There are several ways to describe energy content in feeds.
Measured as a percentage, digestibility relates to the portion of food that is not excreted in the faeces and is available for use by the cow.
Digestibility is not a direct measure of energy but it is indicative of overall feed quality.
The greater the digestibility, the more the benefit of that food to the cow.
2) Metabolisable energy (ME)
Metabolisable energy is the energy in a feed that the cows can use for metabolic activities.
Metabolic activities include maintenance, activity, pregnancy, milk production and gain in body condition.
ME of a feed can be calculated directly from its digestibility.
The higher the energy content of the feed, the more energy is available to the animal.
It is measured by MJ ME/kg DM.
3) Total digestible nutrients (TDN)
TDN is an old system used in US and some other Asian countries.
It is less accurate than ME s it does not account energy losses via methane and urine.
TDN content is expressed as a percentage while TDN intake is expressed in kg/day.