Once maintenance requirement is fulfilled, the rest are used to support processes involved in production. Feeding trial is easy to conduct. It requires a large number of animals. 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.