Feeding Soy to Horses: 7 Myths and Facts Every Horse Owner Should Know

Few topics in the horse world stir up as much debate as soy. Some owners swear by it; others avoid it like the plague. But much of the confusion comes from half-truths, social media chatter, or outdated research.

Let’s break down the most common myths about soy so you can make an informed, science-based decision about whether it deserves a place in your horse’s feed bucket.

Myth 1: “Soy contains phytoestrogens, so it messes with hormones.”

Yes, soybeans contain plant compounds called phytoestrogens — specifically a type known as isoflavones. These are weak, naturally occurring molecules that can mimic estrogen in the body, but in horses, their effect is negligible.

In fact, many plants horses eat daily contain phytoestrogens, including grasses, clovers, lucerne (alfalfa), and even linseed. Lucerne, for instance, contains coumestrol — a much stronger estrogen-like compound than the isoflavones found in soy — yet lucerne is one of the most widely fed forages in the world.

👉 The reality: The amount of phytoestrogens in soy is far too low to influence hormone levels in horses when fed as part of a balanced diet. If you’re still cautious, soy hulls (the fibrous outer layer) are an option — they contain almost no phytoestrogens compared to the meal.

Myth 2: “Soy affects fertility in breeding mares or stallions.”

This myth stems from those same phytoestrogens. While trace amounts of isoflavones can sometimes be detected in blood after feeding soy, there’s no scientific evidence linking soy consumption to fertility problems in mares or stallions.

Well-balanced breeding diets around the world include soy as a protein source — with no negative effects on conception, foaling, or sperm quality.

👉 The reality: There’s no proven connection between soy and reduced fertility in horses.

Myth 3: “Horses are allergic to soy.”

True food allergies in horses are rare, but they can occur — just as in people. A very small number of horses may show sensitivities or allergies to soy proteins, resulting in issues such as skin irritation, digestive upset, or changes in behaviour.

👉 The reality: Most horses tolerate soy perfectly well. If you suspect an allergy, consult your vet for dietary testing or an elimination trial before removing soy completely.

Myth 4: “Soybeans are toxic for horses.”

Raw, unprocessed soybeans contain a trypsin inhibitor, which can interfere with protein digestion. However, commercial soybean products are always heat-treated — via roasting, toasting, or extrusion — to deactivate this inhibitor.

This process makes soy protein safe, digestible, and extremely high quality. Feeding raw soybeans is never recommended, but heat-treated soy is entirely non-toxic and widely used in both equine and livestock feeds.

👉 The reality: Properly processed soy is not toxic — it’s a safe, high-quality source of amino acids.

Myth 5: “Soy causes muscle wastage.”

This idea is a misunderstanding of protein digestion. Once soy has been heat-treated, its amino acids become highly bioavailable. In fact, soybean meal is one of the most digestible and balanced protein sources available for horses.

Soy provides high levels of lysine, a key amino acid for muscle repair and topline development — particularly useful for performance horses, young stock, and those recovering from illness or injury.

👉 The reality: Rather than causing muscle loss, soy supports muscle development when fed as part of a complete ration.

Myth 6: “Soy has no nutritional value.”

Soybeans are nutritional powerhouses! Here’s what they offer:

• ✅ Complete protein: Contains all nine essential amino acids.

• ✅ High in lysine: Crucial for muscle tone and growth.

• ✅ Rich in oil: Whole soybeans contain 15–20% fat, providing cool, slow-release energy.

• ✅ Excellent fibre source: Soy hulls are highly digestible “super fibres,” low in sugar and starch — ideal for horses needing a low-GI diet.

Depending on the product, soy can serve different roles:

• Full-fat soy meal: Great for performance horses, youngsters, or those needing extra condition.

• Soy hulls: Perfect for horses needing energy and fibre without extra starch or sugar.

👉 The reality: Far from being “filler,” soy is a versatile, nutrient-dense ingredient that supports muscle, energy, and digestive health.

Myth 7: “Soy is inflammatory.”

It’s true that soy naturally has a higher Omega-6 to Omega-3 ratio, and excess Omega-6 fatty acids can promote inflammation. But when soy is used in balanced amounts alongside Omega-3 sources (like linseed or chia), the overall fatty acid profile is healthy and beneficial.

Most commercial feeds containing soy are already formulated to balance these ratios appropriately.

👉 The reality: Soy alone isn’t inflammatory — imbalance is. A well-designed diet offsets this easily.

🌾 So… Is Soy Right for Your Horse?

Soy isn’t automatically good or bad — it’s about context and balance.
When heat-treated and properly formulated, soy is one of the best plant-based protein sources for horses. It’s economical, digestible, and versatile, supporting growth, muscle repair, and overall health.

If your horse thrives on a soy-based feed, there’s no scientific reason to avoid it. If they don’t tolerate it, plenty of alternatives like lupins, sunflower, or linseed meal exist. The key is to choose what works for your individual horse.

🐴 Popular Soy-Based Feeds

You’ll find soy used in many reputable feeds, often alongside other legumes such as lupins for an ideal amino acid balance. Popular options include:

• Benchmark Horse Feeds – Perfect Mash, Super Fibre Mash

• Maxisoy – Soybean hull-based super fibre

• Pryde’s EasiFeed – EasiKeeper, ProteinPak

• Hygain – Balanced, Tru Care

💬 Final Takeaway

Don’t let myths dictate your feeding choices. Instead, rely on evidence and expert advice.
If you’re unsure whether soy suits your horse’s needs, chat with your equine nutritionist or veterinarian — they can assess the full diet and help you make an informed choice.

We’re always happy to help you find the best nutritional balance for your horse.
📞 Visit us in store or browse our range online to explore quality soy-based and soy-free feeds.