Vitamin E deficient myopathy is a cause of muscle loss and weakness in horses. Animals affected may exhibit toe dragging and muscle twitching, in addition to overall poor performance. Vitamin E works in concert with the mineral selenium to ensure that muscles function properly.
If you have any concerns that your horse may not be getting enough vitamin E, consult your veterinarian. Fresh pasture is the best source of vitamin E; it’s also available by supplementation. Affected horses recover after accumulating adequate amounts of the vitamin in their systems, at which point normal muscle mass and strength return.
Source: Kentucky Equine Research