THE TIME OF FOALING OF THOROUGHBRED MARES

Abstract

The length of pregnancy in the mare is known to be influenced both by the genotype of the foetus and the environment of the mother. The heritability of gestation length is 36% (Rollins & Howell, 1951), and male foetuses are carried longer than females (Uppenborn, 1933; Mauch, 1937). Mares mated between December and May have pregnancies that are on average 10 days longer than those of mares mated at other times of the year. This seasonal influence alone accounts for 44% of the observed variance in gestation length, and it appears to to be independent of nutritional factors (Howell & Rollins, 1951). In addition to these genetical and environmental factors that control the day of parturition, there appear to be other environmental factors that can even influence the hour of birth.