How Broodmare Nutrition Affects Her Foal
French researchers are on a quest to better understand how feeding pregnant mares and their growing foals affects the resulting young horse’s health and development. Recently, through an ongoing study, the team found that maternal overfeeding and underfeeding can affect foals’ bone growth, metabolism, and even testicular development.

The series of studies is called the FOETALIM project and is funded by the French Horse and Equitation Institute (IFCE) and the Fonds Eperon. In the most recent study, Pascale Chavatte-Palmer, DVM, PhD, of the French National Institute of Agronomical Research; PhD candidate Morgane Robles; and their colleagues followed up on Saddlebred mares that had been fed hay plus barley or hay alone during the last half of gestation. They wanted to study those feeding treatments’ effects on the foals as they matured.

While the foals all followed the same weight and height curves regardless of maternal nutrition, the hay-barley group had wider cannon bones than did the hay-only group at 18 and 24 months of age, Chavatte-Palmer said. They did not check the cannon width after 24 months to see if the differences had evened out over time.

In terms of glucose metabolism, the team found slight differences between groups in very young foals, but all differences had disappeared at weaning. Later on, however, the hay-barley group showed greater susceptibility to metabolic disorders related to insulin resistance when they went through an “overfeeding challenge,” said Chavatte-Palmer. When the foals were between the ages of 20 and 24 months, both groups received 140% of their recommended feed rations, with the goal of increasing weight gain by 500 grams (about a pound) per day. The hay-barley group foals showed a significant increase in insulin resistance, whereas insulin sensitivity did not change in the hay-only group foals during the overfeeding period, she said.

Surprisingly, however, the hay-barley foals did not have a greater incidence of osteochondrosis than the hay-only foals, even though osteochondrosis is known to be associated with enhanced insulin resistance, Chavatte-Palmer added. How can this phenomenon be explained?

The researchers have come up with a plausible theory, though it still needs testing: The foals received the rations at least four times a day through automatic feeders during winter, instead of the traditional two meals a day. Dividing up the rations that way could have kept the glycemic peaks to a minimum, she said. While insulin resistance was down, individual after-meal sugar highs were lower than they would have been with less frequent, larger meals. It’s possible, they suggested, that those glycemic peaks are behind the osteochondrosis. Moreover, as osteochondrosis only develops until a foal is about a year and a half old, overnutrition after 20 months could not affect this pathology’s development in the two groups.

As for sexual maturation, the prepubertal colts in the hay-barley group had more mature testicles at castration (12 months of age) than the hay-only group. What effects that earlier maturation might have on fertility are still unknown, but future studies should give more insight.

With that in mind, how should breeders feed their mares? “Above all, avoid concentrated feeds as much as possible in gestating mares because that’s associated with an increased risk of osteochondrosis in their foals as well as an upset in glycemic regulation in youngsters,” said Chavatte-Palmer. “It also seems to affect sexual maturity, although we don’t yet know how puberty as a whole, or fertility, is affected.”

Even so, the researchers question how underfeeding a pregnant mare could affect her subsequent fertility for the next season, Chavatte-Palmer said. That’s yet another topic to explore as the FOETALIM project advances.

The study, “Maternal Nutrition during Pregnancy Affects Testicular and Bone Development, Glucose Metabolism and Response to Overnutrition in Weaned Horses Up to Two Years,” was published in PLoS One.