Greenhouse gas emissions from a diversity of sheep production systems in the United States

There is a diversity of sheep production systems in the United States (US), from intensive operations which house animals year-round with concentrated feeding to extensive operations that practice transhumance grazing with minimal inputs. The main objective of this study was to quantify the amount and variability of greenhouse gas (GHG) emissions from four distinct production categories that typify US sheep production: Intensive, Intensive Grazing, Extensive Grazing, and Range. An additional objective was to compare emissions estimates produced via three different methods for determining methane (CH4) production and nitrogen (N) excretion. A cradle to farm gate life cycle analysis (LCA) was conducted on 17 sheep operations to determine their GHG emissions for producing sheep and wool. An LCA was also conducted on four feedlots to determine emissions related to finishing lamb from Range operations. The more intensive operations kept ewes for less time on pasture, purchased more feeds, and produced more weaned lambs/ewe/year (P < 0.01). Emissions intensity was lower (P < 0.05) in more intensive operations and ranged from 12.8 to 20.1 kg carbon dioxide equivalents (CO2e)/kg lamb liveweight (LW) or 10.5–13.3 kg CO2e/kg sheep LW. Enteric CH4 was the major source of GHG across all categories, with the proportion of GHG as enteric CH4 declining from most extensive (79%) to most intensive (46%) categories. More intensive operations emitted 44% more kg N2O/sheep LW and 67% more CO2/kg sheep LW than more extensive ones. Breeding ewes accounted for over 60% of total enteric CH4 emissions across all categories. Incorporating a Feedlot component for Range lambs resulted in an emissions intensity of 15.1 kg CO2e/kg lamb LW, 25% lower than that at the Range farm gate. The number of lambs weaned/ewe/year accounted for the greatest variation in emissions intensity on a lamb LW basis (37%), followed by ewe replacement rate (27%), while on a sheep LW basis, age at first breeding accounted for 56% of variation. Yearly ewe enteric CH4 emissions and N excretion were higher using the NRC and the IPCC Tier 2 methods compared to the IPCC Tier 1 method (P < 0.001). Ewe productivity was the main driver of GHG emissions intensity in diverse US sheep production systems; therefore, its improvement will be crucial in reducing emissions. Further gains in extensive operations can be realized by a reduction in preweaning mortality while intensive operations have opportunities to improve manure management and utilize feeds produced with lower GHG emissions.