By Jess Hallas-Kilcoyne

Good nutrition throughout pregnancy is essential to the health of both mare and foal. Adjust your broodmare’s diet as gestation progresses to meet her changing nutrient needs.

The optimal feeding program for a broodmare during pregnancy is designed with two primary considerations in mind – maintaining the mare’s health, and supporting the growth and development of the fetal foal. Both of these considerations are affected greatly by the rate of fetal growth, which varies significantly from early gestation (0 to 4 months) to mid-gestation (5 to 8 months) to late gestation (9 to 11 months, also known as the final trimester).

Fetal Growth During Gestation 

During the early months of pregnancy the fetus grows very slowly, and so the nutritional requirements of the mare remain similar to those of a non-pregnant mare. Starting in the fifth month of gestation, the fetal growth rate begins to increase, resulting in corresponding increases in the mare’s nutrient needs that continue on a month-by-month basis throughout mid-gestation. However, by seven months, the fetus is still very small – less than two percent of the mare’s body weight and only approximately 10 to 15 percent of its weight at birth.

Fetal growth is most rapid in late gestation. During the final trimester the fetal foal, growing now at a rate of approximately one pound per day, gains about 70 to 75 percent of its birth weight. The fetus’s accelerated growth rate, and the increased nutrient uptake required to support this growth, result in greater demand on the broodmare’s body.

Energy & Protein

A broodmare feeding program is ideally developed with the goal of feeing as much forage as possible while limiting grain intake to the minimum amount required for satisfying the mare’s energy needs. In early to mid-gestation, good quality forage (pasture or hay) and a vitamin/mineral supplement may be sufficient to meet the mare’s nutrient requirements and allow her to maintain condition, bearing in mind that recommended changes in the dietary level of certain minerals around the fifth month may require adjustments to the size of the daily supplement ration.

Maintenance of a body condition score between 5 and 7 on the Henneke body condition scoring system throughout the pregnancy contributes toward a broodmare’s ability to carry, deliver, and nurse a healthy foal. - Photo: Robin Duncan Photography

Over-feeding a mare in early pregnancy is a common mistake, often causing her to become overweight. A body condition score between 5 and 7 (according to the Henneke body condition scoring system) is considered optimal for a broodmare throughout the pregnancy.

At around the seventh month of pregnancy, some mares may require the addition of concentrates to the diet in order to maintain condition as the demands of the fetus increase. Alternatively, increasing the grain ration may be avoidable if higher quality forage with a higher caloric and protein content, such as alfalfa or alfalfa grass mix, is introduced to the diet as a means of helping the mare maintain condition. But from the ninth month of pregnancy on, concentrates will almost certainly be needed in the diet to satisfy increased energy and protein requirements during the final trimester and to combat other factors that tend to arise at this time and threaten her with weight loss. Protein needs increase from 8 percent crude protein in early and mid-gestation to 10 percent in the ninth month, and 11 percent in the 11th month; energy requirements increase by about 20 percent from those of early pregnancy.

Another challenge to the broodmare’s ability to maintain condition during late gestation is the fact that the fetal foal, as it begins to grow more rapidly, will increasingly compress the mare’s digestive tract, causing her appetite to decrease at a time when she most needs the nutrients from the food she consumes. Smaller meals fed with greater frequency can help keep her feed intake steady because they won’t overfill the mare’s digestive system, making her uncomfortable. 

The quality of the feed, and its effect on broodmare diet, should be taken into consideration. Broodmares typically enter the final trimester during the winter months, when available forage tends to be of lower quality. Good quality forage should be the basis of all feeding programs, but it is especially important when the mare’s diet is supplying nutrients to the fetal foal as well as to her own body.

A body condition score that remains within the optimal range of 5 and 7 is usually a good indication that a broodmare’s energy and protein requirements are being met by her feeding program, but it doesn’t necessarily mean that all of her nutrient needs are being satisfied. Deficiency in certain minerals and vitamins may not be reflected in the mare’s appearance.

Minerals & Vitamins

The most crucial macrominerals in a broodmare’s diet are calcium, phosphorus, and magnesium. Calcium and phosphorus, in particular, are critical to the healthy development of the fetus. In the final trimester, an 85 percent increase in the amount of calcium and a 100 percent increase in the amount of phosphorus in the mare’s diet are recommended to meet the fetal foal’s increased uptake of these minerals in late gestation. Phosphorus deficiencies are more common in late pregnancy while calcium deficiencies are more likely to occur during lactation.

Trace minerals copper, zinc, manganese, iron, and selenium are also essential for the foal’s bones and muscles to grow and develop properly. Because trace minerals are only present in low concentrations in mare’s milk, it is important that these nutrients be supplied in late gestation so that the fetal foal can store them in its liver in adequate amounts to be available for use over the first few months after it is born.

The most commonly deficient vitamins in broodmares in the final trimester are A and E, both of which are present in pasture and fresh hay; however, the amount in the hay deteriorates the longer the hay is stored. When the increased fetal uptake of these nutrients in late gestation coincides with unavailability of pasture and good quality hay that has been storaged for less than three months (common in winter), vitamins A and E may need to be supplemented. 

Generally speaking, adequate amounts of the minerals and vitamins that are essential to the health of the broodmare and developing foal can be ensured with the addition of a good quality vitamin/mineral supplement to the broodmare’s diet. This supplementation is typically not required if the mare is being fed five or six pounds or more of a properly fortified concentrate feed per day. 

Main Photo: In early and mid-gestation, many mares may be able to comfortably maintain condition with a diet of good quality forage (pasture or hay) and a vitamin/mineral supplement. As early as the seventh month of pregnancy, and almost certainly by the ninth month, a broodmare may require the addition of concentrates to the diet to help maintain condition as the growing fetus increases the demands on her body, resulting in a corresponding increase in her energy and protein requirements. Photographer: ©iStockphoto.com/tfoxfoto

This article is for information purposes only and should not be relied upon as a substitute for professional veterinary advice. No liability will accrue to the publisher or author of the article in the event that a user suffers loss as a result of reliance upon this information. 

This article originally appeared in the October 2013 issue of Canadian Horse Journal.