A new draft report on greenhouse gas emissions lists “enteric fermentation,” primarily in cattle, as the leading cause of methane emissions. The report also, however, shows agriculture overall produces a small percentage of total greenhouse gas emissions and provides significant sequestration of carbon.

EPA lists cattle as primary methane sourceThe draft 1990-2012 Greenhouse Gas Emissions Inventory is now available for public comment.  An EPA web page includes links to each section of the report, an executive summary and instructions for submitting comments.

The draft report attributes 25 percent of U.S. methane emissions to enteric fermentation. The next-leading source of methane emissions is natural gas systems, at 18 percent of the total. Although a small percentage of total greenhouse gas emissions, methane is considered 20 times more effective than CO2 at trapping heat in the atmosphere. The report shows methane emissions from ruminant livestock declining in recent years due to the decline in cattle numbers. The authors also note that feeding practices can influence emissions due to enteric fermentation, as high-quality, concentrated feeds such as grain-based rations reduce methane emissions relative to forage diets.

Carbon dioxide by far accounts for the largest volume of greenhouse gas emissions at 82.7 percent of the total, and 94 percent of carbon dioxide emissions come from fossil fuel consumption, according to the report.

Agricultural overall is responsible for 8.1 percent of total U.S. greenhouse gas emissions, according to the report. Agricultural soil management is the largest source of emissions in the agricultural sector, in terms of carbon dioxide equivalents, as nitrous oxide (N2O) emissions from fertilizing activities are considered approximately 300 times more powerful than CO2 at trapping heat in the atmosphere. The report lists enteric fermentation as the second-largest agricultural source of greenhouse gas emissions, followed by manure management.  

The report also estimates carbon sequestration, or “sinks” where carbon is removed from the atmosphere and stored. Forests, grasslands and farmlands, and their associated soils, serve as sinks by removing carbon dioxide from the atmosphere and storing it in plant tissue and organic materials returned to the soil. “In agricultural soils, mineral and organic soils sequester approximately four times as much carbon as is emitted from these soils through liming and urea fertilization,” the authors note. “The mineral soil carbon sequestration is largely due to the conversion of cropland to permanent pastures and hay production, a reduction in summer fallow areas in semi-arid areas, an increase in the adoption of conservation tillage practices and an increase in the amounts of organic fertilizers (i.e., manure and sewage sludge) applied to agriculture lands.”

View the full draft report from EPA.

Editor’s note: The EPA report focuses on “anthropogenic” or human-caused sources of greenhouse gas emissions. Cattle or other ruminants realistically should not be included in this category, primarily because they have been part of the earth’s ecosystem since before the time of humans. Cattle have essentially replaced populations of other ruminants. Prior to the arrival of Europeans in North America for example, there were an estimated 60 million bison on the continent, and some estimates run significantly higher. Add large herds of elk, deer, antelope and other wild animals that roamed the plains, forests and mountains of North America before Europeans turned up, and the historical number of ruminants in North America probably was similar to that of today. And all those ruminants lived their entire lives eating forage diets exclusively, likely producing significantly more methane emissions per pound of animal than today’s beef and dairy herds. So, is enteric fermentation a “natural” source of greenhouse gasses, like volcanoes and forest fires? Is it an anthropogenic source? Or does it fall somewhere between the two?

JM