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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46 THE HORSE September 2018 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 restric- tion (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 re- vealed broodmare obesity downregulates genes involved in the development of skel- etal 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 Thor- oughbreds 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 con- sider 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 usu- ally 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 inter- vals (time from foaling to next concep- tion), more cycles per conception, lower conception rates, and increased early embryonic mortality." Morley cautions that even brood- mares 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. Calorie and Nutrient Requirements A 1,200-pound mare requires approxi- mately 20,000 calories per day in early gestation, 26,000 during late gestation, and a whopping 38,000 calories per day during early lactation (the first three months after parturition). To encourage a broodmare to lose or gain weight, you must restrict or provide her with fewer or more calories, respec- tively, than she requires per day. Each increase in BCS might equate to feed- ing around 5,000 calories more per day, depending on the mare's age, breed, and health, among other factors. The type of calories in the diet can potentially affect rate of gain, too. That is, calories from fat might be more efficient at increas- ing body weight than calories from fiber. NUTRITION The average broodmare requires 38,000 calories a day during early lactation Body condition scoring, particularly over the back, is the best way to assess broodmares' fat cover. PAULA DA SILVA

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