Protein in horse feed: crude protein, lysine and excess

Protein is the body's building material, not its fuel: it supplies the amino acids for muscle, connective and other tissue as well as for enzymes ². Some of these amino acids are essential – the horse cannot make them itself and has to take them in through the ration. The most important of these in practice is lysine ¹.

Two things are regularly confused when it comes to equine protein: how much protein is listed for the feed and how much of it actually arrives. That is exactly what this article is about.

Crude protein is not the same as usable protein

The classic Weende analysis gives “crude protein” as nitrogen × 6.25 – that is the total nitrogen in the feed, not the share the horse absorbs as amino acids ³. That is why, since the GfE (German Society of Nutrition Physiology) recommendations, equine protein has been assessed as small-intestine digestible (prececal digestible) crude protein, abbreviated in German as dvRP or pcvXP (pcd CP): only what is digested before the large intestine and absorbed as amino acids is available to the metabolism as building material ⁴².

The difference is large and depends on the type of feed. Prececal nitrogen digestibility is only around 27–41% for forage, but around 58–72% for concentrates ¹. Most of the remaining forage nitrogen is broken down by microbes only in the large intestine and absorbed mainly as ammonia – the horse cannot make essential amino acids from it, and this nitrogen is lost again as urea ¹.

How much does a horse need?

As with minerals, it is worth looking at both reference works here – with an important warning up front. The NRC gives the crude protein requirement (CP) for maintenance, graded by temperament: 1.08 g (calm), 1.26 g (average) and 1.44 g (active) per kg body weight per day ¹⁵. The GfE, by contrast, relates its requirement to metabolic body weight and to digestible protein: 3.0 g dvRP per kg BW^0.75 per day ⁴⁶⁷.

The fact that the GfE gives “364 g” and the NRC “756 g” is not a contradiction and does not mean that the GfE assumes half the requirement. They are two different values: dvRP is a subset of crude protein – only the digestible share. You must therefore not convert the two figures into each other or average them ⁴¹. The maintenance share stays the same across all life stages; allowances for growth, pregnancy, lactation and work are added on top. The concrete calculation logic is set out in the article on calculating requirements.

Lysine, threonine and the question of what runs short first

A limiting amino acid is the one whose shortage is the first to limit how well the rest of the protein can be used – like the too-short roof beam that determines how high the whole roof can be. In pigs and poultry on cereal-based rations this is lysine, and this rule of thumb is often transferred to horses without question.

For horses, only one thing is initially robust: lysine is the only amino acid whose requirement is based on direct dose-response studies – nitrogen retention and growth ¹. This requirement is 36 mg per kg body weight as a minimum and 54 mg per kg as an optimum, or alternatively around 4.3% of the crude protein requirement ¹.

For methionine and threonine, the German-language requirement standard does list tabulated values (in which threonine, as mentioned, is above lysine), but for both there is still no requirement figure backed by direct feeding studies in horses; threonine estimates range between 53 and 80% of lysine intake ¹⁶. Which amino acid actually limits first in the high-fibre equine ration has therefore not been conclusively settled. Only the practical consequence is clear: no effect on hoof, horn or coat can be derived from a single amino acid – such promises are not supported ¹.

Quality before quantity

A survey of 201 horses described in the review found an average crude protein intake of 157% of the NRC recommendation. That is a finding for this group, not a statement that every horse gets too much protein. In a trotter study discussed there, the higher protein supply did not lead to greater gain, but it did lead to higher water intake, nitrogen excretion and urine volume ¹.

Energy supply is part of the picture too: the review describes weight loss with energy deficiency despite protein intake. For checking a ration, this means assessing energy and usable protein together. Excesses are not a proven muscle-building strategy ¹.

What forage and concentrates provide

Grass hay usually covers the maintenance protein requirement of adult horses; what matters, however, is the combination of quantity and digestibility. The crude protein content of grass hay varies greatly with botanical composition, cutting date and cut, and falls as the grass becomes over-mature – late, stalky hay is lower in protein and digestibility ²¹.

In terms of protein quality, legumes such as lucerne (alfalfa) and soya bean meal provide more and higher-quality protein than grasses; soya bean meal is the classic lysine-rich supplementary component – in the growth trials from which the lysine evidence comes, lysine was supplemented precisely in this way ¹². This is exactly where targeted amino acid supplementation comes in: with rations that are short in digestible protein or lysine, for example with over-mature hay.

Can too much protein explain skin or stomach problems?

The nutrition sources cited here do not establish a causal link between a protein excess and sweet itch or gastric ulcers. A suspected oversupply needs a ration check; the symptoms need a veterinary investigation regardless. The skin guideline distinguishes between insect hypersensitivity and suspected food allergies, and the veterinary information on the stomach describes non-specific signs and a separate diagnostic process ⁸⁹.

• What do analysis values and actual intake amounts show?
• Which protein reference value is being compared with which requirement?
• Which veterinary findings exist independently of the feeding suspicion?

• Putting sweet itch and feeding into context
• Having stomach problems professionally investigated
• Checking feeding for muscle building

Requirement peaks and limits of interpretation

Horses need considerably more protein and lysine during growth, pregnancy and lactation – there is even a separate requirement formula for lactating mares, and the robust lysine evidence comes precisely from growth ¹. These are also the groups for which no automatic recommendation is given: foals, young horses during rearing and pregnant and lactating mares belong in specialist and veterinary advice, because this is where requirements are highest and errors weigh most heavily.

What can be managed and documented is the intake through the ration – for that you need a feed or hay analysis ³¹⁰. Plasma amino acid levels are a research method for estimating requirements, not a routine test for the yard ¹.

Key takeaways

• Crude protein and digestible protein requirements must not be offset against each other as if they were the same value ¹.
• Protein cannot make up for an inadequate energy intake at will ¹.
• A suspected protein excess is not a skin or stomach diagnosis ⁸⁹.

Frequently asked questions