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Broodmares: Worth Their Weight

By Kristen M. Janicki
 
A mare’s body condition can affect pregnancy outcome and set her foal up for success or failure later in life.
The apple doesn’t fall far from the tree. A chip off the old block. Logistically, these endearing idioms about parent-child similarities make sense because progeny receive their genetic blueprint from each parent, half maternal and half paternal. But a variety of other factors also dictate characteristics of offspring—including maternal diet during ­gestation—and horses are no exception. 
 
 
A pair of trailblazers from the University of Cambridge, in England, conducted one of the first studies looking at how maternal environment can affect foal development in 1938. They found that newborn Shire-Shetland Pony cross foals were proportional in weight to their dams and almost the same weight as purebred foals of the maternal breed. That is, crossbred foals with Shire mothers were similar in weight to full Shire foals, and crossbred foals with Shetland mothers were similar in weight to full Shetland foals. The authors summarized their findings as an illustration of the interplay between nutrition and genetic factors, both of which are involved in fetal ­development.
 
During pregnancy the mare’s daily nutrient requirements increase to allow her to maintain body condition while supporting the growing fetus. During gestation she might gain 12-15% of her initial body weight, mostly attributable to fetal and placental tissues. The amount of energy, or calories, she needs above maintenance (when she’s not pregnant) levels typically doesn’t rise until the fifth month of gestation, after which the majority of fetal development occurs. After foaling, a broodmare must produce enough nutritious milk for the foal, while maintaining her own energy requirements for metabolism, digestion, activity, thermal regulation, and waste production.
 
Mare health and nutrition during conception and pregnancy have been a recent focal point of research into foal growth and development, both in utero and after foaling. In this article we will focus on broodmare weight: What effect does her weight and, ultimately, energy or calorie intake have on her efficiency as a broodmare and on her foal?
 
Underweight Issues
 
Low body condition score (BCS) can affect several reproductive parameters in broodmares. Barren (nonpregnant mares that have foaled previously) and maiden (never carried a foal to term) mares entering the breeding season in thin condition, or less than 5 on the 1-to-9 BCS scale, experience lower pregnancy rates and require more estrous cycles to get pregnant. Also, barren and maiden mares entering the breeding season in thin condition appear to have delayed estrus and ovulation (Henneke et al., 1984).
 
“The effects of maternal malnutrition also have serious health implications, leading to incomplete ripening of (ovarian) follicles, delayed onset of estrous cycles in spring, irregular estrous cycles, and, therefore, reduced conception rates, which all affect a broodmare’s fertility,” says Sarah Morley, MS, an equine science and virology researcher who’s based in the U.K.
 
Restricting a broodmare’s calorie intake while she’s in foal can alter uterine prostaglandin production, inducing uterine contractions, and prolong gestation time (Hines et al, 1987). What little we know suggests that low broodmare BCS during pregnancy most likely results in an underdeveloped fetus and reduced placental function; the placenta provides oxygen and nutrients to the fetus, while removing waste products.
 
Although researchers have not yet established an optimal BCS for pregnant mares, breeding farms do find it more challenging to manage mare weight throughout pregnancy and lactation than during maintenance. However, this information is important because mare weight prior to foaling can influence neonatal birth weight. For instance, after a 2006 Streptococcus equi outbreak infected Thoroughbred mares midpregnancy, causing them to lose about 10% of their body weight, their foals weighed around 5% less at birth than foals from healthy mares (Wilsher et al., 2006). This is a severe case of weight loss, though, and moderate weight reductions during pregnancy don’t appear to have such drastic effects on foal birth weight.
 
Scientists haven’t confirmed whether mare weight influences the foal’s growth rate, though some studies suggest it. Foals born to pony mares fed a restricted calorie diet grew at the same rate as foals out of non-calorie-restricted mares, but restrictions only occurred through the last trimester of pregnancy (Pagan et al.,1984). When mares lost weight postpartum, their foals did not put on as much weight as foals from mares that gained weight postpartum (Pagan et al., 2006). The researchers believed the thinner mares produced less milk for their foals. Breeders can easily remedy this by allowing underweight broodmares to consume more calories and gain weight to assist their milk production.
 
In cases of undernutrition, broodmares need supplementary nutrition at least three months before and two months after foaling, to meet the minimum BCS to become pregnant again upon rebreeding.
 
Overweight Obstacles
 
To account for rapid fetal growth, many broodmares receive a high-­concentrate diet throughout pregnancy that they remain on as long as they’re lactating. Concerns over increasing foal birth weights due to overfeeding Thoroughbred mares prompted Celia Marr, BVMS, MVM, PhD, DEIM, Dipl. ECEIM, FRCVS, specialist in equine internal medicine with Rossdales LLP, in Newmarket, U.K., to study how mare obesity can affect foal size. “In a study recently published in Equine Veterinary Journal by our group, mare BCS correlated with foal birth weight; that is, obese mares had heavier foals,” she says.
 
