The formula
dry matter = live weight × % · as fed = dry matter ÷ dry matter % × 100
Where it comes from
A ration is worked out in dry matter, not in what goes into the trough, because what goes into the trough carries water and water does not feed. A silage at thirty per cent dry matter is three kilos of product for every kilo that counts; the same animal on hay at eighty-five per cent would take barely over one. Confusing the two is what makes a ration that works on paper fall short in the yard. Dry matter intake is expressed as a share of live weight and is the one stable part of the sum: around three per cent for a dairy cow, less in finishing, more in growing animals.
How to work it out by hand
- Multiply live weight by the intake percentage: that is the dry matter eaten a day
- Split it between forage and concentrate in the ratio you want
- Divide each part by its dry matter percentage and multiply by a hundred: that is what goes into the trough
- Multiply by head and by thirty to plan the month’s buying
What is worth knowing
This sum says how much, not what. The balance of energy, protein, fibre and minerals is what decides whether a ration works, and that needs a forage analysis and a nutritionist: two silages at the same dry matter can differ widely in energy. The default intake figure is a mature-cow reference, not a recommendation for a particular farm.