1. a quantity added; e.g. to make up for a deficiency
This alone should set you on the track of better understanding. Take into a consideration the meaning of the word alone, in which case if your horse is doing well and is healthy, he has more likely no need for any additional supplements to his diet. In my travels Iíve seen folks feeding horses many varieties of supplements, where their feed rooms looked like some ancient alchemist workshops. I believe, if supplements are fed, when not truly needed, they will put the animalís body chemistry out of balance. As a result horses can have various digestive problems and as often seen by me, horses ate dirt, their own manure which often looked and smelled abnormally, and/or their over all condition was poor despite the great efforts by the horsemen. Though they may do these latter things for other reasons as well, when seen, one should take a closer look at his feeding program. One thing should be also noticed, that often the use of several supplements will cover up a poor health condition of the animal.I believe that over all good health of the horse, achieved by proper care for the animal, is lasting and real. Our webs have more on the feeding of horses on horse-care site, see "Feeding Page". Also remember that grass or good hay have all the supplements that the horse needs, except sufficient amount of energy supplements if and when the animal is working (grain, salt etc.) This is why we give horses grain products.
All my comments are merely my opinions and beliefs gained from 40 years of professional life with horses. All drugs should be used only by the consent of a veterinarian and according to his instructions. A person who is with the horse everyday, should know him better than anyone else.