GENOTYPE DIFFERENCES IN RESPONSES OF GROWTH AND CARCASS CHARACTERISTICS TO THE INTRAUTERINE COHABITANT PHENOMENON IN TWIN LAMBS

The preweaning growth and carcass characteristics of four lamb genotyp es were analysed for variation attributable to the intrauterine cohabi tant (IUC) phenomenon, where within-sex variation is attributed to the sex of a twin's womb-mate. The four genotypes resulted from the follo wing matings: Poll Dorset rams X Merino ewes, Poll Dorset rams X (Bord er Leicester X Merino) ewes, Texel rams X Merino ewes and Texel Yams X (Border Leicester X Merino) ewes. Four hundred and twenty twin lambs participated in the preweaning study and of these, 209 were slaughtere d to generate carcass data. Some of these genotypes displayed within-s ex variation in weaning weight and fat score, preweaning average daily gain, skin-fold thickness, carcass fatness and eye muscle dimensions attributable to the IUC phenomenon. Genotypic differences in the respo nses of lambs to the IUC phenomenon were also observed. The IUC phenom enon appears to involve prenatal programming, an hypothesis that attri butes postnatal characteristics to events during differentiation. The IUC phenomenon is analogous to the intrauterine position phenomenon in fecund mammals, where variations in prenatal steroid concentrations p rogramme for permanent alterations in postnatal reproductive character istics. The growth responses reported in this paper provide evidence o f variation due to prenatal programming While the magnitude of the res ponses to the IUC phenomenon were not large, the data presented indica te that under field conditions, the magnitude of growth responses to t he IUC phenomenon may be as great as those observed between breeds. If so, further examination of the role that an animal's IUC may have on its subsequent performance could aid the development of more sensitive indices for breed evaluation and progeny selection.