The Horse

SEP 2018

The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.

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Page 44 of 59

45 September 2018 THE HORSE Also, barren and maiden mares entering the breeding season in thin condition ap- pear 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 (ovar- ian) 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 gesta- tion 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 neo- natal 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 preg- nancy 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 thin- ner 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 af- ter foaling, to meet the minimum BCS to become pregnant again upon rebreeding. Overweight Obstacles To account for rapid fetal growth, ma- ny broodmares receive a high- concentrate diet throughout pregnancy that they re- main on as long as they're lactating. Con- cerns 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 Vet- erinary 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 developmen- tal 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 develop- ing metabolic disorders later in life. In a recently published study Robles et al. compared insulin sensitivity (cell re- sponse 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 developmen- tal 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. Low body condition during pregnancy likely results in an underdeveloped fetus and reduced placental function. BIANCA MCCARTY

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