Genetic associations of ewe body condition score and lamb rearing performance in extensively managed meat sheep

Abstract

Many small ruminant production systems are relatively low input and rely heavily on animals grazing pastures often in extensive and harsh environmental conditions, unsuitable for any other type of agriculture. New breeding goal traits for lifetime resilience for maternal sheep breeding programmes have been investigated, focusing on body tissue mobilisation phenotypes and their genetic relationships with maternal production traits. Performance records of 8,355 Scottish Blackface ewes, from 2 extensively reared hill sheep flocks, collected over a 20-year period, were used to quantify relationships between body condition score (BCS) and the ewe's ability to successfully rear lambs. Between 14,000 and 25,000 data records per trait were available, measured across the annual sheep reproductive cycle. The pedigree file used for the analyses included sire and dam information for 50,207 animals. Most heritability estimates for the BCS traits - measured at key time points within the annual production cycle, or considering BCS changes between time points - were low, but significantly different from zero, ranging between 0.07 and 0.17. The heritability estimate for the number of lambs at pregnancy scanning was 0.09. Heritabilities for foetal loss from pregnancy scanning to lambing, lamb loss from lambing to weaning and number of lambs weaned were 0.02, 0.02 and 0.06, respectively. These estimates indicate that genetic control over the reproductive and lamb survival traits (expressed as a trait of the dam), is lower than that for BCS (and BCS changes) over the reproductive cycle. The genetic relationships amongst the BCS of ewes at pre-mating, pregnancy scan and at pre-lambing, with litter size at pregnancy scan and number of lambs weaned, were consistent. They indicate that ewes of higher body condition score (potentially over-fat, within the range recorded in these flocks) conceive, and rear, fewer lambs, with genetic correlations ranging between -0.18 and -0.58. However, post-weaning, animals with genetic propensity for greater gain in BCS from weaning to pre-mating produce larger litters and rear more lambs. In addition, the selection of ewes that lose less condition during pregnancy will reduce foetal losses. The results from this study paint a complex picture that should be interpreted in the context of specific management practices for extensively managed hill sheep, at critical times in their reproductive cycle. The traits relating to body tissue mobilisation, as assessed using BCS, are mostly heritable and their inclusion in future breeding programmes, to aid future selection for resilience, should be considered.