Aspects of reproductive performance in small ruminants--opportunities and challenges

Ovulation rate is the principal source of variation in the reproductive rate of small ruminants. There is extensive variability in this trait both in terms of major genes and polygenes. Identification of the DNA changes responsible for the Booroola and Inverdale effects enabled testing for these mutations in any population. Surprisingly, in only one of six populations, with segregation of major genes for ovulation rate, could the effect be attributed to one of these mutations. Current evidence shows that there are five distinct loci that have large effects on ovulation rate. Selection on ovulation rate in Finn sheep produced a 2.2-fold change without any evidence for major gene involvement. The response appears to reflect changes in the thresholds that control the number of ova shed, indicating that a fundamentally different mechanism is involved from those shown for Booroola and Inverdale genes, and probably other major genes. The results also indicate that the variability of ovulation rate, for a given mean value, can be reduced by genetic selection. This has direct implications for neonatal survival, which declines rapidly as the size of the litter at birth increases above two - a major limitation on the exploitation of the known major genes. The effectiveness of genetic improvement programmes, especially in sheep, would be greatly enhanced if the problem of poor pregnancy rate from cervical insemination of frozen-thawed semen was solved. Recent studies reveal major differences among ewe breeds and large ram-to-ram variation in pregnancy rate from cervical AI with frozen-thawed semen. Identification of the basis for ram-to-ram variation could be exploited immediately in selecting rams for AI. Progress in this area would greatly facilitate the exploitation of molecular genetic information in the genetic improvement of small ruminants.