Just as omega 3 fatty acid is essential for our human wellbeing it is also necessary for horses and horses who are deficient in omega 3 fatty acids tend to display symptoms such as a "dull coat with dry itchy skin, skin allergies (including "sweet itch"), cracking thin-walled hooves, joint stiffness and excitability" (http://www.infohorse.com/html/flaxseed.asp)
Flax seed is high a rich source of omega 3 essential fatty acids and by supplementing your horses diet with flax seed you are able to replenish these vital levels of omega 3. Stabilized ground flax for horses can be added to your horse's diet in order to ensure that he gets the supply of omega 3 that he needs as well as a number of other nutritional substances such as lignan and mucilage.
Scientific research has also been done to test the effect of flax seed supplementation on horses to test their allergic response to certain insect bites. Horses allergic to these insect bites often present with skin eruptions due to excessive itching. These studies showed a noticeable reduction in allergic reaction when horses were fed with the whole flax seed.
Omega 3 has also been found to be beneficial for thoroughbred racehorses in that they play an "important role in the structure and formation of the wall of the red blood cells -conferring upon them increased suppleness and flexibility. This red cell flexibility is advantageous during exercise - the more flexible the red blood cells the more easily and efficiently they pass through the narrow capillaries in the lungs and muscles, making oxygen uptake and delivery and waste product removal, more efficient." (Stewart, J., Omega 3 Oil benefits for Horse and Rider)
In order to add flax seed to your horse's diet you should add ½ to 1 cup of ground, soaked or cooked flax seed to your horses diet each day. The whole seed will not provide the same benefits as it will not be properly digested and so all the goodness taken out.
Just like humans need to get a good balance of essential fatty acids in their diet and flax seed has been found to be beneficial for human consumption, horses can also benefit from taking flax seed in their diet. Flax seed is generally given to improve the horse's coat but can also be used to improve allergic reaction and the flexibility and suppleness of red blood cells in horses which improves their racing ability. If you start noticing that your horse has a dull coat, dry itchy skin cracking hooves and joint stiffness then it is likely that it may be suffering from an omega 3 fatty acid deficiency and would benefit from a diet supplemented with flax seed.
Glossary:
Lignan: chemical compound found in plants that acts like estrogen as well as providing antioxidant properties
Mucilage: gel like substance in plants that contains proteins and polysaccharides