Forage requirement calculator

Digestible energy, not kilograms of feed

The calculator gives the requirement in megajoules of digestible energy (DE). That is the unit NRC 2007 and the GfE 2014 recommendations work in — and it is independent of which feedstuff the energy comes from.

The detour via energy is necessary because kilograms say nothing. A kilogram of late, stemmy hay carries considerably less energy than a kilogram cut early. Planning in kilograms means planning in a unit that means something different from batch to batch.

Why body weight is raised to the power of 0.75

Maintenance requirement is derived from metabolic body weight: body weight to the power of 0.75, multiplied by a factor. For a 500-kilogram horse of average temperament that yields roughly 61 MJ per day.

The exponent is not an arithmetic trick. An animal's energy turnover does not grow in proportion to its mass but more slowly — a 300-kilogram pony needs more per kilogram of body weight than a 600-kilogram warmblood. Scaling linearly underfeeds small horses and overfeeds large ones.

Temperament shifts that baseline. A horse that is constantly moving in the field has a measurably higher maintenance requirement than one that stands. The calculator reflects this in three steps.

Work and life phase are not multiplied

A horse in moderate work that is also in early lactation does not get both factors stacked. The calculator takes the higher of the two.

That is a deliberate decision, not a convenient simplification: a mare in early lactation is not in fact ridden in moderate work. Multiplying both factors would produce a figure that does not occur in practice — and in this species an energy recommendation that is too high is not a harmless error.

What the calculator does not know

Forage quality. Hay varies in energy and protein more than most owners assume — depending on cutting date, sward, weather and storage. Without an analysis everyone works with average values, including this calculator.

A hay analysis costs a two-figure sum and is the only input in the whole calculation that can be measured rather than estimated. For yards with several horses it is usually the single most worthwhile investment in feeding.

Also not included: individual metabolism. Two horses of the same weight in the same work can differ markedly in actual requirement. The calculated figure is a starting point for observation, not a target.

Forage first

The horse is a continuous feeder with a digestive system built for continuous fibre intake. The recommendations therefore state a minimum forage amount relative to body weight before concentrate is discussed at all.

Gaps of more than four hours without feed are viewed critically in the specialist literature. Covering the energy requirement through concentrate while economising on hay solves one arithmetic problem and creates another.

From megajoules to kilograms of hay

Nobody weighs hay in megajoules. The calculator therefore also translates the total requirement into an amount of hay: it divides the digestible energy by a guide value of 7.5 MJ DE per kilogram of average-quality meadow hay, as fed, with about 87 percent dry matter. The result is rounded to half a kilogram — a more precise figure would suggest an accuracy the guide value does not have.

Two limits frame this number. At the lower end is the minimum roughage amount of 1.5 kilograms of dry matter per 100 kilograms of body weight, as given by the GfE. For 500 kilograms that is about 7.5 kilograms of dry matter, or just under 9 kilograms of hay. For a pure maintenance requirement of about 61 MJ the energy calculation alone would give only a little over 8 kilograms — the calculator then shows the minimum amount and says so.

At the upper end is the usual intake limit of about 2 percent of body weight as dry matter, which for 500 kilograms is around 11.5 kilograms of hay. If the energy requirement is higher, for example 100 MJ in heavy work, the amount of hay is capped at this limit and the remainder is shown in MJ. How that remainder is covered is deliberately left open by the calculator; it belongs in ration planning with professional advice.

The 7.5 MJ are an average. Early-cut, leafy hay lies above it, late, stemmy hay below — anyone with a hay analysis can check the figure against it. The amount in kilograms is therefore a guide value for planning, not a target for the individual horse.

• NRC 2007 — Nutrient Requirements of Horses, 6th revised edition, National Research Council
• GfE 2014 — Recommendations for energy and nutrient supply of horses, Gesellschaft für Ernährungsphysiologie
• Harris et al. 2017 — Review: Feeding conserved forage to horses — recent advances and recommendations. Animal 11(6), PMID 27895255
• BMEL 2020 — Guidelines for assessing horse husbandry from an animal welfare perspective (German Federal Ministry of Food and Agriculture)