Editor's note: This article is part of TheHorse.com's ongoing coverage of topics presented at the 2012 Kentucky Equine Research Conference, held May 17-18 in Lexington, Ky.
Developing a feeding plan for a single horse takes careful consideration. But planning a feeding schedule for broodmares necessitates even more though, as these horses are often consuming nutrients for two.
During a presentation at the 2012 Kentucky Equine Research (KER) Conference, held Mat 17-18 in Lexington, Ky., Peter Huntington, BVSc (Hons), MACVSc, MRCVS, director of equine nutrition at KER Australia, discussed some keys to address to make when feeding and managing broodmares at all stages of pregnancy.
"The nutrition of the mare makes a different in the health, growth, and performance of her foal throughout its life," Huntington noted, as an adverse fetal environment caused by poor broodmare nutrition can cause reduced or abnormal fetal growth.
"Broodmares are often 'forgotten' until the third trimester or lactation, but they might be overfed in early pregnancy," he added, stressing the importance of a balanced broodmare diet throughout the year.
Huntington pointed out several areas in broodmare nutrition that need special attention.
Body Condition--When feeding broodmares, aim to keep them at a body condition score of 5 to 7 on a 9-point scale, Huntington said. Thin mares that have a body condition score of less than 5 tend to have longer gestation periods, longer time between estrus cycles, and lower pregnancy rates. Some very thin mares also have decreased milk production, he noted. The mares also tend to produce smaller foals, he said.
On the other hand, Huntington said a study carried our using two groups of pregnant mares--one group had healthy body weights while the other group of mares was considered fat--showed no difference in foaling or pregnancy rates, but the feed costs for the overweight mares were twice as high as costs associated with mares at a healthy weight. Additionally, overweight mares run the risk of laminitis development, he added.
Energy Balance--Most broodmares need balanced energy in their diets, Huntington said. A positive energy balance means the mare is gaining or maintaining weight, while a negative balance means she is losing weight, he said. Monitoring a mare's body condition and weight on a regular basis will help an owner determine if her energy requirements are being met.
Mares in up to four months of gestation have similar energy requirements to a horse consuming a maintenance diet, Huntington said. After that their energy needs will increase as the fetus begins to grow larger, he said, and when she begins lactating, her energy needs will nearly double compared to mid-pregnancy.
Ensuring a mare has access adequate energy source to meet her increased requirements will help ensure a healthy foal with a good start to life, Huntington said.
Protein--"Protein is one of the most important nutrients for a successful breeding program," Huntington said. Previous research has shown that a protein deficiency in broodmares can result in reduced reproductive efficiency, early embryonic death, and reduced foal size, he said. Additionally, mares will draw upon protein reserves in their bodies to "feed" their fetus and aid in milk production if appropriate dietary protein isn't provided.
Mares that are open, barren, and in early pregnancy have similar protein requirements to horses on a maintenance diet. Beginning around five months of gestation, the mare's protein requirement increases monthly through to foaling to account for fetal protein accumulation and then has a dramatic rise after foaling to supply protein lost in milk production.
Huntington discussed one study that showed supplementing a mare's diet with 500 grams of soybean meal (a quality source of protein) two weeks prior to foaling and 750 grams of soybean meal 7 weeks after foaling increased the mare's milk crude protein content and increased foal growth rate.
Minerals--During pregnancy and lactation, broodmares' calcium, phosphorus, and magnesium requirements increase as well, Huntington said. All three macrominerals are important to aid in fetal bone growth in late gestation, and for foal bone development prior to weaning, he noted.
The National Research Council (NRC) recommends increasing broodmares' calcium intake from 20 grams per day in early gestation to 28 grams per day at seven months of gestation, and again to 36 grams per day in the final three months of pregnancy. Huntington relayed that lactating mares have a calcium requirement three times greater than nonpregnant mares, so ensure broodmares consume enough calcium while lactating.
In a similar pattern to calcium requirement increases, broodmares' phosphorus needs increase during gestation and lactation. Huntington said that during the last three months of gestation, mares' phosphorus requirements double over early pregnancy, and mares then see a 50% increase in phosphorus requirements during early lactation.
Finally, magnesium requirements are also believed to increase during late gestation and early lactation. Huntington estimates mares should consume 10 to 12 grams per day during gestation and upwards of 15 grams per day during lactation.
Trace Minerals--Zinc, copper, and manganese are all important trace minerals a fetus stores in its liver for use after parturition, Huntington said. As such, he discussed pregnant mares' nutrient requirements for these trace minerals.
The NRC recommends providing 500 kilogram (1,100 pounds) pregnant mares with 100 milligrams of copper daily until eight months of gestation. At that point the NRC suggests upping the amount to 125 milligrams per day until foaling. He also noted that one study suggested oral supplementation of copper was more effective than injecting the mare with the substance intramuscularly.
