Having being involved in the horse industry for well over 50 years and seeing different methods, trials, methods and formulas work to some degree and others not work, and combined with my real life job of Extension Specialist Equine Nutritionist, I am now firmly convinced that when your horse, just like any other species of animal including humans; has a well balanced metabolic system and feeling good from within, it is only natural it will show on the outside. I have proven this concept time and again over the years simply through eliminating a lot of problems many horses have or are prone to. Pinched nerves, pinched or bunched muscles, poor feet (generally triggered through a poor diet or improper hoof care practice), parasite burdens, improper dentition (through lack of attendance) and poor diets.
Once the horse has been properly tended to in the feet, parasite and muscle areas it is time to begin concentrating on the diet.
When the diet is formulated in the correct manner and the horse is being fed according to its work levels or production levels, with particular attention paid especially to Digestible energy:Protein ratio, mineral balances and ratios between complimentary macro and micro minerals, then you will find the horse's natural show of well being will quickly take over. This in turn gives the sheen to the coat that can not be obtained through a can of spray or wash or whatever 'product' is out there to suck money from already overtaxed horse owners.
The use of vegetable oils in the diet is another coat enhancer. I have used and advocated the use of Canola oil for many years with no ill or side effects despite continued resistance from the 'natural' group of horse owners that continually bleat about it being genetically modified and all the other non-proven or thought about reasons. When blended with Soybean and Wheatgerm oils in the correct ratios, canola oil will provide no more digestible energy than any other vegetable oil, however will provide higher levels of essential fatty acids (EFAs) Omega3 and Omega6 in the correct ratio as to allow the natural anti-inflammatory factors to be enabled within the horse's system thereby enhancing not only haircoat, but at the same time helping prevent skin conditions and alleviate other conditions such as Queensland Itch (known as 'sweet itch' in other countries).
When all the balances are right, other extenuating problems such as muscle freedom, hoof care and dentition has been attended to and the diet is right you will find a natural gloss to your horse's coat that you will not believe and can not be emulated through artificial means.
Another thing that must also be covered in this article is the fact that owners (especially show people) wash their horse just prior to showing them or taking photos. This is terribly detrimental to a horse's coat and strips away natural oils from the coat leaving a dull non-reflective hair fibers. If you feel you need to wash your horse before a show, then do it the week before and allow the natural body oils to come through again. I know top halter horse trainers in the USA that never wash their horses. Granted, the horses are rugged and stalled on a continual basis, but these superior showmen employ people to groom the horses for hours on end. This stimulation not only removes dirt and detrius build up on the coat, but the grooming also stimulates the oil flow to the coat.