Methane emissions from bison—An historic herd estimate for the North American Great Plains

Enteric methane (CH4) emissions were estimated from 30 M bison (Bison bison) across the North American Great Plains before contact with European settlers. We compiled the first historic emissions inventory using an Intergovernmental Panel on Climate Change (IPCC) Tier 2 method. The emissions were governed by the energy requirements for grazing, growth and reproduction. A sex/age distribution accounted for the net effect of births, development and deaths. The CH4 yield was based on calorimeter measurements. The average bison's weight, feed (dry matter, DM) intake and emissions were 411 kg, 3.4 t DM head−1 year−1 and 72 kg CH4 head−1 year−1, respectively. The historic herd's emissions were 2.2 Tg CH4 year−1. On 1 January 2008, 36.5 M cattle were located in 10 American states occupying the historic bison range. Cattle emissions were 2.5 Tg CH4 year−1, estimated using an IPCC Tier 1 method, adjusted by comparison with a mechanistic model and food gathering energy required by 77% of the cattle fed by grazing.