Zinc is important to foals; however there are no studies evaluating the relationship between mares' zinc intake and fetal bone development, Huntington said. Manganese deficiencies have not been observed in horses, he noted.
Two other important minerals to consider when formulating broodmare rations are iodine and selenium. Selenium supplementation has been shown to increase antibody levels in foals when mares consumed 0.3 milligrams per kilogram of the diet of the mineral when compared to mares consumed the NRC-recommended maintenance concentration of 0.1 milligrams per kilogram.
Iodine, while important to daily bodily functions, can cause goiter in foals if broodmares consume an excess amount during pregnancy, Huntington said. "It appears (from published reports) that around 50 milligrams of dietary iodine is required in the daily rations of mares to produce any incidence of goiter in their foals," he said. He also noted that iodine toxicities typically result from oversupplementation, so he recommended using caution when adding iodine to the diet.
"The best way of ensuring your mare gets adequate amounts of these minerals is to use the recommended intakes of a scientifically formulated fortified feed balancer pellet or broodmare feed," Huntington said.
Keep your horse healthy with the information in Understanding Equine Nutrition.
Vitamins--Not unlike minerals, vitamins are important to fetal growth and broodmare health. Huntington reviewed which vitamins are most crucial to consider when feeding pregnant and lactating mares:
- Vitamin A--Huntington said that while vitamin A is an essential part of numerous equine growth processes, both toxicities and deficiencies can be dangerous to young horses. The good news is that green forage and most prepared feed and supplements contain sufficient amounts, but not excessive amounts of vitamin A; thus, mares on green grass or eating recommended amounts of fortified broodmare feed generally consume adequate amounts of vitamin A in her daily feed without supplementation. But if your mare is in a dry field over winter or summer and getting hay then the vitamin A intake needs to be monitored.
 - Vitamin D--Vitamin D's main function, Huntington relayed, is maintaining a calcium balance in the bloodstream. Pregnant and lactating mares require about 3,300 international units (IU) of vitamin D daily, he said. Since vitamin D is synthesized and absorbed from the horse's skin when the animal is exposed to ultraviolet light (from sunshine), no additional vitamin D supplementation is generally required. Broodmares kept in stalls might necessitate artificial light or supplementation due to a lack of exposure to natural ultraviolet light sources.
 - Vitamin E--Huntington said that vitamin E is an important antioxidant for horses. "If adequate amounts of fresh pasture are available to the broodmare, then her vitamin E requirements are likely to be met," he said. He referenced one study, however, that indicated mares that consumed 2,500 IU of natural vitamin E and a fortified broodmare feed daily during the last month of pregnancy had increased total milk vitamin E levels and elevated IgG and IgM levels immediately postpartum, when compared to mares that only consumed the broodmare feed. "Owners should seek professional advice as to the estimated intakes of vitamin E their broodmares have in late pregnancy before considering specific supplementation," Huntington advised.
Feeding Recommendations Finally, Huntington rendered some general feeding recommendations for broodmares before, during, and after pregnancy:
- Mares in Early Gestation and Barren Mares--"Barren, early pregnant, and nonlactating mares have the same nutrient requirements as the mature horse at maintenance," Huntington said. He noted that a common mistake is overfeeding mares at these pregnancy stages, so mares' body condition and weight should be monitored closely to ensure she doesn't gain too much weight during these time periods.
 - Mares in Midgestation--Mares' energy and protein requirements begin increasing during the fifth month of pregnancy, he said. Thus, the NRC recommends raising her protein and energy intake by 5% to 8% during midgestation. Huntington recommended basing the diet on good quality hay and/or pasture at a consumption rate of 1 to 1.5% of bodyweight per day. He noted that additional energy and protein requirements can be met via a fortified broodmare feed, or for the easy keeper, a ration balancer pellet, both fed to label specifications.
 - Mares in Late Gestation--Starting at the eighth month of pregnancy, broodmares' nutritional needs increase significantly as a result of increase fetal growth rates, Huntington said. She requires increased amounts of energy, protein, vitamins, and minerals to help maintain her condition while supplying her fetus with adequate nutrients. Good quality forage should again be the basis of the diet. However most mares will require supplementation with 13% to 15% protein feed (such as a specially designed broodmare feed), he said. Vitamin and mineral increases should be met by the concentrate supplementation.
 - Lactating Mares--Finally, Huntington relayed that lactating mares have the highest nutrient requirements of any horse, aside from racehorses in heavy training. These mares require additional feed supplementation to keep both her and her foal healthy. Huntington recommended providing these mares with a high-quality forage source and a high-quality, energy-dense feed according to label directions for the lactating mare. If additional energy is needed in the diet, he suggested employing a fat source such as oil, sunflower seeds, or stabilized rice bran to provide extra calories.
Take-Home Message Developing a feeding program for broodmares can be a daunting task. A good understanding of her nutrient requirements throughout pregnancy, however, can make the process easier and more successful for all involved.
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