It is well understood that lactation puts great nutritional stress on the mare. Next to lactation, the nutrient requirements of the mare are the greatest in the last trimester of pregnancy. Rather than putting the mare on a nutritional roller coaster with large changes in the condition of mare occurring throughout the year, it is wise to establish an optimum condition and try to maintain that condition throughout the year. To maintain this "mare optimum condition" there are several factors and times throughout the year that must be considered including late pregnancy. During the last 4 months of pregnancy 80% of the foals in-utero growth occurs. At this time energy and protein needs increase by 15 – 25%, and the late pregnant mare in winter can loose condition. In contrast a late pregnant mare in spring that is also being hard fed can easily gain too much weight. You need to vary the amount of hard feed accordingly to avoid these changes.

It may be tempting to think the mare should be maintained in fat condition after weaning. This approach has some drawbacks even though it may look good for the amateur mare owner. The fat mare has an increased risk of having a foal with bent legs (angular limb deformities) that could limit the foal’s value and athletic potential. The other drawback of having your mare too fat is that she may be difficult to get back in foal. Consider the situation of the fat mare that has been pampered at home with 3 feeds a day. She is sent to the stud just before or just after foaling and suddenly she’s mixed with a new group of mares, is in a new environment and on a new feed program. The mare looses weight and doesn’t cycle as well or may shut down entirely. So avoid getting your mare too fat even though it may look more pleasing to have her in show condition.

Minerals are vital in late pregnancy nutrition. The mare’s requirements for calcium and phosphorus increase to support the development of bones in the rapidly growing foal. Milk is a poor source of trace minerals so some of the key trace mineral needs of the foal must be met from liver stores accumulated prior to birth. This means the late pregnant mare must be fed extra trace minerals to supply the foal. Recent research in New Zealand has shown that the foals of mares supplemented with extra copper in late pregnancy had a significantly lower incidence of physitis and OCD lesions at 5 months of age. All mares in this study were also given extra zinc and selenium. So the incidence of DOD in the weanling can be directly related to a mineral deficiency in the diet of the late pregnant mare. However supplementation of the foal with copper after birth did not reduce the risk of OCD and physitis.

A late pregnant mare needs supplements in a premixed feed or a supplement to supply her trace mineral needs.

Recent research has shown that supplementation of selenium and vitamin E to the late pregnant mare has enhanced immunity in the mare and the foal. Antibody levels were higher in foals of mares receiving 3 mg selenium compared to those getting only 1 mg. In the vitamin E study mares receiving 1600 IU per day provided better immune transfer compared to mares getting 800 IU. This may help the foal ward off infection in the critical first few months of life. Another reason for feeding added selenium may be to reduce the incidence of retained afterbirth. In selenium deficient cows there is a higher incidence of retained foetal membranes.

Another vitamin that needs consideration is Vitamin A. Research by Dr Kathleen Crandell has shown that mares without access to green grass and a vitamin A supplement can have smaller foals and these foals do not have the compensatory growth to catch up after birth. Whilst these mares were fed hay to replace pasture, the stored hay had lost it’s vitamin A and the diets were vitamin A deficient resulting in smaller foals.

There is no magical feeding program for the late pregnant mare, but there are a couple of vital considerations that must be incorporated into your program to allow optimum development of the foal inside the mare. These are summarised below in table 1. The critical concern is to establish an optimum for each mare and try to stay as close to that condition as possible year-round.