 
Could maternal obesity play a role in the foal’s risk for developing developmental orthopedic diseases (DODs)? After reviewing more than 400 foaling records Marr found that high foal birth weight was associated with a higher prevalence of neonatal conformational defects.
 
Marr says she and coauthors noted “all forms of orthopedic problems not related to infection, including angular limb and flexural deformities.”
 
Plus, in a recent study out of France, researchers found that yearlings from obese mares were at a higher risk of developing osteochondrosis (OC, an umbrella term for abnormal cartilage development) lesions than yearlings from normal mares (Robles et al., 2018).
 
Mare obesity also appears to affect foal hormone concentrations, which might increase youngsters’ risk of developing metabolic disorders later in life. In a recently published study Robles et al. compared insulin sensitivity (cell response to release the hormone insulin to control blood glucose levels) and glucose tolerance (the ability to absorb and use it) in response to different diets in foals from normal or obese mares at 6, 12, and 18 months of age. Obese mares produced more foals that were insulin resistant at 6 and 18 months of age. Also, more yearlings from obese mares than normal mares were affected by OC lesions, but the research team found no developmental differences between the offspring at 6 or 18 months of age.
 
Researchers haven’t shown broodmare obesity to affect many reproductive parameters. Breeders, however, have reported that obese mares have shorter anovulatory (nonovulating, typically in winter) periods than normal mares.
 
Scientists have found links between obesity and associated insulin resistance and altered breeding season duration, notably causing an unusually long estrous cycle that can delay pregnancy.
 
“Age of the broodmare is an important factor and can be linked to obesity-related diseases,” says Morley. “In young mares in early pregnancy, placental restriction (when the placenta fails to properly support the growing fetus) may occur, leading to lowered birth weights, whereas obesity in the later stages of pregnancy may increase the risk of developmental orthopedic disease in weanlings.”
 
She says study results have also revealed broodmare obesity downregulates genes involved in the development of skeletal muscle and liver metabolic function in the fetus, resulting in increased fetal adiposity (fat). “Although birth weight in the overnourished offspring does not change, the body composition is altered to more fat and less muscle, which will possibly affect growth and performance of the adult horse,” she says.
 
Ideal Condition
A mare’s weight while pregnant can give you some information about fetal growth rate. Body condition scoring, however, is the best way to evaluate her fat cover. “We have studied Thoroughbreds specifically, but our research suggests that we need to develop body condition targets specifically for pregnant mares, rather than apply the guidelines that are used for nonpregnant horses,” says Marr.
 
Nutritionists agree: You should consider both BCS and body weight when making feeding management decisions. A research team from Egypt recently tried to correlate obesity—as determined by BCS—with several morphological (body structure) measurements, as well as blood and ultrasound markers. They found that back fat and several other physical measurements were the easiest ways to determine obesity in broodmares. Blood insulin, glucose, and triglycerides (a type of fat) were elevated slightly in obese mares, whereas leptin (a satiety hormone that basically tells the horse he’s full) concentrations were not valuable markers of obesity (Abo El-Maaty et al., 2017). For now, body condition scoring is the best way to assess broodmares’ fat cover.
 
Again, researchers have yet to establish an ideal BCS for broodmares, but we do know that mares entering the breeding season or foaling at a BCS of 5 or higher get pregnant on fewer estrous cycles than lower-BCS horses (Henneke et al., 1984).
 
“There have been many studies on broodmare BCS and fertility, and although horses with a BCS of 7 are usually classed as overweight, a broodmare requires a BCS of between 6.5 and 8 to maximize fertility and reproduction,” says Morley. “This initial adiposity at the start of the season may well enhance her fertility.”
 
In addition, feeding more energy before conception might optimize fertility rates and stimulate follicular growth in broodmares with a BCS of at least 6.
 
“Usually, broodmares with a BCS greater than 7 have normal pregnancies with no significant effect on gestation length, length of the foaling process, size of foal or placenta, or measures of foal viability,” says Morley. “Whereas mares with a BCS less than or equal to 4 might experience delayed postpartum intervals (time from foaling to next conception), more cycles per conception, lower conception rates, and increased early embryonic mortality.”
 
Morley cautions that even broodmares with a BCS of 6-7 can be at risk of reduced fertility and embryo resorption if their dietary energy is reduced abruptly. Therefore, it’s safe to allow broodmares to gain some weight, she says, although it might not be cost-effective.
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