The efficiency of utilization of intestinal digestible indispensable amino acids in growing bulls

There are problems to determine the efficiency of protein deposition of intestinal 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 of 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 cannulas 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 decreased 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 retention was highest for His and Met and amounted to 33 (Ile), 34 (Phe and Thr), 38 (Lys and Val), 41 (Leu), 52 (Met) und 76 (His) %, respectively. The intersection of the regression curve (AA-deposition against apparently digestible AA) with the abscissae resulted in the maintenance requirement of absorbable 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 necessary in this field.