Mares bred using artificial insemination have about a 50% chance of producing a live foal per cycle. Moreover, 60% of early equine embryonic losses occur within the first six weeks of pregnancy. While the causes of pregnancy failure and embryonic loss are many, one easily recognizable and avoidable reason is mare obesity.

Tom Stout, VetMB, PhD, of the Department of Equine Sciences at Utrecht University, in the Netherlands, and colleagues conducted a study in obese mares looking at early embryonic death (D’Fonseca NMM et al., 2021).

“It has long been suspected that body condition plays an important role in optimizing fertility, although the emphasis has generally been on mares in poor condition,” he said. “And while it has long been suspected that obesity is also undesirable in broodmares, it has been more difficult to prove that it compromises fertility.”

Embryo Transfer in Obese and Normal Mares

To improve our understanding of obesity’s effects on mare reproductive performance, Stout’s team fed a group of Shetland ponies 200% of their net energy requirements. Their diets included concentrate feed, grass hay, and a ration balancer, with hay making up 42.5% of the diet and concentrate feed the rest. Control mares received only 100% of their calculated net energy requirements, consisting of primarily hay (85%) and a small amount of concentrate (15%). The researchers adjusted the control group’s feed as needed to maintain moderate body condition (4 to 6 on a 9-point scale).

Stout’s team artificially inseminated obese and normal mares on Day 0. On Day 7, they flushed the embryos and transferred them to recipient mares as one would in a typical embryo transfer program. Recipient mares—that were also either obese or in good body condition—underwent ultrasound examinations to assess pregnancy on Days 21 and 28 of the study (i.e., Day 14 and 21 after embryo transfer). The researchers recovered all embryos on Day 28 of the study (21 days after embryo transfer) and examined them microscopically.

“The most important finding of this study was that when embryos recovered from obese donor mares were transferred into recipient mares, only around 60% survived up to Day 28,” said Stout. “This included 56% in obese recipients and 63% in normal weight recipients. In contrast, when the embryo was recovered from a mare in normal body condition, 100% survived to Day 28 irrespective of the body condition of the recipient mare. Thus, donor mare obesity appears to negatively affect early embryonic development, presumably as a result of lipid accumulation or effects on metabolic function of the embryos.”

Other parameters, such as the occurrence of hemorrhagic anovulatory follicles that fail to produce on oocyte (egg) and microscopic analysis of embryonic development, did not show obvious differences between obese and normal donor mares.

On the other hand, obesity did result in mares having a considerably shorter period of winter anestrus (when they’re not cycling), meaning they would have extra opportunities to get pregnant early in the breeding season.

“Nevertheless, this study shows for the first time that excess body weight can negatively impact early embryonic survival in mares,” Stout said.

Obese Mares Might Also Be at Increased Risk of Laminitis

During the three-year study period, the researchers also noted that obese mares “exhibited a slightly stiff gait and, on a hard floor, walked with short strides, raising the suspicion of subclinical laminitis.”

The altered gait could be caused by endocrine-associated laminitis due to increased insulin levels circulating in the horse’s bloodstream (i.e., hyperinsulinemia).

“Treating both laminitis and hyperinsulinemia during pregnancy may be challenging,” said Stout. “Progesterone seems to increase insulin resistance and, as gestation proceeds, it may become more challenging to balance weight loss with sufficient nutrition to support fetal growth. Moreover, pregnancy-related weight gain is likely to exacerbate the hoof pain.”

In addition to insulin dysregulation and laminitis, other obesity-related consequences of importance include the development of osteoarthritis, gestational hypertension (high blood pressure), elevated systemic inflammation, and a potentially higher rate of dystocia (difficult birth) due to obese mares having heavier foals.

Take-Home Message

“The incidence of obesity in horses is at an all-time high and is actually considered an epidemic and major welfare issue by many equine experts,” said Stout. “Because early embryonic loss is one of the major contributors to a disappointing foaling rate, managing a broodmare’s BCS to remain within a healthy midrange is an important part of optimizing success, even when the mare is used as an embryo donor.”

The study, “Overfeeding Extends the Period of Annual Cyclicity but Increases the Risk of Early Embryonic Death in Shetland Pony Mares,” was published in 2021 in the journal Animals (Basel).