Today, many horse owners express concerns about feeds containing soy. Across social media and many message boards, there is much discussion about soy and its effects on horses. However, much of the information about soy is not based on any scientific data in horses and may contain misinformation that can be detrimental to your animal.
 

Here we will discuss what we know about soy and its use in equine diets, as well as validating true concerns for feeding soy to certain types of horses.

Soy has been chosen as the main source of protein in equine feeds for decades. Soy is also an affordable choice for providing a vegetative source of protein and has proven to be safe for many horses. More importantly, soy, or more precisely soybean meal, which is the most common form of soy mixed into horse feeds, contains many important amino acids. Specifically, soybean meal contains the limiting amino acid lysine, which horses need to ingest daily.

The simple truth about soy and any potential negative side effects is that we just do not know what is true or not true for many of the claims out there. Research into soy and its effects on horses is lacking. For most of the claims, there is just not any scientific data to prove or disprove them. However, there are some truths that may help.

Some opinions expressed by horse owners is that soy has been the source of their animal’s allergies. Many owners have claimed that when they removed their horses off the feed containing soy and went to a soy-free diet, their horses improved, or their allergies completely disappeared. This may be valid as a small percentage of horses can develop allergies to soy. However, the percentage of horses allergic to soy is estimated to be very low (< 5%). It could be that when these owners changed their animal’s diets, they also were changing other ingredients that were the cause. Further, there could have been other underlying health conditions that contributed.  

If a horse has a sensitivity to soy, typically they would also be sensitive to alfalfa and clover forages. This is because soy, alfalfa, and clovers all are high in phytoestrogens, which has been expressed as another concern from many owners. This is because phytoestrogens are “estrogenic-like” compounds that are present in these feedstuffs and act like estrogen in the body after consumption. Typically, this should not be a worry in any gelding or non-breeding mare. A recent study conducted at the University of Florida demonstrated phytoestrogens were detectable in mare serum after being fed a diet of coastal bermudagrass and a 14% crude protein (soy) commercial feed. However, there have been no studies conducted to demonstrate that diets high in phytoestrogens are detrimental to broodmare conception rates or stallion fertility. To date, no data exists to show any detrimental effects of phytoestrogens to any class of horse.

The next big objection to soy in the diet is that it’s high in Omega 6 Fatty Acids and thus, is proinflammatory. This is complex because inflammation is not always a bad thing and there are many causes of inflammation to horses, such as daily exercise, transport, social stress, and others. Yet, it is worth mentioning that within the diet of a horse, it is ideal to have a ratio of Omega 6 and Omega 3 Fatty Acids to be 1:1. Soy generally does have a high Omega 6 to Omega 3 ratio on its own. Within any individual feed, this ratio will always depend on the percent of protein in the feed and its other ingredients. However, within the total diet for the horse, the fatty acid ratio is usually balanced because green pasture and fresh forage are higher in Omega 3 Fatty Acids.

If any owner is ever concerned about inflammation, supplements like Tribute’s Natural Remedy® offer a great source of dietary Omega 3 fatty acids and other ingredients that can be added to their diet to reduce inflammation.

As a side note, horses should never be fed raw soybeans due to their ability to inhibit digestion of protein. Rather, the soybean or soy by-products need to be heat treated to denature the enzymes responsible for inhibiting digestion.

Overall, when it comes to soy in the diets of horses, much is not known because the research is just not there. While studies in other species can be helpful, generally they cannot be applied to horses due to such massive physiological differences. If your horse requires a soy-free diet, check out our Wholesome Blends™ line, which consists of three soy-free horse feeds that are also fortified with whole seeds and vegetables.
 

As always, every horse is different and since feeding situations are dynamic, we encourage you to reach out to us directly to develop a personalized feeding plan tailored to each of your horses.
 

Chris J. Mortensen, Ph.D.