Sex of co-twin in utero environment does not alter reproductive fitness of Australian Merino sheep

Context Reproductive efficiency in ewes can vary widely depending on their fertility (pregnancy rate) and fecundity (litter size) and on their ability to rear multiple litters. Litter size and sex are important factors that contribute to lamb mortality, and hence reproductive efficiency. However, little is known about their interaction when fetuses of either sex grow in utero as co-twins, nor the impact that this interaction has on subsequent performance in domestic flocks.Aims and methods We examined the effect of type of birth (single, twin) and of sex of co-twin (F, F[F], F[M], M, M[M], M[F] where the symbol within brackets indicates sex of co-twin) on birth weight, survival to weaning, weaning and hogget liveweights and scrotal circumference. In addition, reproductive performance (fertility, fecundity, lamb survival, weaning rate) of female progeny was examined as well as the performance of their lambs (birth and weaning weight). This large-scale study (>13 000 lambing observations) was conducted with Australian Merino sheep from four studs, mated over nine annual cycles.Key results Sex of co-twin had no effect on birth weight and on later stages of growth. Sex of co-twin altered survival, with fewer of the M[M] class surviving to weaning compared with other co-twin classes (P<0.05). Scrotal circumference did not vary between the male classes. Although reproductive performance (fecundity, lamb survival, weaning rate) was greater in ewes twin to a male (F[M]) than in single females (F) (P<0.05), this difference could be due to greater genetic merit of twin versus single-born progeny.Conclusions Reproductive performance of females with a male co-twin exceeded that of single females; the difference could be due to greater genetic merit of twin-born ewes rather than a direct effect of in utero environment. There was no difference between females with a male co-twin and females with a co-twin of the same sex. Reproductive capacity of males was not affected by co-twin class or type of birth.Implications These results suggest that an increase in twinning rate by managerial or genetic means will not result in undesirable side effects due to sex of co-twin in utero.