Ovulation Rate as the Selection Criterion for Litter Size in Sheep

Egg transfer experiments have consistently failed to find differences among sheep breeds in their ability to support a given number of fertilised eggs. The literature on egg survival rate in pregnant, multiple ovulating ewes is also remarkably consistent across a wide range of breeds and environments in terms of the ratio of foetuses (or lambs) to corpora lutea. Two reports on egg survival in ewes of the same breed but genetically different ovulation rates have also failed to demonstrate differences in egg survival. It is concluded that there is no gentic variation among ewes in their ability to support a given number of fertilised eggs (within the range l-6). It follows, therefore, that the genetic correlation between litter size and ovulation rate is unity. Consequently, selection for increased litter size is really using litter size as an index of ovulation rate. It is shown that the efficiency of this index is a function of average ovulation rate and declines as ovulation rate increases.