Sustainability & efficiency

Effect of differences in modelling ewe weight on methane intensity

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Abstract

Ewe weight is a key trait that affects methane emissions. Improved production efficiency reduces environmental impact by decreasing methane intensity (MI) (kg CH4/kg of carcase). Ewe weight evidently increases until five years of age. However, it is unlikely that most carbon calculators account for ewe liveweight changing with age. The objective of this work was to compare different methods of modelling ewe weight on MI and investigate the effect of genetic changes in ewe weight. This supports environmental selection index development. Average pre-mating weight of 3,197 Aberfield ewes, at each age, was estimated using multiple linear regression analysis, incorporating significant (P< 0.05) model-terms (age, mating body condition score, ID, lambing year, flock). Mean ewe weight increased until 5-years-old by 31.93kg from year-one weight (8 months-old) (Table 1); suggesting inaccuracies if ewe weight is modelled as constant over all 'adult' ages. Three methods of modelling ewe weight in emission models were compared: 1. Ewes reach 'ewe weight' at 12 months and remained constant weight ('ewe weight' = 3-year-old pre-mating weight), 2. Ewe weight by age (Pre-mating weight annually increased by one increment) and 3. Ewe weight by age, growth accounted throughout each year (including growth energy requirements). Changes in MI, energy requirements and carcase output (prime lambs and cull ewes) of a 100-ewe flock were modelled. Average Aberfield performance for key traits was used as model foundations and the three methods of modelling ewe weight were compared. Ewe carcase output was estimated using a killing-out percentage of 46%. Gross energy requirements, methane emission factors and methane conversion factors were calculated for each stock-class, to estimate total flock methane emissions. These values were divided by flock carcase yield to estimate annual MI. Genetic change in ewe weight over 10-years was calculated from EBVs, and the effect on flock MI estimated, modelling ewe weight by method 3. Modelling ewe weight using method 1 (constant ewe weight), overestimates MI by 3.7-3.8% compared to modelling by age (Table 2). This is largely due to a greater proportion of the flock being under 3-years-old, therefore overestimating methane emissions. Accounting for ewe growth (method 3 vs. method 2) slightly increases MI by 0.9%, but also increases accuracy of carcase weights and energy requirements for growth, maintenance and activity. A significant (P< 0.05) annual genetic trend of -0.238kg in ewe weight, reduced MI of a 100-ewe flock over 10-years by an estimate of 0.9%, despite reduced cull ewe output of 61.36 kg/annum, by modelling ewe weight with growth (method 3). In conclusion, modelling ewe weight with a constant weight overestimates MI. Accounting for ewe growth throughout the year further increases accuracy. Negative genetic trend of ewe weight predicts to improve MI.

Keywords: 2026

How to Cite:

Dods, E., Lambe, N. & Roden, J., (2026) “Effect of differences in modelling ewe weight on methane intensity”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2285976. doi: https://doi.org/10.31274/wcgalp.23828

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