chooses to calculate digestibility values once feed has passed through the small intestine, but before it enters the large in- testine. As a result, he uses piglets surgically fitted with a T-cannula at the distal ileum, when the feed is about to enter the colon. Digesta are collected from the ileum cannula. Pro- tein and amino acid content both in feed as well as in the ileal digesta are then analysed and compared, providing the most accurate figure for true protein and amino acid digestibility for animal or plant protein ingredients. In research, often a distinction is made between three differ- ent types of digestibility, which will be briefly discussed be- low. What matters is that standardised ileal digestibility is im- portant for dietary formulation; the others are steps in the calculation process.
Apparent ileal digestibility (AID) AID is calculated by deducting the total ileal outflow from the dietary amino acid intake. In simple words: what goes in, mi- nus what comes out. The value in itself mostly functions as a starting point for further research. As the name suggests, ‘ap- parent’ ileal digestibility appears to represent the fraction of amino acids that has been digested, but there are a few other things to consider.
True ileal digestibility (TID) The fact is that total ileal outflow consists not only of
non-digested amino acids. Another component is what is known as ‘endogenous losses’, or the total of proteins that are derived from the animal itself and are released into the gas- trointestinal tract during digestion. These include mostly di- gestion enzymes, bile, mucus and dead intestinal epithelial cells. To calculate the TID, the AID is corrected for these endogenous losses.
Standardised ileal digestibility (SID) Endogenous losses can also be subdivided into two types: ‘basal losses’, or losses occurring anyway; and ‘specific losses’, those that are dependent on feed ingredient composition. Elements such as anti-nutritional factors and dietary fibre lower the digestibility, causing the endogenous losses to be higher. When AID is only corrected for the basal losses, the SID is what is left.
Once the correct in vivo digestibility values for proteins and amino acids have been established, only then can feed com- panies and pig farmers formulate their diets more accurately. Providing a well-balanced feed will ensure maximum growth, most efficient feed conversion and a healthy pig. After all, any protein and amino acids that are not digested and absorbed in the small intestine are lost for the pig and contribute to more nitrogen excretion.
▶PIG PROGRESS | Volume 36, No. 2, 2020 29
Piglets during a digestibility trial.
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