Total Digestible Nutrients Supplied

Total digestible nutrients is the oldest surviving measure of feed energy — the sum of the digestible protein, fibre, nitrogen-free extract and fat, with the fat multiplied by 2.25 because it yields that much more energy per unit than carbohydrate. It is expressed as a percentage of the feed's dry matter, so multiplying it by a feed weight gives the mass of digestible nutrients delivered. Twenty kilograms of dry matter at 65% TDN supplies 13 kg of TDN.

The mistake this page exists to prevent is applying that percentage to an as-fed weight. TDN percentages come from feed tests, and feed tests report on the dry matter, always. Twenty kilograms of maize silage as fed at 32% dry matter carries 6.4 kg of dry matter, and at 68% TDN that is 4.4 kg of TDN — not the 13.6 kg a careless multiplication would suggest. The error runs in the generous direction, which means a ration built this way looks adequate on paper while the animals lose condition in the field.

Typical values place grass hay in the mid-50s, alfalfa around 58%, maize silage around 68%, barley about 84% and maize grain about 88%. Fats exceed 100% by construction, which is a good reminder that TDN is a calculated index rather than a measured energy: the 2.25 multiplier on fat pushes a pure oil above the ceiling a percentage would seem to impose. Forages above the mid-60s are unusual, and a forage test reading in the 80s almost always means a column was read wrong or a grain sample was mislabelled.

TDN is a serviceable index and an imperfect one, which is why modern ration software works in net energy instead. TDN treats all digestible nutrients as equally useful once absorbed, and they are not — the heat lost in fermenting fibre means a forage and a grain with equal TDN do not support equal production, and the gap widens as the production level rises. Net energy systems split maintenance, gain and lactation into separate values precisely because of this. TDN survives because it is simple, universally reported and adequate for maintenance-level feeding, and it is worth knowing exactly how far that adequacy extends.