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Micronutrient Nutrition and the Third Trimester Mare – Part 2: Vitamins

by Dr Clair Thunes PhD Nutrition
Consulting Nutritionist for EnviroEquine & PET
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There are two classes of vitamins water soluble (B) and fat soluble (A,D, K and E), 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.  In fact, hay has been found to lose 9.5% of its possible vitamin A per month so that by 6 months after cutting, more than 50% of the possible vitamin A will have been lost.  At the same time winter pastures have less beta-carotene than summer pastures and therefore even on pasture mares may have vitamin A insufficiencies.  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 is a sensible precaution to insure the mare’s vitamin A status is adequate prior to foaling.  This could be especially true for mares that foal early before spring pastures and new hay becomes 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 and so it is 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.

EnviroEquine GastroBalance and GastroBalance Plus both provide supplemental sources of vitamins A and E. The form of vitamin E in GastroBalance Plus is natural d-alpha tocopherol to aid in absorption. Additionally EnviroEquine’s OmegaBalance also provides a rich source of natural tocopherols and other polyphenol antioxidants.

Consulting Nutritionist for EnviroEquine Dr. Clair Thunes is passionate about her profession—one that she decided upon at the age of 14. After earning a Bachelor of Science with Honors from Edinburgh University, and a Master of Science in Animal Science and a PhD in Nutrition from the University of California, Davis, Dr. Thunes went on to found Summit Equine Nutrition LLC an independent consulting company in 2007. An experienced nutritionist and accomplished scientist, Dr. Thunes understands the vital role that nutrition plays in managing horses today. Most importantly, she believes in making nutrition accessible to everyone and removes the guesswork so that owners have the peace of mind that their horse’s diets are optimal for maximum health and peak performance.  Her clients include all horses from competitors at the 2016 Rio Olympics to retired pasture friends, mules and miniature donkeys. She writes a weekly online commentary for and her nutrition articles have been published in noted publications including: The Horse, Equine Wellness, Trail Blazer, Horse & Rider and The Horse Report. Besides consulting she teaches equine nutrition and equine exercise physiology in the Animal Science Department at UC Davis and equine health at Cosumnes River College.  Clair continues to be involved with The United States Pony Clubs, Inc. and she is currently the Regional Supervisor for the Sierra Pacific Region.