As the mare progresses into her final “trimester,” or the last three to four months of pregnancy, we need to start increasing her nutritional plane. It is during this last phase of pregnancy that we observe the most fetal growth, at approximately 1 pound per day. A mare must consume enough nutrients to meet her needs as well as those of the fetus and, in the latter weeks, to support mammary development. If a mare is due while the temperatures are still cold, she will also have increased nutritional demands for thermoregulation. Nutrient requirements for calories (energy), protein, calcium, and phosphorus will increase.
We should be familiar with the appropriate weight and body condition of our mares. Most feed companies have nutritionists on staff who can visit your farm and assess your mares. Many have a portable scale they can bring along to weigh your mares. They can also assess body condition while weighing to make sure your mares are neither too fat nor too thin. Having the mare in moderate to fleshy condition will give her some fat reserves for foaling and early lactation. Weight gain of 140 to 200 pounds is common.
We must also ensure our mares receive adequate forage. Forage provides the mare with not only nutrients but also heat when her body ferments/digests it in the hindgut. This process can help the mare maintain core body temperature during colder months. Oftentimes weight loss during winter is due to the horse trying to maintain body temperature. We want our mares to receive at least 1% of their body weight (or about 10 pounds of hay for the average 1,000-pound mare) in good-quality forage per day. The mare might be able to meet her dietary needs with good-quality forage alone, at an amount of 2-3% of body weight, or 20 to 30 pounds of dry forage. As the mare progresses into late gestation, the growing fetus takes up more room in the abdomen, and the mare might not be able to eat enough forage to meet her needs. Adding a concentrate designed for pregnant mares can help. To ensure you are meeting dietary requirements, follow the directions on the feed tag. Feeding less of a concentrate than what is recommended on the tag can lead to nutritional imbalances or deficiencies.
When we are looking at forage types, we want to make sure the hay is of good quality. A mostly grass hay with some legume content, in addition to concentrate feed, will likely meet most of the mare’s needs in late gestation. Avoid endophyte-infected tall fescue. Consumption of this forage can cause a variety of complications in the late-gestation mare, including low or no milk production, delayed parturition, retained placenta, and a dysmature foal.
Water intake requirements also increase during late gestation as the mare drinks to meet her needs (about 1 gallon per 100 pounds of body weight) and for fetal development. An average 1,000-pound broodmare needs 10 gallons to meet her water requirements but could need an additional 5 gallons for the fetus. Water intake is especially important when most of the diet is dry feedstuffs, such as hay and grain (as opposed to pasture), to help prevent colic.
In summary, make sure the late-gestation mare is receiving enough nutrients to meet both her needs and the needs of the developing fetus. Monitor energy, protein, calcium, and phosphorus levels in her diet, and ensure she has access to good-quality forage and water.