Soy is a common ingredient in many equine feeds. Different parts of the soybean can be used to provide protein, energy, and fiber in your horse’s diet.
Soybeans are popular because of their versatility and affordability. Soy oil is a palatable fat source for horses who require additional calories.
Soybean meal and roasted soybeans also have a superior amino acid profile compared to other commonly fed protein sources. This makes soy a desirable addition to the high-protein diets required by lactating and growing horses.
However, there is a concern in the horse community about whether soy products are harmful. If you are concerned about your horse’s soy consumption, read on to learn more.
Soy Products in the Equine Diet
Soy can show up in your horse’s diet in many forms, including soybean meal, soy oil, and soy hulls.
These different forms vary in their nutritional profile and can be used to balance your feeding program based on your horse’s individual needs, including:
- Physiological status of your horse
- Forage quality
- Composition of the rest of their diet
- Health concerns
An equine nutritionist can help you determine if soy products would be suitable for your horse and how much to add to their diet.
Roasted soybeans are whole soybeans that are roasted to enhance their digestibility. This means the horse can absorb and use more of the nutrients in the soybean.
Roasting destroys endogenous trypsin inhibitors, which are anti-nutritional factors that can inhibit protein digestion. 
This form of soy is a great source of protein and fat. Feeding roasted soybeans is beneficial in diets where extra protein and energy are needed, such as for growing or lactating horses.
Soybean meal is a dense source of protein, and a particularly good source of the limiting amino acid Lysine.
Low protein or lysine intake can lead to several issues including loss of muscle mass, weak hooves, poor growth and poor performance.
Soybean meal is a by-product of soy oil production in which the oil is extracted, leaving the protein-rich meal.
Compared to alfalfa, soybean meal is just as effective for supporting growth in yearling horses and results in better protein utilization. 
Nutritionists often recommend soybean meal for growing or lactating horses with high protein requirements. When weighing the use of soybean meal against other sources of protein, we consider some key nutrients:
Soybean meal typically has a crude protein content of around 44 – 48%. This exceeds other protein sources including:
- Alfalfa (17 – 25% crude protein)
- Canola meal (36 – 41% crude protein)
- Ground flax (26% crude protein)
The high concentration of protein makes it a good choice for horses with elevated protein needs, but limited feed intake such as in young, growing horses.
Calcium and Phosphorus:
Soybean meal has relatively low calcium to phosphorus ratio (approximately 0.45 : 1). This makes it a good choice to balance diets that contain alfalfa for growing horses.
Alfalfa is an energy and protein-rich forage with a calcium to phosphorous ratio of 5 : 1.
Feeding a diet that provides an optimal calcium to phosphorus ratio is extremely important for development in growing horses. Soybean meal balances the high levels of calcium in alfalfa to provide growing horses with a healthy ratio of these minerals.
Soybean oil is a dense source of calories for horses that need more energy in their diets.
Soy-based oils are affordable, palatable and a good option for horses that are exercising or need to gain weight.
The oil does have a high ratio of omega-6 to omega-3 fatty acids. But since it is usually fed in small proportions out of the total diet, it is easy to balance with plenty of access to pasture.
In research studies, soy oil:
- Was found to have similar digestibility compared to coconut oil 
- Was found to have a similar effect to fish oil when fed to horses on a high starch diet 
- May not be as beneficial as flax oil for stimulating antioxidant activity and maintaining circulating lipid profile 
Some soybean oil products contain added omega-3 sources to improve their fatty acid balance and support anti-inflammatory properties.
Mad Barn’s w-3 oil contain omega-3 fatty acids from flax oil and DHA-rich microalgae.
Oils that contain DHA or EPA provide superior anti-inflammatory benefits compared to plant-based oils such as flax or camelina oil, which only contain alpha-linolenic acid.
Soybean hulls are low in non-structural carbohydrates (sugar and starch) but contain highly digestible fiber.  This makes them a gut-friendly source of calories for horses.
Nutritional Profile of Soy Products
The following values are on a dry-matter basis.
|Roasted Soybeans||Soybean meal||Soy oil||Soy hulls|
|Digestible energy (mcal / kg)||4.36||3.74||9.5||1.49|
|Crude protein (%)||41.7||51.5||0||12.1|
|Crude fat (%)||18.8||2.8||100||2.96|
|Alpha linolenic acid (% total fatty acids)||6.99||8.43||8.23||13.1|
|Linoleic acid (% total fatty acids)||52.4||54.2||53.7||42.7|
Common Concerns with Soy
Critics of feeding soy to horses list many reasons to avoid it, such as concerns over genetic modification, phytoestrogens, and inflammation.
While some of these aspects have been investigated in humans and other animal models, there has been very little research on these effects in horses.
Overall, research suggests that soy is safe and even beneficial for horses.
Given the high percentage of horses that have soy in their diet, if soy were problematic to feed to horses we would expect to see more evidence of this in research and clinical reports.
Genetically Modified Soy
In a recent review, it was estimated that 77% of the soybeans produced globally are genetically modified.
The most common genetic change is to make them resistant to the herbicide glyphosate (Round-Up). This modification was first developed in 1985 and approved for use in 1996. 
Since then, several studies have been conducted to evaluate its health and environmental impacts. The main concerns regarding any GM crops are:
- Cross-pollination: The transfer of a new gene to wild relatives. To mitigate this, fields of GM soybeans and conventional soybeans must be at least 10 metres apart.
- Horizontal gene transfer: The transfer of a new gene to other species (i.e. from plant to animal). This would require a series of biological processes resulting in DNA incorporation into the host genome. The probability of this occurring from plant to animal and impacting health is extremely low and therefore not considered a risk