• horse;
  • Thoroughbred;
  • conception rate;
  • pregnancy loss rate;
  • twin conception;
  • foaling rate


Reasons for performing study: Previous surveys of reproductive efficiency in British Thoroughbreds included only mares and stallions standing on studfarms in and around Newmarket. The present study was widened to compare Flatrace (FR) (Group A) and National Hunt (NH) (Group B) mares and stallions on studfarms throughout England.

Objectives: To assess the influences of mare type, status and age, and veterinary manipulations on reproductive efficiency parameters. To compare the inherent fertility of stallions, based on singleton and twin pregnancy rates and pregnancy loss rates, in Groups A and B Thoroughbred breeding stock.

Methods: Managers of 24 FR and 9 NH public studfarms were asked to complete a questionnaire for each mated oestrous cycle shown by 2321 Group A and 1052 Group B mares throughout the 2002 mating season. Parameters such as per cycle singleton and twin pregnancy rates, and pregnancy loss rates were noted, and the success of hormone treatments to induce oestrus and ovulation assessed. The number of matings per oestrus and per pregnancy were recorded, together with the incidence and effectiveness of uterine and other veterinary treatments. The inherent fertility of 84 Group A and 43 Group B stallions in the study, as measured by the singleton and twin early pregnancy rates and the pregnancy loss rates recorded in the mares they mated, was also estimated.

Results: Per cycle early pregnancy (Days 13–16) was 63.2% for Group A and 65.3% for Group B mares; and 10.3% and 13.1%, respectively, of those pregnancies were twins or triplets. Early, middle and late pregnancy loss rates were 7.2% vs. 8.0% (Days 15–42), 3.6% vs. 6.1% (Days 42–1st October) and 2.7% vs. 2.1% (October-foaling), respectively. Matings per oestrus and per early pregnancy were significantly higher in Group B vs. Group A mares. For stallions that mated ≥30 mares, overall early pregnancy rates per cycle in mares mated ranged from 30–89% across the 2 groups.

Conclusions: No major differences in reproductive efficiency were identified between FR and NH mares and stallions. Increasing mare age was the single biggest limiting factor to an otherwise high rate of fertility in well-managed English Thoroughbreds.

Potential relevance: This study identified factors that influence reproductive efficiency in the Thoroughbred.