The Kester News Hour at the annual American Association of Equine Practitioners Convention showcases key developments around the globe in equine research. In the 2024 edition Maria Schnobrich, VMD, Dipl. ACT, of Rood & Riddle Equine Hospital in Lexington, Kentucky, highlighted three recently published equine reproduction studies.

Factors Affecting Mare Pregnancy Rates

The researchers’ objective in the first study Schnobrich described (Pasch et al., 2024) was to evaluate how the number of straws provided as a breeding dose—as well as other mare, stallion, and management factors—affected pregnancy outcomes with cryopreserved semen.

“The key takeaway message was that mares may not need to be checked as frequently for ovulation as previously thought,” said Schnobrich. “The pregnancy rates were similar for mares bred one time up to six hours post-ovulation compared to mares bred within three hours of ovulation.”

The researchers conducted this retrospective study over 11 years and included 624 estrous cycles. They induced ovulation in the mares with deslorelin (a synthetic analog of gonadotropin-releasing hormone), which is a common technique. The authors compared a single insemination post-ovulation versus a timed protocol with insemination occurring approximately 24 hours after they administered deslorelin and pre ovulation, and again within six hours post-ovulation.

“These are exciting findings because it means we don’t need to be as panicked to tighten the ovulation to insemination window post-ovulation,” said Schnobrich. “And, for the stallions in this study, there was no significant effect of the number of straws used for insemination on pregnancy rate.”

Effects of Obesity During Pregnancy in Horses

Schnobrich presented a study (Robles et al., 2023) in which the researchers described the effects of obesity during equine pregnancy on placental structure, gene expression, and colostrum and milk fatty acid concentration.

“This study built on the findings from a previous study demonstrating a negative impact of obesity in pregnant mares on their offspring (Robles, 2018),” said Schnobrich. Specifically, foals born to obese dams had higher inflammation and increased prevalence of osteochondrosis lesions (53%) at 6 months of age compared to foals born from normal body condition mares (10%).

In this equine reproduction study the researchers compared placentas and milk quality in obese mares and normal mares. They did not observe any significant differences in the placentas between the two groups, but they found the colostrum from obese mares lower in medium-chain fatty acids, and their milk had a more proinflammatory profile.

“These findings again emphasize that obesity in the pregnant mare should be avoided,” said Schnobrich. “As we see an increase in obesity in our population of broodmares, this further highlights our need to be diligent about a healthy body condition during pregnancy.”

Equine Pregnancy Success Rates After Twin Reductions

Veterinarians eliminate equine twins by manual crush of one twin approximately 14 to 16 days post ovulation with minimal adverse effect on the remaining pregnancy. For those that either go undiagnosed until later in pregnancy or have not naturally reduced on their own, the options for elimination include transvaginal aspiration. This method, however, offers approximately a  50% chance of success.

Schnobrich presented a retrospective study (Sielhorst et al., 2024), in which the researchers examined nine years of data on dizygotic twins (the result of two separate eggs) reduced using transvaginal ultrasound-guided aspiration. They studied a variety of factors (i.e., position of pregnancy, medications used, technique of procedure), and considered pregnancy outcomes at three different time points following the pregnancy (five to seven days, three to four weeks, and at live foal delivery).

“This study helps us understand when to utilize this procedure and what helps optimize outcomes,” said Schnobrich. “Intervention in most cases of twins is required as in cases of bilateral twins (one pregnancy in each uterine horn). Most that are there at 16 days post-ovulation will be there at 40 days.”

The authors recommended veterinarians reduce one twin between Days 28 and 32, and definitely before the endometrial cups form at Day 35, so the mare can be bred back if she loses the pregnancy.

Take-Home Message

Schnobrich said these equine reproduction studies offered practical implications for veterinarians and breeders and that, collectively, the findings emphasize the importance of timely interventions, careful monitoring of broodmare condition, and evidence-based decision-making.