The effect of age on D20, D40 and live foal rates in the Clydesdale mare

Reproductive performance is an important aspect of the equine industry. Breeders ideally want to increase the number of successful or valuable horses, whether these horses are racehorses, eventers, or show horses. This is especially important for breeds like the Clydesdale given the significant decline in breed numbers over the last century from over 20 000 registered horses in the 1920 s to less than 5 000 worldwide today. Mare age is a factor which has been shown to influence reproductive success in Thoroughbreds. However, limited work has been done on other breeds to investigate whether a similar association exists. The aims of this study were to determine the effect of mare age on (1) Day 20 pregnancy rates, (2) Day 40 pregnancy rates, and (3) live foal rates. Breeding records (n = 441) for 135 Clydesdale mares (aged 3–22) were reviewed. Generalised linear mixed models were used to determine the Day 20, Day 40, and live foal rates. Rates for country and farm were determined for live foal rates only. Mare was added as a random effect in all models to account for the fact that mares had multiple pregnancies. Mare age was categorised into four age groups: 3–6 years, 7–10 years, 11–14 years, >14 years and included as a fixed effect to look for differences across the age groups. Of the 441 pregnancies, 26 involved the use of hormone treatment. As a result, all models were re-run excluding any pregnancies where hormones were used to ensure consistency of results. The overall foaling rate (mean ± ) was 71% (66–76%). There was considerable variation in overall foal rate across farms (35% (13–65%) to 87% (66–96%)) but there was no significant difference ( P = 0.177). Country was significant ( P = 0.029). Mare age had a significant ( P = 0.021) effect on live foaling rates. Mares aged over 14 years had a decrease of 65% in the odds of having a successful pregnancy compared to a horse 3–6 years of age. In conclusion, age was a significant factor for reproductive success in Clydesdales, with mares 14 years of age or younger having the highest reproductive success.