Litter size, ovulation rate and prenatal survival in relation to ewe body weight: genetics review.

Lamb's birth weight and litter weight were found to be related to ewe weight. Deviations from generally applicable formulae indicated an interaction between the genotype of the lamb and the maternal environment, as suggested in egg transfer experiments. Land's (1977) hypothesis on the spare conceptus capacity, when comparing large and small breeds within a species, with respect to prolificacy, was more applicable to genetically homogeneous groups within a breed than between breeds and lines within a breed. A hypothesis suggests that variation in the within breed genetic correlation between ewe weight and prolificacy may largely depend on the relative uniformity of selection criteria applied. It can further be argued whether selection based on body weight or growth rate, compared to selection on reproductive performance, could be important sources of within breed as well as among breed variations in the genetic correlation estimates between ewe weight and prolificacy. The dynamics of the genetic correlation between ewe weight and prolificacy has an effect of long-term selection for body weight, growth rate and reproduction traits. This leads to the problem of the relative incompatibility of selection of an association between body proportions with reproductive traits, especially in meat type breeds. There is a parallellism or antagonism in the genetic correlations between ewe weight and prolificacy as compared to those between ewe weight and other reproduction traits. The within breed relationship between ewe weight and ovulation rate was found to be positive in some breeds. The effects of body condition at mating were contradictory. Genotypexenvironment interactions were observed in certain cases. Among most breeds, differences in ewe weight were unrelated to differences in ovulation rate. There were no relationships between ewe weight and prenatal survival within breeds or in lines within a breed. However, there exists a differential relationship between ovulation rates and prenatal survival rates in relation to ewe weight, depending on the selection criteria applied. A clearcut relationship between litter weight components and ewe weight cannot be generalized but may vary among differentially selected breeds and lines within them. This is a final, unifying and unresolved question.