Determinants of gestation length in Thoroughbred mares on German stud farms.

The aim of the present study was to analyze the effects of stallion and mare, their ages, and maternal lineage on the gestation length (GL) in Thoroughbreds. In addition, additive genetic effects of the dam, stallion and fetus were analyzed. Data were taken from 1993 through 2009, and included 16,226 pregnancies from 5959 Th oroughbred mares mated with 290 different stallions. All analyses were performed using linear mixed models. The GL ranged from 306 to 390 days, with a mean length of 347.0 ± 14.4 days. Mating of mares with stallions aged 17 years and older resulted in a significantly longer GL compared to younger stallions. Furthermore, the GL significantly increased with the increasing age of the mares, and the GL was longer with male foals. The month and year of breeding, as well as the mare´s breeding history (parity and reproductive status) also affected GL. The mare and stallion themselves explained 18% and 4% of the variance in GL. Coefficients of inbreeding of mares and foals had no significant effect on GL. The heritability for the GL was 0.17 for the dam and 0.006 for the fetus, whereas an additive genetic paternal effect was not estimable. The relative proportions among the additive genetic and permanent environmental contributions of the dam were 76.5% and 23.5%. A maternal lineage effect was not obvious.