Ideally, none of our horses should need any supplementation to their diets. In a perfect world, our horses would have a constant supply of good grazing and the concentrated feed that we give them would be tailored specifically for their own particular needs. But we don't live in this perfect world and our grazing quality is variable at best (some of our horses don't have access to grazing at all). Exercise and weather affect our horse's energy requirements and horse feed quality can be good, bad or variable. Without a chemist to analyse every component of our horses diet, every day and relate the results to his energy requirements, we are, to a large extent, guessing when it comes to feeding our horses. Realistically, all we can hope to do is to provide all the nutrients, vitamins and minerals that he may or may not need during the day.

Wastage
It has been argued (in humans at least), that supplementation with vitamins is unnecessary and results in nothing more than yellow urine. But this is like arguing that water is unnecessary because it also ends up in the urine! Taking vitamins is like drinking a glass of water - as they are needed in the body, they are used. If a need for a certain mineral or vitamin exists, and none is available, signs of deficiency will manifest themselves.

Quality
All sorts of variables play a part in whether or not supplementation is necessary. The quality of the grazing and/or the hay is important. So is the quality of the concentrate fed. And even a very good quality hay or concentrate deteriorates on storage. Also, even if a nutrient, vitamin or mineral is present in the diet, this does not mean that it is present in an easily digestible form. Some chemical forms of iron, for example, are very difficult to absorb.

Special Requirements
Then there are the horses with special requirements. Older horses have digestive systems that are less efficient than younger horse's, so they don't absorb the nutrients as well as they should. Horses who have had severe worm problems or who have been starved may have damaged digestive systems. Other horses have weak hooves and require more of certain nutrients to correct this. Show horses need to be in exceptionally good condition.

Sport horses, like endurance horses, eventers or racehorses have much higher energy requirements than horses in normal work. They also need more protein and amino acids for muscle-building. Muscles are made up mainly of proteins, which in turn are comprised of strings of amino acids joined together. Certain amino acids, e.g. lysine, are known as essential amino acids. This means that they can't be synthesised by the horse's own body. So if they are not supplied in the diet, and a certain protein needs to be made, when it comes to the place in the protein chain where that particular amino acid fits, protein synthesis
(and therefore muscle development) just stops.

Disadvantages
What are the disadvantages of supplementation? Some people argue that our horse feeds are balanced, so supplementation with vitamins etcetera, unbalances the feed. Here again, we can return to the "glass of water" analogy - if the horse doesn't need what we are supplying, then it just passes straight through. So at worst, we are just wasting money! But if the horse's body does need what we are supplying, it is there and can be used.

Over dosages and side-effects
This is, of course provided that we don't go completely overboard and overdose with vitamins and minerals. This is possible, especially with the fat-soluble vitamins, such as vitamin A (Retinol). So, it is of utmost importance to feed only as directed by the supplier and not to use too many different supplements at once.

Herbal supplements, especially, must be regarded as medicines. One can overdose on herbs and side effects are also common. In some countries, certain herbs e.g. Valeriana, are banned and are tested for when drug testing takes place. So be sure to choose a reputable supplier of herbal supplements.

Conclusion
So, overall, the choice to supplement or not is up to the individual horse owner. Keep a close eye on your horse's condition - he may need extra supplements only at certain times of the year, or only during periods of hard work. Or he may have one of those digestive systems that need extra supplementation all year round. Every horse's situation is unique and each horse needs to be treated as an individual.