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Dry matter intake is 2.5-3.5% of body weight for a dairy cow and up to 4% for a high yielder. The ration splits between forage and concentrate; a forage share below about 40% risks rumen acidosis. The production allowance is roughly 400 g of concentrate per litre of milk above maintenance.
Dry matter intake
DMI (kg) = body weight x DMI %
Ration split
Forage DM = DMI x forage share; concentrate DM = DMI - forage DM; production concentrate = 0.4 kg per litre of milk
Green fodder is 75-85% water and silage 65-70%. Only dry matter carries nutrients, so rations balanced on fresh weight can be wildly wrong.
Rumen pH falls, fibre digestion suffers and the cow risks subacute acidosis, laminitis and a drop in milk fat. Keep effective fibre above the minimum for the animal's stage.