English to German Science Livestock / Animal Husbandry animal feed

From the formulation data of a certain type of animal feed; the term appears in a table of amino acids. The table has the different acids in the rows and the columns are labelled as follows (three columns, each separated into two sub-columns):

Content:
G/kg "As is"
% of protein

Poultry:
Digest % TRU
Digest g/kg

Piglets:
Digest % SID
Digest g/kg


Unfortunately, Google mistakes TRU to mean true, which makes searching for this abbreviation very difficult. Does anyone have a clue what it stands for?