Influence of birth weight and litter size on lamb survival in high fecundity Booroola-Merino crossbred flocks

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Abstract
Lamb survival and birth weight data were collected from 3 Booroola-Merino crossbred flocks from 1975 to 1982. The mean litter sizes for these flocks were 1.79, 1.99, and 1.80, with birth rank ranging from 1 to 4 within each flock. Birth weight was the most important factor affecting survival; other apparent effects, such as that of year, ewe age, and birth rank could be largely explained by birth weight differences. Using a quadratic function relating chances of survival to birth weight for single-born lambs, optimum birth weights for the 3 flocks were estimated as 5.45, 4.73, and 4.27 kg. A hyperbolic relationship between survival and birth weight was used for multiple-born lambs, which rarely had higher birth weights and associated dystocia problems. At any given birth weight within a range common to all litter sizes, lambs born in large litters (⩾ 3) had a lower probability of survival than contemporary lambs born as singles or twins. Age-at-death data indicated that, in all birth ranks, a large proportion of deaths occurred within the first 12 h post-partum.

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