Editor's note: This article was written by Alexander Hristov, associate professor of nutrition, Penn State Department of Dairy and Animal Science and first appeared in the Penn State Dairy Digest.

In relation to our research at Penn State, I’m being asked now and then to give my opinion about domestic ruminants greenhouse gas (GHG) emissions. The last one was a piece for Penn State’s Live (March 3rd, 2011; Probing Question: Are cow burps contributing to global warming?). Soon after that, I received an e-mail from a Penn State alumnus raising the question of the historical wild ruminants’ contribution to GHG in North America. This gentleman had been unsuccessfully trying to get an answer to his question through contacting various Penn State’s Departments and even EPA. I thought this was an intriguing (and important) issue worth researching.

In North America, there are many native ruminants, i.e. herbivore animals with a complex digestive system, a major compartment of which is the rumen. Some present-day examples are the bison (Bison bison), the elk (or wapiti, Cervus Canadensis), or the deer (white-tailed, Odocoileus virginianus or mule, Odocoileus hemionus). The common feature of all ruminants is the microbial fermentation and degradation of fibrous feeds occurring in their forestomachs, primarily the reticulorumen (i.e., the joint compartments of the rumen and the reticulum). The results of this fermentative process are products benefiting the host animal (microbial protein, used by the animal as a source of amino acids and volatile fatty acids, used as an energy source).

Rumen microbes, on the other hand, thrive in the “temperature-controlled”, oxygen-free (the typical rumen microbes are anaerobes), substrate-rich environment that the host animal provides for them. Indeed, this symbiotic process is one of Nature’s wonders and has allowed ruminants to dominate the natural world. Ruminal fermentation, however, is not a very efficient process (at least from a biochemical point of view) and results in several by-products considered a waste, mainly carbon dioxide and methane. Methane is a potent GHG and domestic ruminants have been blamed by many for a large portion of the global GHG emissions, thus having a significant impact on climate change. The importance, or rather unimportance, of livestock to global GHG emissions has been discussed by us and by others (see, for example, Hristov, 2008); here, we’ll focus on wild ruminants.

Attempts to quantify methane emissions from wild ruminants have been made in the past. Crutzen et al. (1986), for example, estimated that wild ruminants produce about 0.37 Tg/yr (1 teragram = 1012 grams) of methane. McAllister et al. (1996) estimated wild ruminants (bison, elk, caribou, deer, sheep) in Canada alone produce 0.15 Tg/yr, which on average comes to about 49 g of methane (or about 1 kg of CO2 Eq.; the global warming potential of GHG is expressed as CO2 equivalents)/animal/d, a figure close to the 41 g (or 861 g of CO2 Eq.)/animal/d estimated for deer (various species of deer and caribou) by Crutzen et al. (1986). For comparison, the average U.S. car emitted between 15 and 22 kg of CO2/d in 2010 (http://www.fueleconomy.gov/feg/pdfs/guides/FEG2010.pdf).

There are several challenges in calculating methane emissions from wild ruminants (or any wild animal): (1) the population size is not exactly known and (2) the emission per animal or per unit of feed intake is also not exactly known. The reasons for #1 are obvious. The uncertainty about emissions per animal or unit of feed intake stems from uncertainties about: (1) the type of feed consumed by the wild animal, (2) the amount of methane produced per unit of these feeds, and (3) the daily amount of feed consumed by the animal. All these combined make the estimations of methane production from wild ruminants only an approximation. For example, in the study that most closely resembled a scientific effort to estimate methane emissions from bison, elk, and deer, that of Galbraith et al. (1998), the animals were fed sun-cured alfalfa pellets. This kind of diet, of course, is not even close to the natural diet of these species in the wild. In addition, in this study methane production was measured in chambers, the effect of which on a wild animal behavior and metabolism is unknown. 

The analysis we present here is not an exception and we had to make several assumptions and approximations to be able to compute wild ruminants’ emissions for the United States in the past and present. Table 1 footnotes summarize most of these assumptions. For example, historical population numbers are approximations, at best. Present day populations are known with a much greater degree of certainty and still are approximations. Historical (pre-European settlement) population estimates were mostly from the 2003 edition of the Feldhamer et al. book “Wild Mammals of North America” (The John Hopkins University Press, Baltimore, MD) - in our humble opinion, an excellent, encyclopedic review of the biology of North American mammals.

Pre-settlement elk population estimates were from the Rocky Mountain Elk Foundation (RMEF; Mr. Tom Toman). Average body weights (BW; female weights were used) were from the Feldhamer book (bison), Wikipedia (http://en.wikipedia.org; deer), and the RMEF (elk). A number of wild ruminant species, such as mountain sheep, goat, and antelope, for example, were not included in the analysis, mostly due to relatively small population sizes, i.e. impact on methane emissions, and lack of reliable population and feed intake data. Some species, such as moose and caribou, were not included because their habitat is primarily outside of the contiguous United States (i.e., Canada and Alaska).

