The effects of breed, breeding system and other factors on lamb mortality: 3. Factors influencing the incidence of weakly lambs as a cause of death

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Abstract
The incidence of lamb mortality from weakness at birth, underdevelopment, exposure and starvation was examined over 6 years in an upland flock and analysed in relation to breed, breeding system and other factors. These causes accounted for more than a quarter of the deaths and contributed to many more.The frequency of death due to these causes was greater in lambs light at birth. The incidence of these causes of death differed significantly between the breeds, Scottish Blackface, Cheviot and Welsh Mountain and their crosses, but for the separate categories breed variation was significant only for the incidence of death by starvation, which was also less frequent in cross-breds than purebreds. Mortality from several of these causes was increased by inbreeding (25–59%), but decreased by crossing inbred lines. Lambs born to line-cross or other non-inbred dams had a relatively low incidence of these various causes of death. For underdeveloped lambs variation due to maternal inbreeding was significant. When classes were adjusted for difference in birth weight, breed variation in mortality was greatly enhanced but variation due to inbreeding much reduced.Slightly more twins than single-born lambs died from these causes and triplets fared worst, but such variation was significant only for death from exposure. Mortality from these causes varied significantly with parity of dam, being highest in the first, and mortality in males was slightly but not significantly higher than in females.