Updated predictions of enteric methane emissions from sheep suitable for use in the New Zealand national greenhouse gas inventory

Abstract

Enteric methane (CH4) emissions make up approximately one-third of all New Zealand’s carbon dioxide equivalent greenhouse gas emissions. In current national inventory calculations, fixed values are used to estimate emissions from sheep; 20.9 g CH4 per kg dry matter intake (DMI) for sheep <1 year old and 16.8 g CH4 per kg DMI for sheep >1 year old. These values have been principally derived from trials where intake was estimated, and CH4 emissions were measured indirectly using the sulfur hexafluoride tracer technique. Using New Zealand sheep data collected between 2009 and 2015, where intake was accurately measured and CH4 emissions were measured for a minimum of 48 h in respiration chambers (n = 817), updated sheep methane prediction algorithms suitable for use in the national greenhouse gas inventory were derived. A single equation for all sheep based on daily DMI (kg) alone (ln(g CH4/day) = 0.763 × ln(DMI) + 3.039) explained 76% of the variation in CH4 emissions. Splitting the dataset into two age classes (sheep <1 year old and sheep >1 year old) provided two alternative equations; (sheep >1 year old), ln(g CH4/day) = 0.765 × ln(DMI) + 3.09 and (sheep <1 year old), ln(g CH4/day) = 0.734 × ln(DMI) + 0.05(metabolisable energy) + 2.46. An analysis of concordance suggests that a better fit to the data is obtained by using a two-algorithm approach. The use of these updated algorithms in the national inventory resulted in small changes to estimated emissions both within and between years.