The following information on nutrition of late-gestation broodmares was provided by the University of Kentucky College of Agriculture.
Late-pregnancy mares need to be fed adequately so they are not undernourished, because the last few months of fetal development see the most growth, tissue accumulation, and weight gain. This growth particularly accelerates in the last two months, according to Laurie Lawrence, PhD, a professor in the department of animal and food sciences at the University of Kentucky, who, along with research staff and students, oversees a broodmare band of 20 at UK's Maine Chance Farm.
Because a mare will rob her body to feed the fetus first, it's important she maintain a healthy weight during pregnancy. When considering her calorie needs, make sure her Henneke body condition (BC) score remains stable. BC scores range from 1 to 9, with 9 being obese and 1 malnourished. In late gestation, aim for a score between 5 and 6.
"It's not an issue if she's a slightly higher score, but a lower score can compromise a mare's ability to get re-bred," Lawrence cautioned. "With an appropriate body condition, you can't see the ribs but you can feel them, and there is a fat cover over the topline. The mare will appear pleasingly plump.
"It's important that mares receive adequate feed to fuel fetal growth," Lawrence continued. "To accomplish it, they can use a combination of body stores and diet. Ideally, a mare will get sufficient feed and use the nutrients from her diet to supply the fetus's needs. That way she retains her own body stores for herself."
Lawrence points out that in late gestation, a mare's voluntary feed intake does not increase with her body's needs. Thus, owners should feed mares higher amounts of grain at more frequent intervals because the mare might not be able to manage large amounts of feed as the foal fills her belly. She instead needs to nibble throughout the day to meet her nutritional requirements.
The quality of feed is also very important and determines what mix a mare is fed. Lawrence recommended that each mare be evaluated individually.
"Professionals will look at a mare every day and make a judgment about her condition, adjusting feed up or down, as she needs it. That might not be necessary for everyone, but a weekly check should be routine," she said.
As mentioned, owners should feed enough grain to maintain the mare's body condition. The amount fed depends on the protein percentage of both the grain and forage used. If it's more than four pounds, divide it into multiple feedings. Keep in mind that if you overfeed protein, mares will excrete the excess.
Choose a concentrate designed for broodmares that contains an appropriate percentage of protein and mineral content. Adjust the amount of concentrate fed according to the amount and type of hay fed. For instance, if you feed timothy hay, the mare will need a higher protein concentrate. If you feed alfalfa, which has a high protein content of 16-18%, the mare can have a lower protein feed. If a mare is maintaining body condition on forage alone, consider feeding a balancer pellet, which is a concentrated source of minerals. Mares need adequate amounts of copper, zinc, calcium, and phosphorus, as well as other trace minerals, during gestation.
"Have water and a salt block available at all times. Animals will usually regulate their salt intake, but they will not recognize the body's need for trace minerals," Lawrence said. The mare's water intake will increase dramatically as soon as lactation starts.
If horses are fed in a herd, monitor the heavily pregnant mares to see where they fall in the herd's pecking order. They might compete well in the beginning and defend their feed, but as they near term, they might not eat their feed as quickly and more dominant mares might move in to finish their portion. BC score these mares regularly to be sure they don't lose ground.
Research shows that both inadequate and excessive feed are detrimental to broodmares. Underweight mares, in particular, have longer gestation periods. As Lawrence put it: "If you turn down the oven, it takes longer to bake the cake."
Veterinarians and nutritionists do not recommend a "fat," or high, BC score, but mares do need a buffer for the beginning of lactation. During pregnancy most mares should consume 2% of their body weight per day (for example, a 1,200 pound horse requires 24 pounds of grain and hay/pasture). After foaling, total feed needs will increase (that same mare would now require 30 to 35 pounds of total feed) because of the increased demands of lactation. (Plan for variation if feeding moderate- or low-quality hay.)
Lawrence reminds owners to transition the mare's feed intake gradually as she approaches her foaling date. "Don't increase her feed by dramatic amounts. Do it slowly, over seven to 14 days. Ideally, you'll stay with the same hay, but if you have to change, start seven to 10 days before she foals. She will voluntarily eat more food, but you have to be careful not to upset her GI tract in the days before or after foaling," Lawrence cautioned.