Studies in flock mating of sheep. 2. Fertilization and prenatal mortality

Abstract

Two flocks of 100 and 200 Merino ewes were used to assess the distribution of sperm in the ewe genital tract, the proportion of eggs fertilized, and early loss of embryos, and to relate these to the number of services. The number of sperm recovered from the genital tract was, in general, higher in ewes served more than once than in ewes served once only, but variation between ewes was high. In the flock of 100 ewes it was estimated that 96 per cent of the mated ewes had fertilized eggs, but only 76 per cent were pregnant 20 days post coitum (p.c). In the other flock 90 per cent of the mated ewes killed 1-2 days p.c had fertilized eggs, and 86 and 84.5 per cent of the mated ewes killed 30 and 60 days p.c. respectively, were pregnant. In both flocks the proportion of ewes pregnant 20-60 days p.c. was higher in ewes served thrice or more than in those served once only. Ejaculates were collected from two of the rams with the artificial vagina each morning during the mating periods. The number of spermatozoa m the ejaculates fell precipitately during the first two days, but thereafter was relatively constant. Subsequently, the mating periods were simulated by the collection from the same rams of 66 ejaculates over four days. The mean number of spermatozoa per ejaculate fell to 250-400 million during the second day and was maintained at about that level through the next two days.