There are problems to determine the efficiency of protein deposition of int
estinal absorbed amino acids, because it cannot be excluded that the amount
of single or some indispensable amino acid (IAA) can influence the growing
performance of the animal. The dependence of amino acid gain on the flux o
f essential absorbable amino acids into the duodenum was studied in growing
bulls to calculate the utilization rate of these amino acids. Four growing
bulls (Schwarzbuntes Milchrind, BW: 240 - 310 kg) with duodenal T-type can
nulas were fed diets varying in crude protein content. Energy level was 11,
2 MJ ME/kg DM. The efficiency for the different absorbable IAA was decrease
d with increasing nutritional level. This is in contrast with the constant
utilization factor of the protein evaluation system (AfB, 1995; 1997). The
efficiency of utilization of apparently digestible IAA for protein retentio
n was highest for His and Met and amounted to 33 (Ile), 34 (Phe and Thr), 3
8 (Lys and Val), 41 (Leu), 52 (Met) und 76 (His) %, respectively. The inter
section of the regression curve (AA-deposition against apparently digestibl
e AA) with the abscissae resulted in the maintenance requirement of absorba
ble amino acid. For growing bulls (BW: 300 kg) a value of 4,7 g/d histidine
and 4,2 g/d methionine was determined, but more investigations are necessa
ry in this field.