Abstract

Lamb survival and birth weight data from a Booroola x Romney flock and a Booroola x Merino flock were collected between 1975 and 1981. The mean litter sizes for these flocks were 1.96 and 1.76 respectively, with litter sizes ranging from 1 to 4 within each flock. Effects of year, ewe age, birth rank and birth weight on lamb survival were assessed by fitting a sequence of logit models to the data. Birth weight was identified as the key parameter, with year and birth rank effects being markedly reduced once the variability due to birth weight was accounted for. An apparent ewe age effect was completely explained in terms of lamb birth weight. At any given birth weight lambs born to large litters (>/-3) had a lower probability of survival than contemporary lambs born as singles or twins.

RW, Moore, RMW Sumner, JJ Bass, and H-UP Hockey

Proceedings of the New Zealand Society of Animal Production, Volume 43, , 21-24, 1983