To produce a healthy foal, a broodmare must take in adequate nutrients prior to and throughout gestation, but nutrition is especially critical during the first and last trimester of pregnancy. In horses, much of the fetus’s development takes place in latter gestation.
The National Research Council (NRC) notes that during gestation, dietary energy provides not only for the mare’s maintenance but also for deposition of fetal and placental tissues, uterine hypertrophy, mammary development and fetal maintenance.1
As gestation proceeds, the mare gains weight—0.3 to 0.8 lbs per day. Mares have been observed to gain a lot of weight during midgestation (second trimester), and some gain little or no weight during latter gestation yet deliver a healthy, normal foal. At parturition, fetal tissue generally accounts for about 10% of dam weight.
During most of pregnancy (from conception to eight months), food intake and body weight should be monitored carefully; most equine nutritionists and practitioners don’t believe pregnant mares need an excess of dietary energy. Once a mare reaches late pregnancy (months nine to 11), her needs change. At that time proper foal development requires a modest increase in energy content and protein concentration, which is best achieved by adding some grain and maximizing protein rather than feeding solely timothy or other grass sources.
During the final trimester, a fetus increases its weight by 1 lb per day, accounting for two-thirds of fetal growth. Naturally, mares are inclined to eat sufficient amounts when available to build up fat stores to be used during latter gestation. However, some tend to back off feed due to the lack of room in the gastrointestinal tract from the fetus’s presence in the body cavity.
Although it’s been suggested that one can feed a mare at maintenance energy during the first two-thirds of pregnancy, recent data suggests that the first trimester is a critical time for the fetus and certain nutrients may need to be monitored carefully.2,3 For one thing, underfeeding may cause embryonic loss in the first 36 days of gestation. As Dirk Vanderwall, DVM, PhD, DACT, of Utah State University states, “Proper nutrition is important for optimum fertility in the mare. All indications are that to maximize reproductive efficiency and minimize early embryonic loss, mares should receive good quality feedstuffs in sufficient quantity to maintain optimum body condition.”3
Body condition score
Here’s what to consider regarding body condition score (BCS) when you’re examining a mare:
- A BCS of 5 to 6 (moderate to fleshy) means a mare is not too thrifty nor overweight and is at an optimal weight; the mare has a better chance of conceiving.
- During pregnancy, a BCS significantly more than 6 is not ideal. A BCS greater than 6 is apparent in extra rump fat in a mare; this is as unhealthy as if the mare were underfed.
- Prior to breeding and in pregnant mares, a BCS of 7 to 9 is a definite concern. With a score of 7 a mare feels “fleshy”; fat begins to fill in among the ribs, around the tailhead, along the withers, behind the shoulders and along the neck.
- A BCS of 8 indicates fat, with a crease down the back. It’s difficult to feel the horse’s ribs and the fat around the tailhead is very soft. The area along the withers is filled in with fat and there is a thickening of the neck and along the inner buttocks. This is undesirable.
- A BCS of 9 indicates extreme fat and is highly undesirable for a mare before and during pregnancy.
Although the impact of maternal nutrition on fetal development in the equine has not been thoroughly investigated, overnutrition is a common occurrence in the industry.2 According to Josie Coverdale, PhD, associate professor of equine science at Texas A&M University (TAMU), and her colleagues, permanent changes in the fetus due to improper nutrition of the mare during gestation can lead to abnormalities in the adult horse: “The long-term influences of maternal over-nutrition can affect fetal development, and impact placental function and transfer of nutrients to the fetus. The resultant reduction of transfer of nutrients to the fetus may result in slower cell division, which can reduce organ cell number.”2
They note particularly that mare glucose and insulin dynamics are influenced by maternal nutrition. In their study, mare glucose and insulin increased with more concentrate supplementation, resulting in altered glucose tolerance in the mare and foal. In mares, a high-starch diet promotes higher levels of insulin and may lead to unfavorable metabolic programming of the foal. Obesity in mares due to feeding of high-energy (high-concentrate) diets may lead to reduced glucose tolerance, altered pancreatic function, reduced insulin sensitivity and modified body composition in the foal.
In addition, improper micronutrient levels may affect the balance of antioxidants and free radical formation; therefore, copper, zinc, iron, selenium, vitamin E and beta-carotene may have significant roles to play in mitigating intrauterine growth retardation (IUGR).2
“The concept of fetal programming is not new,” Coverdale says. “IUGR was first a clinical observation in Dutch women who experienced famine at various times during gestation.”
The children of the women who endured the Dutch famine had higher incidences of obesity, cardiovascular disturbances and health-related problems later in life, she explains. “We know from that initial Dutch observation that there’s an effect of nutrition of the dam, regardless of species, that iwmpacts fetal growth and development,” Coverdale says.
Of course, overnutrition is more likely to a problem than undernutrition in thoroughbreds and quarter horses. “Wanting to take good care of those mares, we overfeed them rather than underfeed them,” she says.
Coverdale has investigated whether the equine fetus’s metabolism (e.g., glucose and insulin levels) can be permanently altered based on the mare’s diet. “The general premise is that the diet the dam receives results in long-term changes in the fetus,” she says. “We’ve yet to see a substantial difference on gestation length, birth weight or growth patterns in horses. But we have seen differences in metabolism, especially alterations in glucose and insulin.”
So what take-home message did Coverdale find for equine practitioners and horse owners? That “the equine nutrient guidelines we have from the NRC are spot-on,” she says. “The BCS system developed during the mid-1980s is still fairly accurate. The only correction I would make is that while a BCS of 6 is ideal, a BCS of substantially greater than 6 is most likely detrimental.” While a BCS below 6 might harm a mare’s reproductive efficiency, she says, a BCS greater than 6 can harm fetal development.
