A: First, there really are no "bad" oils or fatty acids for horses. Horses must consume both linoleic acid (omega-6) and alpha-linolenic acid (omega-3), because their bodies lack the enzymes needed to synthesize them. Any oil, if used properly, will be oxidized for energy and incorporated into cell membranes as required. However, some oils are definitely considered better than others.
The ratios of these two fatty acids differ among various fat sources---many oils and foods are much higher in one type than the other. As many people know from their own nutrition, corn oil and some other vegetable oils contain a higher proportion of omega-6s, which are not as good for us because these fatty acids can contribute to inflammation and reduced cell-mediated immunity. Similarly, we know that fish contains higher levels of the omega-3s, which have many health benefits. Flaxseed and linseed oils are also rich in omega-3s. But because omega-6 and omega-3 work hand-in-hand within the horse's body, equine nutritionists place more emphasis on maintaining the proper balance or ratio between the two rather than focusing on the total quantity of either one.
What some horse owners don't realize is that pasture grasses, especially cool-season grasses like timothy or orchardgrass, have ratios of omega-6 to omega-3 that are similar to flaxseed oil. Studies from the University of Florida found that two to three cups of flaxseed oil has the same amount of omega-3 content as 22 pounds of grass hay. This means that horses on an all-pasture or hay diet would have an omega-6:omega-3 ratio of approximately 0.3:1 to 0.6:1. Adding an omega-6-rich fat concentrate, such as corn oil, to that type of diet would therefore increase the ratio to about 8:1.
That number is still well below the recommendation of an omega-6:omega-3 ratio of less than 10:1 for people. However, the recommended ratio for horses has not yet been confirmed---this is because we've seen conflicting results in studies on the effects that omega-3 fatty acid supplementation has on reducing inflammation and enhancing cell-mediated immunity, especially in horses with chronic inflammatory diseases like arthritis, heaves and obesity.
Based on the findings we have so far, I can offer two general suggestions: For a horse fed lower-quality forage, it might be beneficial to invest in a high-quality fat supplement that is rich in omega-3s, such as flaxseed (with a ratio of 0.26:1) or fish oil (0.13:1). However, if the horse is eating good-quality pasture or a cool-season grass hay that is harvested at early to mid-maturity, he may already have a low ratio of omega-6s to omega-3s, and spending the money on an expensive supplement might not be worth it. In instances like this, feeding rice bran (19:1) or soybean oil (7:1) versus corn oil (53:1) would be other options to consider for that "little bit extra" a horse might need.
Carey Williams, PhD
Rutgers, the State University of New Jersey
New Brunswick, New Jersey