Reassessing Ruminant Methane Contribution

The environmental impact of livestock production, especially ruminants, has received a lot of attention in both the scientific community and popular media. One of the most discussed aspects of ruminants’ environmental impact is the production of the greenhouse gas, methane. Methane is produced as a natural byproduct of fermentation in the ruminant stomach during the process of feed digestion. The production of methane is not a man-made process and occurs naturally in all wild and domestic ruminant animals.

Wild ruminants in North America include deer, moose, elk, big horn sheep, antelope and bison with bison having the largest population. Estimates of the bison population prior to European settlement of North America varies greatly ranging from 21 to 88 million. And estimates of the total wild ruminant population prior to settlement ranges from 83 to 133 million. Due to lots of factors chief among them the growth in human population, the wild ruminant population has decreased to 30.5 million today and have been replaced by 90 million domestic ruminants.

Do domestic ruminants produce more methane than wild ruminants? Methane emissions factors for bison are similar to that of domestic cattle when fed the same diet, and both are greater than deer and elk. However, diets of wild and domestic ruminants are not necessarily similar. Diets of domestic ruminants are managed by humans and are typically of greater nutritive value than wild ruminants consume, especially during the winter months when vegetation is dormant.

Attempting to account for differences in methane emissions from wild and domestic ruminants, recent research compared the amount of methane from wild ruminants prior to European settlement of North America and current wild and domestic ruminant populations (Figure 1). Due to the wide variation in estimates of bison population, results were computed for low, medium and high bison populations. Based on these data, the amount of methane from domestic ruminants contributing to the increase in global atmospheric methane concentration is less than 100% because a fraction of that methane is replacing naturally produced methane from pre-settlement wild ruminant populations. Doing the math, the proportion of methane emissions from domestic ruminants in North America that is contributing to atmospheric methane concentrations ranges from 50 to -19% depending upon the pre-settlement bison population with an average of 35%.

Several feed additives have been investigated for their ability to reduce enteric methane emissions from domestic ruminants; the most effective include methane inhibitors, electron acceptors, hydrogen sinks, and plant extracts. These feed additives can reduce enteric methane emissions from 10 to 50% depending upon domestic ruminant species and diet, indicating that implementation could mitigate the 35% of domestic ruminant methane emissions that is new to North America since the European settlement. Although most of these feed additives have adverse effects that may hinder their use, one, 3-nitrooxyproponal, reduces methane emissions without negatively affecting animal performance and is in the process of commercialization. 3-nitrooxypropanol also shifts rumen VFA profile toward higher proportions of propionate making the ruminant animal more feed efficient, which is very similar to another feed additive, monensin, which has been widely adopted in ruminant livestock production. Thus, the use of 3-nitrooxypropanol looks very attractive for producers to economically include in livestock rations and could significantly mitigate enteric methane emissions from domestic ruminants.

In conclusion, the extent of domestic ruminants’ contribution to greenhouse gas emissions is not as great as once thought, although livestock production has more environmental impact than methane alone. It appears that we are on the verge of balancing the methane scale as far as domestic ruminant emissions are concerned.

Estimated methane emissions from wild ruminants prior to European settlement of North American Continent based on 3 estimates of the American bison herd (30, 50 and 75 million bison) compared with methane emissions from current population of wild and domestic ruminants. Adapted from Hristov, 2012