Protein quality instead of protein quantity — why lysine decides the ration

About protein one usually speaks in quantities in the horse: crude protein, per cent in the ration, grams per day. That falls short. A protein is only as valuable as its scarcest building block — and in the horse this scarcest building block is almost always a particular amino acid. The quality of the protein decides, not its mass alone.

The principle of the limiting amino acid

The body builds protein according to a fixed blueprint — if a single essential amino acid is missing, the whole synthesis stalls, no matter how abundantly the others are present. This scarcest amino acid is called the first-limiting one. In the typical horse ration of grass, hay and grain this is lysine. Once it is used up, the remaining protein can no longer be fully utilised — the horse excretes the surplus instead of building it in. After lysine follow methionine and threonine in the discussion as second- and third-limiting.

Exactly here the term biological value comes in: it describes how well the amino acid pattern of a feed fits the requirement of the horse. A feed with high crude protein content but unfavourable pattern is worth less than a more sparingly measured one with a balanced profile. It becomes visible where protein is needed: in the musculature, in the regrowing hoof horn and in the coat — all tissues with a high requirement of the matching building blocks.

When it counts particularly — and where the gaps lie

The requirement of high-quality protein is not constant. Growing horses — foals, weanlings, yearlings — have by far the highest relative lysine requirement, because they actively build tissue. In sport with muscle build-up too and in old age, when protein utilisation declines and muscle loss threatens, quality gains weight. A pure pasture or grass ration often does cover the crude protein quantity, but can leave a gap precisely with lysine — especially when the grass is overmature and thinned in its amino acid profile.

The research thereby urges differentiation. A controlled study in yearlings found with staggered lysine intake no linear rise in whole-body protein synthesis — a hint that more of a single building block does not automatically translate into more build-up. Amino acids act in interplay, and to supplement beyond the requirement brings no additional benefit. The lesson from this is not less lysine, but to close the gap in a targeted way instead of blindly topping up.

Protein quality is thus no academic detail but the actual adjusting screw. Whoever knows the first-limiting amino acid and aligns the ration to it feeds more precisely and often more sparingly than whoever looks only at the crude protein value. With protein the rule is: first the pattern, then the quantity.