In this comprehensive blog post, you will get a complete overview of soy, soy meal, soy hulls, phytoestrogens, GMO cultivation, alternative protein sources and specific feeding plans.
Use the headings below to jump directly to the topic you want to read more about.
Table of contents
1 – Soy and phytoestrogens: Myths and facts
Introduction
For many years, myths have abounded that soy is “hormone-disrupting” for horses. But what are the upsides and downsides? In this article, you will get a sober overview of phytoestrogens in soy and other legumes – and why they are rarely a problem in horse feed in practice.
What are phytoestrogens?
Phytoestrogens are natural plant substances found in all legumes – including alfalfa and sainfoin. They got their name because they resemble the estrogen that animals and humans produce themselves when you look at the molecular structure. The name arose before the precise biological functions, receptor distribution in the body and the overall effect in living organisms were known.
The myth of "hormone disruption"
The old claims that phytoestrogens are hormone disruptors primarily stem from laboratory experiments on cell cultures.
In such experiments, extremely high doses can be used, far exceeding what a horse can get through natural feed. At the same time, cell experiments bypass the many regulatory mechanisms that the body has: digestion, microflora, liver function, blood levels and receptor balances in tissues.
Where are they found in the soybean?
The isoflavones (the most important group of phytoestrogens in soy) are found primarily in the germ and cotyledons – not in the shell (soybean hulls).
The level varies with processing and also requires fermentation in the intestine before they are truly released.
How do they bind in the body?
An example is genistein, which can bind to estrogen receptors. It has greater affinity for ERβ (e.g. in bone tissue) than ERα, but the binding is much weaker than that of the body's own estrogen – about 1/400 of the binding to the alpha receptor.
Overall, isoflavones have a more modulating effect than “estrogen-stimulating” effect.
Other sources of phytoestrogens
Alfalfa and sainfoin also contain phytoestrogens, often in higher or more potent forms than soy. Therefore, there is no reason to proclaim soy as unique or particularly “problematic” in this context.
Sources:
- PMC – Genistein/ERβ selectivity
- aocs.onlinelibrary.wiley.com – Isoflavone distribution in soybeans
- scielo.br – Phytoestrogen content in different fractions
2 – Soybean meal: The effective protein source
What is soy flour?
Soybean meal is the part of the soybean that remains after oil extraction. It is a highly concentrated source of protein with high digestibility and a favorable amino acid profile.
Typical content in solvent-extracted soy meal:
| Nutrient | Content per 100 g (dry matter) |
|---|---|
| Crude protein | 45–48g |
| Lysine | 2.7–3.2g |
| Sugar | 9–11g |
| Starch | 2g |
| Fat | 1–2g |
| Digestible energy | 1.5–1.6 MJ |
(Sources: Mad Barn USA, Kentucky Equine Research)
Why is soybean meal used in horse feed?
-
High digestibility: approximately 90% of the protein is utilized, compared to 50–60% for many grass-based sources.
-
Good amino acid profile: Particularly high in lysine, which is often the limiting amino acid in horse feed.
-
Easy dosage: Relatively small amounts can cover a large protein requirement.
Soybean meal for senior horses
Older horses have the same protein needs as younger ones – but the quality becomes even more important. Soy meal is therefore particularly suitable for seniors, as it has high digestibility and a favourable amino acid profile.
-
Many senior horses lose weight easily in the winter, and soy meal can help maintain muscle mass.
-
Alfalfa can be advantageously combined with soybean meal to provide extra fiber and variety.
-
Only horses with diagnosed kidney disease need to have their protein intake adjusted individually. In protein-losing kidney disease, on the contrary, the need is often increased.
-
A healthy older horse thrives best on good quality protein rather than less protein.
3 – Soybean meal and weight loss: Why it works
Protein protects muscles
When you reduce the amount of roughage to achieve a calorie deficit, protein intake also decreases. If the horse gets too little protein, it will start to break down muscle mass to obtain amino acids. This slows down the metabolism and increases the risk of rapid weight gain.
