Figure 2.

Contribution of Livestock to Methane Emissions and the Impact of Parasitism.

(A) Enteric fermentation from ruminant livestock is second only to wetlands as the most important living source of atmospheric methane. (B) Current estimates ignore effects of pathogens on methane release from livestock, but each percentage point increase in gastrointestinal worm infection prevalence might increase this contribution by 0.52% relative to baseline levels, resulting in an increase of up to 52% in a universally infected global livestock population. (C) The total impact of pathogen infection on livestock methane production arises from a combination of effects on processes such as lamb production, methane yield, and time to slaughter. Data in (A) are global estimates of methane emissions from 1961 to 2012 due to combined enteric fermentation of cattle, sheep, and goats (http://www.fao.org/faostat/en/#data/GE). Estimates of methane emissions from wetlands, rice cultivation, termites, and oceans are from bottom-up inventories and atmospheric circulation modeling [15]. For (B), we estimated the net effect of increasing worm infection on enteric methane emissions from sheep as the observed effect of infection on methane yield (+33% [5]) times the effect on time to slaughter (+14% [5]). This resulted in an estimated 52% increase (1.33 × 1.14 = 1.52) in methane release. Next, to illustrate how such an effect could scale up to affect livestock-derived methane emissions globally, we recalculated global livestock methane emissions from 2011 across the full range (0–1) of possible infection prevalence, p, using the equation: Emissions(p) = Emissions₂₀₁₁(1.52 * p + (1 − p)).