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THE INFLUENCE OF BREED OF EWE AND OFFSPRING ON LITTER SIZE AFTER EGG TRANSFER IN SHEEP

Summary.: In order to investigate the influences of maternal and embryonic genotype on litter size, fertilized eggs were transferred, at a rate of five per recipient, from Clun Forest ewes to Romney Marsh, Suffolk and Finnish Landrace ewes, and also from Romney Marsh and Finnish Landrace ewes to Romney Marsh ewes. The numbers of natural ovulations (corpora lutea) were counted in recipient ewes at the time of transfer. All ewes which became pregnant were allowed to lamb.

Differences in conception rate and litter size at birth between the three breeds which received Clun Forest eggs were not significant, but, taken together, these differences resulted in the survival of significantly more of the transferred eggs when the recipient ewes were Finnish Landrace (63%) than when they were Romney Marsh (47%) or Suffolk (43%). In Romney Marsh ewes, the survival rate of Romney Marsh eggs tended to be poorer (33%) than that of alien Clun Forest (47%) or Finnish Landrace (44%) eggs but this effect was not significant.

Irrespective of the genotype of the eggs transferred, the mean size of litters from Romney Marsh ewes following transfer of five eggs was significantly greater than the potential litter size of this breed determined by natural ovulation rate.

It was concluded that ovulation rate was the major factor limiting fertility in the Romney Marsh breed. Differences do seem to exist between breeds of sheep in the numbers of offspring which the uterus is able to support, but, in the breeds studied, this control did not appear to limit litter size below levels which are desirable in practice.