Historic, pre-European settlement, and present-day contribution of wild ruminants to enteric methane emissions in the United States

The objectives of this analysis were to estimate historic (pre-European settlement) enteric CH4 emissions from wild ruminants in the contiguous United States and compare these with present-day CH4 emissions from farmed ruminants. The analysis included bison, elk (wapiti), and deer (white-tailed and mule). Wild ruminants such as moose, antelope (pronghorn), caribou, and mountain sheep and goat were not included in the analysis because their natural range is mostly outside the contiguous United States or because they have relatively small population sizes. Data for presettlement and present-day population sizes, animal BW, feed intake, and CH4 emission factors were adopted from various sources. Present-day CH4 emissions from livestock were from recent United States Environmental Protection Agency estimates. The most important factor determining CH4 emissions from wild ruminants in the presettlement period was the size of the bison population. Overall, enteric CH4 emissions from bison, elk, and deer in the presettlement period were about 86% (assuming bison population size of 50 million) of the current CH4 emissions from farmed ruminants in the United States. Present-day CH4 emissions from wild ruminants (bison, elk, and deer) were estimated at 0.28 Tg/yr, or 4.3% of the emissions from domestic ruminants. Due to its population size (estimated at 25 million), the white-tailed deer is the most significant present-day wild ruminant contributor to enteric CH4 emissions in the contiguous United States.