RESUMO
Introduction of the Inverdale prolificacy gene (FecXI) could markedly improve reproductive efficiency in commercial flocks, but as homozygous carrier Inverdale ewes are infertile, it is imperative that these animals are identified at an early age and excluded from breeding stock. As the ovaries of homozygous carrier ewes are nonfunctional, there are wide differences in reproductive hormone levels between these and other Inverdale genotypes. This study assesses the accuracy of using hormone concentrations alone, to identify infertile homozygous ewe lambs. Ewe lambs were blood sampled at 2, 5 and/or 8 months of age, and plasma analyzed for follicle-stimulating hormone (FSH), luteinizing hormone (LH) and inhibin content. These animals were either the offspring of both known carrier rams and known carrier ewes, and therefore would be either homozygous (II) or heterozygous (I+) for the Inverdale gene (group 1, N = 122), or had one parent that was a carrier and therefore would be either heterozygous or noncarriers (++) of the gene (group 2, N = 32). Animals were designated as either II or I+/++ on the basis of their plasma hormone concentrations. Inverdale genotype was also assigned from laparoscopic observation of the ovaries at each of these occasions. Definitive assignment of genotype was made at laparoscopy as adults during the breeding season. On the basis of laparoscopy as adults, 62 (51%) lambs in group 1 were identified as homozygous and 60 (49%) as heterozygous. At all three ages, both mean FSH and mean LH concentrations were significantly higher in II than in I+ lambs. Mean inhibin concentrations were significantly lower in II lambs at 8 months, but did not differ significantly between genotypes at 2 or 5 month of age. The use of discriminant analysis techniques to segregate individual animals in group 1 on the basis of their plasma FSH and LH concentrations, correctly identified Inverdale genotype in 50/52 (96%) lambs at 2 months, 75/79 (95%) at 5 months and 118/122 (97%) at 8 months of age. Discriminant analysis was equally effective for segregating II ewe lambs (group 1) from fertile ewe lambs of I+ and ++ genotype (group 2, 97% correct at 5 months and 98% at 8 months). At no stage did inclusion of inhibin concentrations into the discriminant function alter the number of homozygous ewes misclassified. This demonstrates that infertile homozygous ewe lambs can accurately be distinguished from their fertile flockmates by using plasma concentrations of gonadotrophins alone, and that this can be achieved from as early as 2 months of age.