overgrazing. This type of grazing has attracted much attention from farmers, among grassroots organizations, and even some mainstream media (3).

Some have hypothesized that these grazing systems could also cause grasslands to draw CO₂ into the soil (a process called soil carbon (C) sequestration), potentially acting to offset some of the GHGs produced. However, most modeling studies attempting to compare the environmental impact of differing beef management systems have only compared continuous grazing to feedlot systems-nearly unanimously excluding potential soil C sequestration. Recognizing the lack of scientific research on AMP grazing lead us to design research taking a more holistic approach to GHG accounting in beef production.

This paper combines data from both feedlot finishing and AMP grazing into a life-cycle assessment (LCA), which is a comprehensive tool for GHG accounting. In addition to the common LCA methodology, 4-year soil C data was taken in the AMP grazing system and added as a potential C sink. Sources of emissions for both systems included those produced during feed production and processing, manure storage and handling, enteric methane, and on-farm processes.

As far as the amount of beef produced, the feedlot cattle gained more weight, in less time, and with less land (due to the high yield of grain) than the AMP grazed cattle. However, compared to the continuously grazed cattle of other studies, AMP grazing produced much more beef, in less time and with less land. Comparatively, AMP grazed cattle was more efficient than continuously grazed cattle but less than feedlot cattle.

In sheer GHG emissions- before any potential reductions due to soil C sequestration- feedlot production resulted in 6.09 kg CO₂-e kg CW^-1, whereas AMP grazing resulted in 30% higher emissions (9.62 kg CO₂-e kg CW^-1). This is common among other LCA findings and is the result of higher feed efficiency (cattle gain more weight on grains than grass, in general) and shorter overall finishing time in the feedlot.

However, soil analyses showed very high rates of soil C sequestration in the AMP grazing system at 3.59 Mg C ha⁻¹. Overall, this level of CO₂ entering the soil more than offset all of the GHGs produced, turning the system into a net C sink of -6.65 kg CO₂-e kg CW^-1. Carbon negative beef- the proof was in the soil.

Methane is commonly to blame for cattle’s environmental destruction. Though ruminants have roamed the Earth for thousands of years, emitting methane as a natural byproduct of rumination, it’s high global warming potential (30x that of CO₂) make it a hot topic in the current context. Though we modeled methane emissions, there has been some controversy over whether these models are accurate for improved grazing systems. Indeed, when we compared the globally used methane model, it overpredicted emissions by 35% compared to actual, on-farm data (not modeled).

Compared to previous studies, this paper highlights the importance of taking a “systems” approach when analyzing the sustainability of any given system. Previous research has all indicated feedlot production to be more sustainable because it produces less GHG emissions. However, looking at the impacts literally on the ground as well as in the air can completely change the concept of sustainability.

We can’t definitively say that AMP grazing will be the silver bullet for sustainable beef production. The jury is still out on how long high rates of soil C sequestration may continue eventually, it will decrease and reach a new level of equilibrium. Also in need of more research is how this type of grazing will influence soil C in other climates and in grass and rangelands with different forage species. Surely there is much more work to be done.

At the very least, the results of this study give weight to what farmers and ranchers have been saying for many years.

These findings are described in the article entitled Impacts of soil carbon sequestration on life cycle greenhouse gas emissions in Midwestern USA beef finishing systems, recently published in the journal Agricultural Systems. This work was conducted by Paige L. Stanley, Jason E. Rowntree, David K. Beede, and Michael W. Hamm from Michigan State University and Marcia S. DeLonge from the Union of Concerned Scientists.

References:

1. OECD. (2016). OECD Meat Consumption. https://data.oecd.org/agroutput/meat-consumption.htm
2. Oates et al., (2011). Management-intensive rotational grazing enhances forage production and quality of subhumid cool season pastures.
3. https://www.nytimes.com/2018/04/18/magazine/dirt-save-earth-carbon-farming-climate-change.html