I am not sure if there is a broad spectrum supplement, but mine get the following: flax seed, garlic (for ticks), probios and MSM. Joe Joe and Rowan also get Smartpak Muscle Mass, Selena gets Smartpak Calm, and Rowan and Selena get Mare Magic (obviously unnecessary for a colt). In winter I do add the dry molasses, mostly because the liquid is so messy and difficult to measure. Their grain is Triple Crown Senior (they range in age from 12 to 23), which you might not need. All this is mixed with timothy hay cubes into a kind of soup, which they love. I spend more time and effort on their food than I do my own, but they are all at a good weight, even coming out of winter (Selena is actually a bit fatter than she needs to be), and healthy. Joe Joe and Selena are Arabians, and Rowan came from a kill pen in December (even the kill buyers did not want her, because she only weighed 712 lbs, which is why I bought her – she needed a safe place to live with food on a regular basis). She has now gained about 150-200 lbs., and is horse shaped.
It is never the horse's fault