If you look at listings or shelves of supplements, there is still the feeling that vitamins are fed for tonic or health-boosting effects, and that minerals are especially for individual horses with deficiencies. Both of these are missing the point.
Grass forage including – even the best-kept – pasture, hay and haylage are all deficient in minerals for horses, without exception. Preserved forages (hay and haylage) are usually deficient in vitamins for horses.
If you feed your horse the full recommended amount of a well-formulated compound feed, they will receive all the vitamins and minerals they need, but if not, they won’t. So if your horse only needs one Stubbs scoop of a regular coarse mix to maintain condition, then they are not receiving a balanced diet and they need a vitamin and mineral supplement as well.
During summer, many horses and ponies will maintain weight and condition on forage alone, if they are out grazing for even just part of the day. Feed a multi-vitamin and mineral supplement to make up for shortages of minerals in the grass. Grass is particularly short of copper, zinc and selenium for horses and ponies. The extra vitamins will do no harm, because the vitamin A in grass is in the form of betacarotenes (provitamin A), which are safe to consume even in large quantities. Fresh grass supplies limited vitamin D, and most of this vitamin comes from the sunlight on the skin, in horses living outside either full or part time in summer. The vitamin E from fresh grass will not oversupply levels even for a horse on a supplement supplying relatively high levels.
Good doers who are fed restricted forage – or even more so – fed 10 hour-plus soaked hay or straw as part of their ration, must be fed a multi-vitamin and mineral supplement to balance their diet. If they are fed soaked hay and/or straw as their total ration, they will also need extra good quality protein, either in the form of a balancer pellet (instead of the vitamin and mineral supplement) or a good source of protein e.g. soy bean meal or flakes (with the vitamin and mineral supplement).
During winter, healthy horses in work who are fed the full recommended amount of a well-formulated compound feed will not require extra vitamins and minerals to balance the diet, but they will required added electrolyte salts. Vitamins and/or minerals can be added for therapeutic effects, e.g. vitamin E for muscle health or magnesium for the hooves.
If a horse or pony requires less than the full recommended amount of a compound feed in order to maintain weight and condition in winter, then they need to be fed a multi-vitamin and mineral supplement, adjusting the amount according to how much compound feed they need.
Multi-vitamin and mineral products are the most underused supplements, because many owners reach for a therapeutic supplement e.g. for hooves, before they have balanced the base diet. There is no point in this because a lack of essential nutrients from a diet that is not balanced will (eventually) cause health problems including poor skin, hooves and coat, poor immunity, poor exercise performance, and general ill-health.