The benefits of adequate protein during weight loss:
-
Preserves muscle mass
-
Keeps metabolism active
-
Provides a better feeling of satiety
-
Supports a healthy intestinal flora
Typical dosage for weight loss (per 100 kg horse):
100–150 g of soy meal daily (Merck Veterinary Manual). No grass during the course, adjust according to roughage analysis and horse's response.
Alternatives to soy flour:
Alfalfa, green pellets or sainfoin can be used, but approximately 3 times as much is required to achieve the same amount of protein, and the calcium-phosphorus ratio must be balanced.
4 – Soybean Husks: The Overlooked Source of Fiber
What are soybean hulls?
Soybean hulls are the shell of the soybean. They are not used for their protein content, but for their content of easily fermentable fiber.
Benefits of soybean hulls in horse feed:
-
Low sugar and starch content
-
Provides a steady flow of energy via hindgut fermentation
-
Supports stable intestinal environment
Phytoestrogens in shells?
Soybean hulls have a very low content of isoflavones because they are primarily found in the germ. Therefore, the hulls are not relevant in the discussion about phytoestrogens.
(Sources: Kentucky Equine Research, Kentucky Performance Products)
5 – GMO and soy: Facts and attitudes
Why was soy GMO grown?
The first GMO crop was soy, modified to tolerate glyphosate, a herbicide. The benefit to agriculture was easier cultivation, not better nutritional value.
Is there a health risk?
Glyphosate has been linked to health problems in humans – primarily through occupational exposure, not through food/feed. EFSA assesses that the permitted residue levels in feed do not pose a risk.
Our choice
Many people still choose GMO-free, especially for environmental reasons. All of Regulator Complete's production is certified GMO-free.
(Source: European Food Safety Authority)
6 – Alfalfa, green manure and sainfoin: Possible alternatives
Protein content and quantity requirements
Alfalfa, green pellets and sainfoin typically contain 16–20% crude protein and are therefore less concentrated than soybean meal. For the same amount of protein, you need about 3× as much.
Important to remember
-
The calcium-phosphorus balance must be correct, especially for young horses and pregnant mares.
-
They also contain phytoestrogens – often of a different and more potent type than soy.
(Source: Kentucky Equine Research)
7 – Feeding plan with soybean meal: An example
Example: When roughage only has 6% protein
An adult horse in light work needs approximately 150 g of crude protein per 100 kg of body weight per day .
If the roughage only contains 6% crude protein , the calculation looks like this:
1. What does roughage provide?
-
1.5 kg dry matter (DM) × 6% = 90 g protein
2. What does the horse lack?
-
Need: 150 g
-
Sheep from roughage: 90 g
-
Lack: 60 g protein
3. How much soy meal is needed?
-
Soybean meal contains approximately 47% protein
-
To get 60 g → approx. 125–130 g soy meal per 100 kg horse per day
👉 Please divide it into 2 meals.
✅ In short:
If your horse only gets roughage with 6% protein, it needs approximately 125–130 g of soy meal per 100 kg of body weight per day to meet its needs.
Example from Lighter Saturday (for weight loss)
(per 100 kg horse, 2–14 years – senior in brackets)
Week 1–4
-
No grass
-
1.0 kg hay/wrap
-
1.0 kg spring barley straw
-
85 g Every Day Happy (100 g senior)
-
90 g +Fiber Mash
-
10g salt
-
10 ml RC O3 algae oil (12 ml senior)
-
100–150 g soy flour
From week 5 to normal weight
-
No grass
-
1.0 kg hay/wrap
-
1.0 kg spring barley straw
-
65 g Every Day Happy (80 g senior)
-
90 g +Fiber Mash
-
10g salt
-
5 ml RC O3 algae oil (6 ml senior)
-
100–150 g soy flour
The ration typically amounts to 1.5–1.7% of body weight per day in dry matter – in line with common recommendations for weight loss (Merck Veterinary Manual).
Comments (0)
There are no comments for this article. Be the first one to leave a message!