Chloride, copper, iodine, iron, sodium, potassium, magnesium, phosphorous and calcium all have an increased requirement for the last trimester. Most hays are high in iron and potassium and the majority of horses are having their requirements met in excess so adding these nutrients to the diet as a supplement is typically not necessary. Similarly if your mare is receiving adequate salt and any alfalfa in the diet the additional sodium, chloride, iodine, magnesium and calcium requirements are also likely being met.
Copper and zinc tend to be low in most hays, and it is not unusual for a typical diet to be deficient in one or both of these. Alternatively, the diet may be so high in iron that the iron out competes the copper and zinc for absorption leading to copper and zinc deficiencies even though there may be enough copper and zinc present in the diet. Therefore it is a good idea to provide a supplement that will provide additional copper and zinc throughout the pregnancy. This is particularly important for last trimester mares because copper is required for elastic connective tissue and collagen formation the main component of bone matrix.< While not the only cause of developmental orthopedic diseases (DOD) in foals, there has been research that suggests that foals born to mares that were given supplemental copper had lower incidents of osteochondrosis even if the foals were given supplemental copper after birth (NRC, 2007). It is thought that supplementing the pregnant mare may promote repair of developing osteochondral lesions in the foal. Therefore it is very important that the mare’s diet provide adequate trace minerals that are correctly balanced to assure absorption and utilization. There are many products on the market containing copper and zinc but many provide insufficient amounts to result in correct trace mineral balance in the diet. You may wish to discuss your supplement choice with a nutritionist who can help you decide whether the supplement you have chosen will bring to the diet what your horse actually needs.
There are two classes of vitamins water soluble (A,D,E and K) and fat soluble (B and C), all but vitamin A and E can be made by the horse. Both vitamin A and E have implications for the brood mare and foal. One of the body stores for vitamin A in the form of beta-carotene (a precursor to vitamin A) is the ovary and namely the corpus luteum where it helps control progesterone secretion and therefore ovulation and embryo implantation. As a result, inadequate beta-carotene in the diet may lead to issues with ovulation and pregnancy maintenance. While beta-carotene in hay may be adequate to maintain vitamin A in the body, it may not be adequate to maintain ovarian stores of beta-carotene. The plasma beta-carotene content of mares fed pasture is 8 to 13 times higher than mares fed hay (Lewis, 1995). In fact, hay has been found to lose 9.5% of its vitamin A per month so that by 6 months after cutting, more than 50% of the vitamin A will have been lost. At the same time winter pastures have less vitamin A and beta-carotene than summer pastures and therefore even on pasture mares may have vitamin A insufficiencies (Greiwe-Crandell et al., 1995). In this same study, foals with respiratory infection were found to have reduced vitamin A status. It was unclear whether the infections caused the low vitamin A status or visa versa, but as foals are born without vitamin A stores and rely on transfer of vitamin A in colostrum from their mothers, it would seem a sensible precaution to insure the mares vitamin A status is adequate prior to foaling. This could be especially true for mares that foal early before spring pastures and new hay become available. NRC requirement for vitamin A is 60 IU/kg body weight for mares and 45 IU/kg body weight for foals.
Vitamin E is not heat stable and levels in hay are therefore very low compared to those in fresh pasture. Research has shown improved immune status (higher IgG levels) in foals born to mares supplemented with 160 IU vitamin E per kg dry matter consumed. It was not confirmed that these foals in fact had improved health, but resistance to systemic infections and respiratory diseases is dependent on the amount of colostrol IgG absorbed (Lewis, 1995) and so it would seem good insurance to assure that mares are consuming adequate vitamin E prior to colostrum production which occurs in the last month of gestation. This is especially true for mares receiving most of their diet as hay.
Mares should be fed as much of their nutrient requirement in the form of good quality hay and pasture as possible. Supplemental grain should be fed as necessary to maintain condition. Quality protein should be provided to insure adequate essential amino acid intakes and supplemental copper, zinc and vitamin E should be supplemented along with vitamin A if there are concerns about the vitamin A status of the diet. Several manufacturers make products that provide fortified minerals and quality protein at approximately 12 or 30% of the product, the lower protein level being ideal for mares receiving some alfalfa and the 30% products being ideal for mares on grass hay. These products are made to be fed at approximately 1-2 lbs per day. Should more calories be required for weight gain than provided by these feeds, a feed specifically manufactured for brood mares and growing youngstock may be fed in higher amounts. You should read the manufacturers feeding instructions to find out the amount to feed and whether the product should be combined with a more fortified feed if fed below their recommended feeding levels in order to insure adequate vitamin and mineral intakes. Alternatively, it is possible to formulate a vitamin and mineral supplement to specifically balance your hay. This may be fed in conjunction with a commercially available fortified grain or with straight grains such as oats.
If you are unclear as to the best approach for feeding your brood mare during the last months of her pregnancy and as she transitions to lactation you may want to consider working with a specialist independent equine nutritionist who will be able to evaluate your horse’s specific needs and help you put together an optimal diet. Providing for your late gestation mare’s dietary needs will insure that her foal is in vibrant health, skeletally strong, and has a robust immune system. Additionally your mare will be in the best health producing enough milk to feed her foal while being reproductively able to breed again should you so desire.
Fowden, A.L., A.J. Forehead, K.L. White and P.M. Taylor. (2000). Equine uteroplacental metabolism at mid and late gestation. Exp. Physiol. 85:539-545.
Greiwe-Crandell K.M., D.S. Kronfeld, L.A. Gay and D. Sklaty (1995). Seasonal Vitamin A Depletion in Grazing Horses Is Assessed Better by the Relative Dose Response Test than by Serum Retinol Concentration. J. Nutr. 125:2711-2716.
Lewis, L.D. Equine clinical nutrition: feeding and care. Philadelphia.Williams and Wilkins 1995.
National Research Council. Nutrient requirements of horses: 6th Revised Edition. Washington D.C. National Academies Press. 2007.
Reynolds, L.P., C.l. Ferrel, D.A. Robertson, and S.P. Ford. (1986). Matbolism of the gravid uterus, foetus and utero-placenta at several stages of gestation in cows. J. Agric. Sci. 106:437-442.
Van Neikirk, F.E., and C.H. van Neikirk. (1997a). The effect of dietary protein on reproduction in the mare: III. Ovarian and uterine changes during the anovulatory, transitional and ovulatory periods in the non-pregnant mare. J.S. Afr. Vet. Assoc. 68:86-92.
Van Neikirk, F.E., and C.H. van Neikirk. (1997b). The effect of dietary protein on reproduction in the mare: I. The composition and evaluation of digestibility of dietary protein from different sources. J.S. Afr. Vet. Assoc. 68:78-80.