While feeding a mare at maintenance energy during early pregnancy is still recommended, the role of functional nutrients such as amino acids, vitamins and trace minerals during early gestation is poorly understood. “We still lack sufficient data on those nutrients that are involved with metabolism, enzyme pathways and so on,” she says. “There’s a lot more complexity to it; they do more than solely meet a mare’s energy requirements.
“Is there a way to enhance embryonic survival and perfect the initial stages of genetic development as well?” Coverdale continues. “That’s what we believe the nutrient demand of the first trimester does—it’s involved in the expression of genes. Certainly a fetus is programmed to be what it is genetically, but is there a nutritional component we’re still not aware of that affects foal phenotype during the first trimester?”
Still, she says, “We’ve got to be cautious not to concede to the adage that if a little bit is good, a lot must be better. Our data provide justification for keeping mares more at the traditionally acceptable BCS of 6. It’s unwise to spend extra money on excessive amounts of high-concentrate diets. It’s not only an unsafe management practice for the mare, it could potentially limit the metabolism of the developing foal.”
Holly Mason, DVM, is an assistant professor at Utah State University School of Veterinary Medicine. In practice she has seen a fair number of acquired contractural limb deformities, physitis (swelling around the growth plates of certain long bones in foals) and osteochondritis dissecans (OCD). “I wonder if we should be doing something different early on with these mares,” she says.
It’s not uncommon for broodmares to inadvertently be fed an unbalanced ration throughout gestation, Mason continues. “What we’re beginning to know about trace nutrients such as copper, zinc, vitamin E, selenium and the calcium-phosphorus ratio is critical,” she says. “I’d like to see more work done with mares during early pregnancy, as well as follow-up with subsequent foal crops with radiographs and clinical exams for any acquired contractural limb deformities, physitis or OCD.”
Mason agrees with Coverdale’s assessment that a mare in early pregnancy should be fed at maintenance energy per NRC recommendations. Additionally, she accedes that the role amino acids, vitamins and trace minerals is not entirely understood in the pregnant mare. “It can be a challenge to differentiate between nutrition, genetics, environment and other factors that impact animal health and development when you are trying to solve a problem or optimize an outcome,” Mason says.
Mason says she tries to work with her clients’ goals, whether they’re conservative or aggressive in exploring new strategies in broodmare nutrition. “I usually start by looking at their hay and evaluate what they’re currently feeding, looking at their concentrate if they’re feeding one or if it’s a vitamin-mineral supplement or a mineral salt block,” she says. “I get them to read the feed tag to ensure they’re providing the proper amount of daily feeding.When we neglect to reference the feed tag, it increases the chance that our horses are not receiving the recommended amounts of that product,” she says.
Mason also recommends a hay analysis. “If a client is feeding a thoroughbred on the East Coast during January through March, they’re likely to not be on the best pasture, and therefore hay quality is critical,” she says. “If owners really want to do what’s right from a nutritional perspective, they need to test hay frequently, testing every new batch or different portions of the batch. It’s not cost-prohibitive.”
Not only is there general variability among types of hay, but also geographic and price variability, she says. It’s common in Texas, for example, to feed coastal Bermuda grass hay. In California, it can be common to feed 100% alfalfa hay because it’s so readily available—even though it isn’t necessarily the best option, Mason says.
She says she likes to see a balanced intake of vitamins and minerals throughout gestation. “When we’re getting toward those last three months, I believe on focusing on the energy intake and protein content,” she says. “I caution my clients that, during a horse’s midgestation, they need to think about the mare’s BCS. And since BCS is a subjective measure, I recommend using scales or body weight tapes. Once we’re heading into the home stretch of gestation, mares should be on an increase of energy/concentrate and protein intake, but not too much.”
During the latter portion of gestation, the reproductive tract can compete with the gastrointestinal tract for space within the abdomen, and some mares won’t consume enough forage to meet their needs. “That’s when we emphasize concentrate feeds to ensure adequate daily energy intake,” Mason states. “If you’re noticing a decline of consumption during month 10, and the mare is leaving the bulkier roughages behind, it’s important to compensate by feeding concentrate or a pelleted or cubed roughage source.”
Mason says she would like to see further research on IUGR and associated endocrine abnormalities that might have lifelong influences. “It could have a significant impact on the industry if we had a better understanding,” she says.
“The work by Dr. Coverdale and her colleagues provokes thought about a lot of things that foals are exposed to in utero,” Mason continues. “There are still many open questions about how the nutritional plane of the mare affects the health of the foal.”
Still, thanks to the work of Coverdale and the practical insights of practitioners such as Mason, wise veterinarians will consider a mare’s nutrition from conception to parturition, keeping an eye on critical fetal programming issues, following the nutrition basics from the NRC and carefully monitoring the mare’s BCS.
1. National Research Council (NRC). Nutrient Requirements of Horses, 6th ed. Washington, DC: National Academy Press, 2007.
2. Coverdale JA, Hammer CJ, Walter KW. Horse Species Symposium: Nutritional programming and the impact on mare and foal performance. J Anim Sci 2015;93:3261-3267.
3. Vanderwall DK. Early embryonic loss in the mare. J Equine Vet Sci 2008;28:691-702.
4. Lawrence L, Camargo F. University of Kentucky Cooperative Extension Service. Feeding the broodmare: Four easy steps. Available at www2.a.uky.edu/agc/pubs/ASC/ASC185/ASC185.pdf.