pregnancy loss

No breeder wants a mare to lose a fetus or foal. But researchers have learned that when pregnancy loss is detected early enough, some mares can be rebred successfully in the same breeding season with decent foaling rates.

In a study of 82 Thoroughbred mares rebred after early pregnancy loss, 57.3% delivered a live foal the following spring, said Yasuo Nambo, DVM, PhD, of the Obihiro University of Agriculture and Veterinary Medicine, in Hokkaido, Japan.

“The observed foaling rate of the mares following rebreeding in the same reproductive season after pregnancy loss was lower than the foaling rates of 69–79% that have been reported in previous studies on Thoroughbred mares,” the researchers noted. “However, it was still relatively good considering that all of the mares had experienced pregnancy loss.”

However, mare age, body condition score (BCS), and when veterinarians detect pregnancy loss could help—or hindered—rebreeding success.

Rebred mares between 3 and 8 years old had a 74.1% live-foaling rate, while mares aged 14 to 18 had a significantly lower rate of rebreeding success at 42.9%.

Meanwhile, foaling rates decreased the later into gestation the pregnancy loss was detected (66.7% when veterinarians dected loss before Day 29 compared to 42.9% when it was dectected after Day 42).

And, mares with lower body conditions were less likely to have rebreeding success. Previous study results have found that nutrition and poor body condition have a significant effect on pregnancy loss in mares.

“To reduce pregnancy loss, mares should be maintained at BCS of greater than 5.5 during breeding season and lactation period,” Nambo said.

“The most appropriate course of action is to diagnose pregnancy loss as soon as possible,” Nambo said.

To achieve this, he said, conduct exams regularly. Veterinarians typically check to see if a mare is in foal around 10 days to two weeks after breeding. Once pregnancy is initially confirmed, Nambo recommended following up with transrectal ultrasounds twice, for example, on Days 28 and 35. This allows for early loss detection and provides an opportunity to rebreed.

The study, “Foaling rate of mares that were rebred after pregnancy loss in Hidaka, Japan,” was published in the Journal of Equine Science.