Soya For Horses: The Science, Myth and Misinformation | Equine Science Matters™

Few topics in equine nutrition spark as much debate as soya. Some horse owners swear by it, others avoid it at all costs, and many are left wondering what to believe. In this article, we’ll look at why soya is used so widely in horse feeds, the different forms it comes in, and the most common concerns around feeding it. 

Soya can appear in horse feeds in several different forms. The most common are soya bean meal or flakes, soya bean hulls, and soya oil. Although all come from the same bean, each provides very different nutrients. One thing they do have in common is that they must be heat-treated before feeding. Raw soya beans contain compounds called trypsin inhibitors, which can reduce protein digestibility. Fortunately, roasting or baking the beans effectively reduces these compounds to safe levels. 

Soya Bean Meal 

Soya bean meal or flakes are made from the main body of the bean, after the fibrous outer hull has been removed and the oil extracted. This type of Soya is particularly valued for its high protein content and, more importantly, its amino acid profile. Soya bean meal is rich in lysine, an essential amino acid that is often lacking in forage-based diets. Typically containing around 44-48% protein, it is a highly digestible and efficient protein source once properly cooked. For this reason, it is widely used to help support muscle development and overall condition in horses. 

Soya Hulls 

Soya hulls are the fibrous outer coating of the bean. Unlike the meal, they contain very little protein, oil, starch, or sugar. Instead, they provide substantial levels of highly digestible fibre. This fibre is readily fermented in the hindgut, producing beneficial short-chain fatty acids that provide energy for the horse. Because of this, soya hulls are a useful way to increase the digestible energy content of a feed without adding starch or sugar. This makes them particularly valuable when energy needs to be increased without relying on additional cereal grains. 

Soya Oil 

Soya oil is the oil extracted from the bean and, like other oils, is a very concentrated source of calories. It contains virtually no protein and no fibre. Because it is so energy-dense, it is often included in feeds to boost calorie intake without increasing meal size. Soya oil is commonly found in conditioning feeds and performance mixes to provide what are often described as “non-heating” calories. It is also frequently included in leisure and pasture feeds, as well as chaffs, due to its content of linoleic acid, a fatty acid thought to support skin health and coat shine. 

Concerns with feeding Soya to horses 

Despite its useful nutritional properties, soya is not without controversy. The main concerns tend to centre around phytoestrogens, inflammation, allergies, and behaviour. 

Phytoestrogens 

Soya beans contain naturally occurring plant compounds known as phytoestrogens. These compounds can, to a limited extent, mimic the action of oestrogen, a hormone produced naturally in the body. The main phytoestrogens found in soya are isoflavones. 

Isoflavones are not identical to oestrogen, but they are similar enough to bind to some oestrogen receptors in the body. By doing so, they can influence how the horse’s own oestrogen exerts its effects. The amount of isoflavones varies depending on the form of soya: levels are minimal in soya oil, slightly higher in the hulls, and highest (although still relatively minimal) in soya bean meal or flakes. Growing conditions, harvesting methods, and processing can all influence the levels of isoflavones in the final feed. 

How these compounds are absorbed also matters. Research in ruminants, such as cattle, does not directly translate to horses due to differences in digestion. Emerging research in both humans and horses suggests that the gut microbiota plays a key role in how isoflavones are metabolised and absorbed. Interestingly, these compounds can be broken down into substances that are either oestrogen-like or anti-oestrogenic, meaning they may have very different effects. 

In humans, individual differences in gut microbiota are thought to explain why research findings can sometimes appear conflicting, although the majority of current evidence suggests these compounds are beneficial. Research into the equine gut microbiota is still in its early stages, but similar mechanisms may exist in horses. 

From a practical feeding perspective, it is important to note that soya bean meal and flakes are included in horse feeds primarily for their protein content. The quantities used are highly unlikely to provide phytoestrogens at levels that would be considered problematic. Soya has been used in equine feeds for many years, and if widespread issues existed, they would almost certainly have become apparent by now. 

Inflammation 

Another concern often raised is the fatty acid profile of soya oil. It contains higher levels of Omega-6 fatty acids than Omega-3s, which is sometimes thought to promote inflammation. This could be relevant if soya oil is fed in large amounts. However, for horses with access to fresh grass, which naturally provides omega-3 fatty acids, the inclusion of soya oil in a balanced diet is unlikely to significantly alter the overall Omega 3:6 ratio. If a horse is receiving soya oil from multiple sources, such as both concentrate feed and chaff, adding a small amount of linseed oil, which is rich in omega-3, may help balance the ratio. 

Allergies and behaviour 

Feed-related allergies are usually caused by a specific protein. When the horse consumes that protein, the immune system recognises it as a threat and triggers a defence response, leading to symptoms. While allergy tests are available, they are generally used as part of a broader diagnostic process rather than as a standalone answer. Results should always be interpreted cautiously and in consultation with a veterinarian, taking the horse’s full medical history into account. 

Some owners also report behavioural changes when feeding soya. From a nutritional and mechanistic standpoint, this would seem unlikely based on the typical nutrient profile of soya derivatives. That said, horses are individuals, and differences in gut microbiota could influence how isoflavones are metabolised, potentially affecting behaviour in some cases. It is also worth remembering that soya is rarely fed on its own. It is usually just one component of a larger mix that includes other fibre sources and cereals. In these cases, it is more likely that the overall nutrient profile of the feed, rather than the soya alone, is influencing behaviour. 

If soya is suspected to be contributing to allergic reactions or behavioural changes, and other factors such as pain or management issues have been ruled out, the most reliable approach is a structured elimination and reintroduction diet. This should always be done carefully, with attention paid to meeting the horse’s nutritional needs throughout the process. 

Summary 

Soya bean meal, hulls, and oil each offer distinct nutritional benefits and can play valuable roles in the equine diet. Their widespread use reflects these advantages, and current safety data shows no cause for concern for the vast majority of horses. If allergies or behavioural issues are suspected, these should be investigated with the support of a qualified veterinarian, using a structured elimination diet as the gold standard for diagnosis.