As you probably know, one of the more established supplements for horses these days is flax, and even some health-minded people are eating it as well. That's not a bad thing. However, changing from flax to fish oil will offer far greater benefits for you, your horse, and probably even your wallet.
For starters, did you know that the more a body needs omega-3 supplementation, the less flax works? That's because flax contains only an immature form of omega-3 (ALA), which must be elongated by the body to be fully potent and requires specific enzymes to do so.
Two things interfere with this process: an aged body, which produces fewer of the enzymes, and a diet rich in omega-6s (grains and most oils), which deplete the enzymes. So aged horses, who may or may not have needed supplemental omega-3 when younger, now have reduced effectiveness processing their natural plant-source omega-3 intake. Performance horses of all ages, fed grain diets and fat supplements, now need omega-3 to balance all the omega-6s they get, as well as the rigors of competition. Yet, those same omega-6s make it very difficult to process plant-based omega-3s.
The end result is an amazing 30x inequity between flax oil and fish oil. In other words, a body could easily require six cups of flax oil to equal the results of one ounce of quality fish oil. That doesn't even take into consideration the usual form of whole or ground flax, of which great quantities are needed to produce the oil. So, in both situations, fish oil is both more effective and more cost-efficient.
Fish oil is omega-3 at its most powerful and one of only two full-spectrum omega-3s in the world (the other is breast milk). Far from being a single substance, omega-3 is a family of at least 15 molecules, and full-spectrum fish oil offers them all, working synergistically. The three molecules that you hear the most about are short-chain alpha linolenic acid (ALA), and long-chain eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).
When you choose a plant-based form of omega-3 such as flax, you choose purely ALA, and likely get only the shiny hair that comes with any added fat source. When you choose an algae source of omega-3, you get DHA, which is great for enhancing fertility and brain focus. When you choose fish oil, however, you get both ALA and DHA, as well as EPA, which researchers have established as the omega-3 molecule responsible for lowering triglycerides and supporting joint health, including reducing degradation of cartilage and easing inflammation and pain.
Perhaps best of all, you are able to benefit from a "whole is greater than the sum of its parts" situation: the natural synergistic power of ALA, EPA and DHA operating as a full-spectrum family.