TDN (Total Digestible Nutrients)
TDN — Total Digestible Nutrients — is the standard measure of digestible energy content in cattle feed ingredients and complete rations. It represents the sum of all the digestible nutrients in a feed and is expressed as a percentage of the feed's dry matter.
The TDN formula
TDN combines four digestible components:
TDN % = Digestible Crude Protein + Digestible Crude Fibre + Digestible Nitrogen-Free Extract + (2.25 × Digestible Ether Extract)
The 2.25 multiplier on fat reflects that fat contains roughly 2.25 times more energy per gram than carbohydrate or protein.
Why TDN matters
TDN is the primary energy metric used in Indian dairy ration formulation. A higher TDN value means more usable energy per kilogram of feed, which directly translates into milk production potential.
| Animal class | TDN target (DM basis) |
|---|---|
| Lactating cow / buffalo | 65–70% |
| High-yield lactating (15+ L) | 70–75% |
| Calf starter | 75–78% |
| Dry cow / heifer | 55–60% |
| Maintenance feeding | 50–55% |
Typical TDN values of common Indian feed ingredients
| Ingredient | Approximate TDN (DM basis) |
|---|---|
| Maize | 78–82% |
| Groundnut cake | 75–80% |
| Cotton seed cake (premium) | 70–75% |
| Soybean meal | 78% |
| Wheat bran | 65–70% |
| DORB | 55–60% |
| Mustard cake (expeller) | 70–75% |
| Maize silage | 65–70% |
| Green fodder (mature) | 50–60% |
| Dry fodder (paddy/wheat straw) | 38–45% |
TDN vs ME
TDN is one of two common energy measures:
- TDN is used in Indian and North American dairy nutrition; expressed as a percentage
- ME (Metabolisable Energy) is used in European systems; expressed in MJ/kg
The two are related: roughly ME (MJ/kg DM) ≈ 0.15 × TDN (%). So a feed at 70% TDN delivers approximately 10.5 MJ/kg ME.
Practical use
When formulating a ration, the target TDN of the whole ration (forage + concentrate combined, on DM basis) should match the animal's production level. For a lactating cow yielding 12 L/day, the ration TDN target is around 67–68%. The DCP and TDN calculator computes the total TDN of any ration from its ingredients and quantities.