Calculating horse nutrient requirements: the formulas

“How much does my horse need?” cannot be answered with one figure per kilogram. Metabolism does not scale linearly with body mass: a 600 kg horse does not need twice as much as a 300 kg pony. That is why requirements are calculated using metabolic body weight ¹².

This article explains the calculation logic behind it. It is deliberately not a substitute for a requirements table and not a ready-made ration calculation – but it is the map that lets you place both in context.

Energy: maintenance plus additions

In German-speaking countries, energy requirements are given in megajoules of metabolisable energy (MJ ME). They are additive: maintenance requirement plus additions for work, growth, pregnancy, lactation and – in unfavourable conditions – thermoregulation ¹³.

The maintenance requirement follows the form ME = k × BW⁰·⁷⁵. The key point is that the factor k is not the same for all horses: the GfE (German Society of Nutrition Physiology) differentiates by type and temperament ¹.

So a calm 500 kg warmblood is at about 55 MJ ME per day before doing any work at all. In practice, work additions are not estimated freely but added as tabulated values for each workload level ⁴¹.

Protein: pcd CP instead of crude protein

Since the GfE recommendations, protein in horses is no longer assessed simply as crude protein but as prececal digestible crude protein (pcd CP) – the share that actually becomes available as amino acids before the large intestine. What is only broken down by microbes in the large intestine is hardly available to the horse as a source of amino acids ¹².

The structure “maintenance plus additions” applies here too: pcd CP requirement = maintenance + work + growth/pregnancy/lactation. The constants come from the requirements tables ⁴.

• Energy-protein ratio = g pcd CP per MJ ME – a quick plausibility check of the ration.
• A ratio that is too narrow suggests a protein deficiency; one that is too wide suggests an unnecessary protein surplus.
• Excess protein is not stored but adds to nitrogen excretion and burdens the stable air ².

Macrominerals and trace elements: where calculation errors lurk

For calcium, phosphorus, magnesium, sodium and the trace elements, there is no universally valid short formula you could memorise. The values are tabulated – by life stage, weight and performance ¹³⁴.

A common mistake: requirements tables distinguish between the net requirement (what the horse actually retains) and the dietary requirement, which takes the absorption rate into account. The two figures differ considerably. Anyone who multiplies a constant found online by body weight without checking it can quickly get the requirement massively wrong ¹².

The Ca:P ratio

With calcium and phosphorus, it is not only the amount that counts but the ratio between them. A phosphorus surplus interferes with calcium absorption – which is why the ratio has a lower limit that should not be undercut ¹³.

Calculation building blocks for the ration

Once the requirement is established, the supply from the ration is calculated against it. This needs three simple but essential building blocks ⁵⁶:

• Summation: nutrient in the ration = sum of (feed amount × content) across all components.
• Dry matter and as-fed basis: analysis values usually refer to dry matter – as-fed amount = DM amount divided by the DM share. Skip this step and you will be out by double-digit percentages ⁶.
• Coverage = supply divided by requirement, times 100. Values well below 100% show gaps; values well above show an oversupply.

What the calculation cannot do

• It does not replace an assessment of body condition – requirements and actual use differ individually ⁷.
• It does not replace a veterinary examination in the case of illness or unusual weight changes.
• It does not provide a product recommendation: whether a gap is closed with mineral feed, a different forage or a change of ration is a professional decision ².