What it does
Magnesium is a macromineral [1]. Working horses need more magnesium than horses at maintenance [2].
Deficiency
Low blood magnesium causing tetany has been reported in lactating mares and acutely stressed horses [2].
Excess and supplements
Intakes above the estimated upper limit have been tolerated without apparent harm, but overdosing Epsom salts (magnesium sulphate) can cause toxicosis [2]. Reports of calming effects are anecdotal [2].
For horses with equine metabolic syndrome, magnesium has some evidence of improving insulin regulation, but other studies have not confirmed this, and the efficacy of such supplements remains questionable or unproven [3].
See also chromium.
Sources
- 1. Arispe S, Filley S. Understanding your forage test results (EM 8801). Oregon State University Extension Service. Revised October 2020, reviewed 2023. https://extension.oregonstate.edu/catalog/em-8801-understanding-your-forage-test-results (accessed 2026-11-07)
- 2. Liburt NR (peer reviewed by Boyle AG). Nutritional Requirements of Horses and Other Equids. MSD Veterinary Manual, Professional Version. Merck & Co., Inc., Rahway, NJ. Last full review February 2026. https://www.msdvetmanual.com/management-and-nutrition/nutrition-horses/nutritional-requirements-of-horses (accessed 2026-11-07)
- 3. Durham AE, Frank N, McGowan CM, Menzies-Gow NJ, Roelfsema E, Vervuert I, Feige K, Fey K. ECEIM consensus statement on equine metabolic syndrome. Journal of Veterinary Internal Medicine 2019;33(2):335–349. doi:10.1111/jvim.15423 https://pmc.ncbi.nlm.nih.gov/articles/PMC6430910/ (accessed 2026-11-07)
Last edited 7 Nov 2026, 1:48 am