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.