Vitamins are critically important organic compounds. They must be present in the body to enable important reactions to take place that allow the animal to live. Vitamins are divided into two categories: the water-soluble group consists of the B-complex vitamins (e.g., B1, B2) and the fat-soluble group is comprised of vitamins A, E, D and K. Some vitamins also have associated names (for example, B1 is also known as thiamine).
It is important to recognize that the horse synthesizes many of the vitamins it needs and therefore does not typically need dietary supplementation of all vitamins. It is important to check your feed and be sure that all of your horse’s vitamin requirements are being met since vitamin deficiencies can lead to various health problems. However, it is also important to realize that extreme excesses in these vitamins are not desirable either, particularly regarding fat-soluble vitamins. Excess water-soluble vitamins are generally excreted in the urine; however, fat-soluble vitamins are stored readily in the animal’s fat tissue and therefore can build up to high levels if fed in excessive amounts. Since excessively high levels of vitamins can lead to toxicity, it is important to use good judgment when feeding nutritional supplements that are high in particular vitamins.
Minerals are critical inorganic materials that must be present in adequate amounts for the body to function properly, mineral needs will change depending on your horse’s age and status (i.e., if the horse is working, gestating or lactating).
Most commercial feed companies balance their feed to meet the mineral requirements of different needs of horses. Forage will also provide minerals. In some cases, additional supplementation of some minerals may provide desirable results. For example, biotin, zinc and copper supplemented above requirements have been shown to improve hoof strength. However, care should be taken because excessive amounts of minerals may also cause toxicities, lead to serious health conditions or interfere with absorption of other minerals.
If your horse does not receive a compound feed or eats very little of it, it may be important to supplement additional vitamins/minerals to his forage diet by feeding a a feed balancer or broad spectrum vitamin and mineral. Feed balancers are designed to be fed at a low level (approximately 100g / 100kg of bodyweight) that contains the needed vitamins, minerals and protein. https://equi-box.co.uk/horse-feed/feed-balancers.html