Nutrient needs change for mares during late gestation and lactation period
story by Laurie Lawrence, Ph.D
Note: This is part 2 of a two-part series on broodmare nutrition. Part 1 was published in the January 2022 issue of Hoof Beats.
Pregnant and lactating mares can use body stores to supply nutrients to the fetus and foal. However, it is better if a mare consumes the extra nutrients needed rather than mobilize nutrients from her muscles and skeleton.
It is relatively easy to meet the additional needs of early and mid-gestation by providing plenty of good-quality forage (hay or pasture) and small amounts of supplemental feeds. Typical feeding rates for commercial broodmare feed range from four to eight pounds per day. For mares that are able to maintain good body condition (i.e., ribs not visible) with pasture/hay alone, mineral needs can be met with one to two pounds of a commercially manufactured ration balancer instead of the commercial broodmare concentrate.
In late gestation, the growth of the fetus escalates, triggering the need for additional dietary nutrients. In late gestation, protein needs will increase about 50 percent while calcium needs almost double. Calorie needs increase about 40 percent unless the climate is harsh; then mares will need even more calories to stay warm.
Although nutrient needs escalate in late gestation, pregnant mares may not increase their voluntary feed intake. In a University of Kentucky study, mares given free-choice access to hay did not consume any more hay at 10 months of gestation than they did at five months of gestation. A decrease in abdominal room in late gestation may limit total feed intake. Anecdotally, as mares approach the end of gestation, they appear to spend more time resting and less time eating.
Feeding good-quality hay will encourage intake, but it is usually necessary to increase concentrate in late gestation. A commercially manufactured concentrate fortified for broodmares should be used. While plain oats or corn can provide adequate calories, they are deficient in calcium and in trace minerals important for fetal development.
Mares will lose body condition if they are using more calories than they are consuming. Mares that lose condition during late gestation may have a longer gestation period and may be less reproductively efficient in the next breeding cycle. Mares may look plump while wearing a winter coat, but then reveal themselves to be thin once they shed out. During winter, regular palpation of the fat cover on the ribs, spine and tail head will provide information about whether a mare is maintaining her body stores or using those stores to fuel fetal growth.
Lactation is more nutritionally demanding than late gestation. Fortunately, voluntary feed intake capacity increases after foaling. In the University of Kentucky study, lactating mares increased voluntary hay intake by about 35 percent on average, but the range for individual mares was wide and some mares increased hay intake by almost 80 percent. It is important to remember that dietary change is a risk factor for colic, so smoothing the transition in feed intake at the onset of lactation may be advisable.
The amount of milk produced varies among mares and will affect the nutrient needs of the mare. Heavy milking mares, in particular, are at risk of using body stores to meet the demands of lactation. Loss of body stores, which translates to loss of body condition, may reduce reproductive efficiency by delaying the onset of the first or second post-partum ovulation. In addition, mares that are in low body condition or are losing body condition may have higher risk for early embryonic loss.
Because lactating mares have high nutrient needs, they are often fed several pounds of concentrate per day. However, the importance of forage (pasture or hay) quantity and quality cannot be overlooked. Well managed pastures with thick plant cover and good soil fertility can provide many nutrients for mares and foals. Pasture growth is dependent on weather and thus pasture availability changes during the year.
Hay should be offered to broodmares when pasture availability declines. When pasture is not available, total hay intakes for 1,200 pound lactating mares can exceed 35 pounds per day.
The nutritional goal is to maintain constant nutrient intakes, so viewing hay as a “pasture substitute” is a useful feeding management concept. Hay will substitute for both the quantity of pasture consumed and the nutrients provided by the pasture. For well managed pastures in cool and temperate climates, the type of hay that substitutes for pasture the most closely on a nutrient basis is typically an early or mid-maturity alfalfa-grass mix.
The fewer nutrients provided by the pasture and hay, the more nutrients must come from the concentrate. Horses are selective eaters and waste will be increased with late-maturity hay that is coarse and stemmy. Lower-quality hay may be less expensive per bale or ton, but if waste is increased and more concentrate must be fed, then the end result may be a higher daily feed cost than expected.
Milk is the primary source of nutrients for foals in the first few weeks of life, but foals begin eating solid food soon after birth. The nutrient needs of the foal start to exceed the nutrients available in milk at two to three months of age. Consequently, the feeds selected for lactating mares are also the feeds selected for the nursing foal. High-quality forage and appropriate amounts of concentrate designed for broodmares and foals should form the basis of the diet for mares and foals until weaning. HB
Laurie Lawrence, Ph.D, is a professor in the Department of Animal and Food Sciences at the University of Kentucky where she is active in equine nutrition research and teaching. Her current research interests include the factors affecting fiber digestion in the horse and the effect of the mare’s diet on milk composition and foal development. To comment on this story, email us at email@example.com.