Genetics of Standardbred stallion reproductive performance

Reproductive performance of 10 Standardbred stallions was related to the probability that the embryo resulting from a given mating would be heterozygous for transferrin or plasma esterase. Fertility, measured by foaling rate per insemination or by foaling rate per year, showed a highly significant regression on the probability of offspring heterozygosity for transferrin and, to lesser extent, for esterase. Substantial differences between stallions in the slope of the regression line and no deficiency of foals homozygous for either protein suggests that the relationship to fertility is indirect, probably a reflection of the amount of outcrossing between lines within the breed. An example of the potential impact of maximizing the probability of foal transferrin heterozygosity on the productivity of an actual farm is presented.