Another critically important factor for estimating methane production is feed (or dry matter) intake (DMI). For all species, DMI was assumed to be 2% of the animal’s BW. This intake was assumed for bison based on the beef NRC model (2000; Nutrient Requirements of Beef Cattle) prediction for a mature non-pregnant, non-lactating beef cow. This intake, 10.6 kg/d in our analysis, is higher than the 7.7 kg DM/hd/d intake of bison (in captivity, fed alfalfa hay pellets) published by Galbraith et al. (1998) and the 9.2 kg DMI/hd/d estimated by Kelliher and Clark (2010). The Kelliher and Clark (2010) average DMI estimate (2.2% of BW) is in fact close to our assumption, but these authors assumed average, herd-weighed BW of 441 kg. The 2% of BW DMI for elk (4.5 kg/d) is close to the calculated DMI based on 87 g DMI/kg BW0.75(metabolic BW)/d reported for elk by Galbraith et al. (1998). Similarly, DMI for deer was close to the metabolic BW DMI reported for that species by the latter authors.

As apparent from Table 1, the bison was by far the most important wild ruminant methane emission source in the pre-settlement period.  The size of the animal, its high DMI, and its sheer numbers were determining its role as the greatest wild ruminant methane emitter. As the bison pre-settlement population size estimates vastly differ between sources (see Reynolds et al., 2003), we calculated emissions for 3 different scenarios: high (75 million), low (30 million), and medium (50 million bison) population sizes. The high population size is based on estimates by the famous 19th century naturalist and writer, Ernest Thompson Seton, and the low population size is based on the number of animals the available range at the time could support (estimated by McHugh, 1972). In all cases, the bison methane emissions represented between 84 and 93% of all emissions from wild ruminants in the pre-settlement period.

Kelliher and Clark (2010) came to similar conclusions; these authors estimated that methane emissions from bison in the 10 states encompassing the historical range of this animal were close to the present-day methane emissions by cattle in these states (2.2 vs. 2.5 Tg methane/yr, respectively). It has to be pointed out that our bison population (plains bison) estimates include bison in the Canadian Prairie Provinces (Alberta, Saskatchewan, and Manitoba), which would likely overestimate methane emission for the contiguous United States. According to Reynolds’ distribution map (Reynolds et al., 2003), however, the Canadian portion of the plains bison range was relatively small compared to the distribution range in the contiguous United States.   

Overall, methane emissions from bison, elk, and deer in the pre-settlement period in the contiguous United States were about 70% (medium bison population size) of the current emissions from farmed ruminants in the U.S.; data for current (2008) methane and GHG emissions in the U.S. are from the EPA “Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 – 2008” report. If the high bison population estimate is taken for this comparison, wild ruminants in the pre-settlement period emitted as much methane as the current domestic ruminants in the United States. Present-day livestock methane emissions are primarily from cattle; the contribution of sheep and goats to the total emissions is miniscule (about 1%). Estimates for methane emissions from horses (about 0.17 Tg/yr) and swine (about 0.10 Tg/yr) were published by EPA but are not included in this analysis. It is worth mentioning that in this most recent EPA report, GHG emissions from agriculture made up about 6% of the total GHG emission in the U.S. for 2008 (427.5 vs. 6,956.8 Tg CO2 Eq./yr, respectively). Methane from enteric fermentation was 140.8 Tg CO2 Eq./yr, representing 25% of total methane emissions in the U.S. Emissions of methane and nitrous oxide from enteric fermentation and manure management (i.e., the total livestock contribution to GHG emissions) was 202.9 Tg CO2 Eq./yr, or 47% of the agricultural emission, but only 2.9% of all GHG emissions in the U.S. 

Present-day methane emissions from wild ruminants (excluding moose, mountain sheep, goat, antelope, and caribou) were also estimated. Population data were from the Feldhamer book (bison and deer), or the RMEF (elk). Body weight, DMI, and methane emission per unit of DMI were the same as for the pre-settlement calculations. With these assumptions, it can be estimated that present-day methane emission from the major wild ruminant species in the U.S. are about 0.23 Tg/yr, which is only 3.6% of the emissions from domestic ruminants. Due to its population size (estimated at 25 million; Miller et al., 2003) the white-tailed deer is the largest present-day wild ruminant contributor to GHG emissions in the contiguous United States. 

Source: Penn State Dairy Digest


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