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.