Duration of gestational length of mares from the East Bulgarian breed and the influence of some factors on it

A study on the breeding age of mares, the duration of gestational length, and the influence of selected factors was conducted in mares of the East Bulgarian horse breed. The data used in the study was obtained from the breeding registers and mating records of mares reared at the State Stud Farm Kabiuk near Shumen and covered a long-term period of breeding activity. The analysis included data from 24 mares with a total of 74 registered pregnancies. The average gestational length in mares of the East Bulgarian breed was 333.8 ± 3.50 days. The results showed that the age of the mare had a highly significant effect on gestational length (p < 0.001), while line affiliation also exerted a statistically significant influence (p < 0.01). Mares from the middle reproductive age group showed a tendency toward shorter gestational periods, while younger mares had slightly longer values and older mares demonstrated similar durations with increased variability. Significant differences in gestational length were observed among the different breeding lines within the population. In contrast, the season of conception and family affiliation did not show a statistically reliable effect on the duration of gestation. These findings indicate that variation in gestational length in the East Bulgarian horse breed is mainly determined by age-related and genetic factors rather than the environmental conditions. Keywords: breeding age, East Bulgarian breed, gestational length, mares, reproductive performance, warmblood horses