However, it does not provide any information on why certain diet is better than the other. Digestion trial involves smaller number of animals. Digestibility is defined as the proportion of the nutrient which is absorbed by the animals. Since not all the nutrient in the feces were derived from dietary source this digestibility is termed apparent digestibility. Fecal nutrients came from sloughed off intestinal cells and digestive enzymes are called endogenous source of nutrients.
Digestibility calculated after removing the endogenous nutrients is called true digestibility.
Calculation of digestibility requires total collection of feed intake and fecal output, which is difficult to do, especially with grazing animals. An indirect method was developed. This dictates that an indicator must be able to pass through GI tract at the same flow rate as other nutrients, and it must not be digested and absorbed by animals. Chromic oxide and some rare earth elements are commonly used external indicators. Lignin has been used as an internal indicator.