Variation in reticulo-rumen volumes between and within sheep breeds and genetic relationships with production traits

Context: Reducing methane emissions from livestock systems is a global priority. Genetic selection could play a vital role, but direct measurement of methane emissions from individual sheep is expensive and difficult. Previous research studies, in the UK and internationally, have identified links between rumen morphology, measured by computed tomography (CT) scanning, and methane emissions from sheep. In particular, larger reticulo-rumen volumes (RRvol) are associated with increased methane emissions. Aims: The aims were to investigate RRvol differences between breeds and sires, in Texel and Scottish Blackface lambs, and to estimate genetic parameters of RRvol and other production traits in Texel sheep. Methods: Scottish Blackface (SBF; n = 151) and Texel (TEX; n = 111) lambs of both sexes, reared together on low-ground pastures, were CT scanned in batches pre-slaughter (average age 20 weeks) and RRvol measured to investigate breed and sire effects. Genetic analysis of RRvol measured in 806 Texel ram lambs (∼20 weeks of age) from the UK's national breeding programme was undertaken to estimate genetic parameters. Key results: SBF lambs had reticulo-rumen volumes 30% larger on average than those of TEX (P < 0.001) at the same live weight. Significant sire differences were identified. RRvol measured from images obtained during routine CT scanning was moderately heritable in TEX lambs (h2 = 0.45, s.e. 0.13). Genetic correlations with RRvol were low for growth traits (rg < 0.1), moderate and negative for carcass composition assessed by ultrasound or CT (rg -0.25 to -0.47) and highly negative for CT-measured carcass weight and muscularity (shape) (rg -0.69 and -0.67, respectively), although standard errors were relatively high (0.17-0.3). Phenotypic correlations were of lower magnitude, but followed similar trends. Conclusions: RRvol is under genetic control, varying between breeds, sires and individual animals. Lambs with heavier, more muscular carcasses have lower volume reticulo-rumens. Implications. These results add to our understanding of the relationships between rumen morphology and production traits. The genetic relationships between reticulo-rumen measurements and other economically and environmentally important traits (including methane emissions) should be fully investigated before recommendations can be made on their inclusion in breeding programmes.