Physiological oxidation happens inside an animal's body through various metabolic pathways. Physical oxidation takes place inside a bomb calorimeter which converts feed energy to heat. Calorie is the unit for energy of our choice. This provides a measurement of total energy content in a feed. A digestion trial is required to collect feed intake and fecal samples.
Fecal energy is the single largest loss for a ruminant.
Not all the DE of a feed can be retained by an animal. The energy loss is urine, mainly urea in the urine. ME represents the retained energy which supports two different functions, maintenance and production. Most of ME values are not experimentally derived. One final energy loss happens when the animal actually uses ME for maintenance or production.