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.