Reproductive performance and factors that decrease pregnancy rate in heavydraft horses bred at the foal heat

Reproductive performance and factors that decrease the pregnancy rate in he avy draft horses bred at the foal heat were investigated. The study was bas ed on a total of 422 mares varying from 4 to 20 years in age, which had foa led during the period 1991 to 1994. Over the four years, the percentage of the mares bred at the foal heat was 62.6% and the pregnancy rate was 60.2%. The pregnancy rate following breeding at the foal heat in 1991 (67.7%) was significantly higher than that in 1992 (44.1%). Significant correlation wa s observed between the pregnancy rate and the mean placental retention time and also between the pregnancy rate and the mean temperature during the wi nter season. When compared on a monthly basis, the pregnancy rate following breeding at the foal heat in February was 53.3% and in May, 48.2%. The mea n number of covers per mare at the foal heat was 2.3; this figure decreased to 1.8 for those bred in May. The mean number of days from parturition to the first breeding date was 11.3 days and the number of days from parturiti on to the last breeding day at the foal heat was 14.0 days. The number of d ays from parturition to the last breeding day at the foal heat decreased to 12.5 days in May. The latter may have been influenced by factors involving the stallion. When compared on a monthly basis, the mean placental retenti on time was longest in January (219 min.) following which it decreased grad ually up to mid-summer. The mean placental retention time of mares which we re not bred at the foal heat was significantly longer (197 min.) than in ma res bred at the foaling heat (143 min.) and also in mares that conceived fo llowing breeding at the foal heat (135 min.). The pregnancy rate was less i n mares bred shortly after parturition. The findings of this study would su ggest that certain factors decrease the pregnancy rate in mares bred at the foal heat. The stress of low temperature in winter, prolonged placental re tention time, breeding too early after parturition and factors pertaining t o individual stallions may be involved. Expulsion of the placenta as soon a s possible after foaling and appropriate breeding timing are significant fa ctors that can influence the pregnancy rate following breeding at the foal heat.