THE USE OF PLASMA-FREE AMINO-ACIDS FOR PREDICTING THE LIMITING AMINO ACID(S) IN DIETS FOR CHICKENS

Three experiments were carried out with male broiler chicks aged 23 days (Experiments 1 and 2) and 28 days (Experiment 3). In Experiment 1 chicks were tube-fed with a nitrogen (N)-free diet for 4 days and then with a diet based on casein. Blood samples were taken by heart puncture for amino acid (AA) analysis at hourly intervals between 0 and 6 h after a meal. In Experiments 2 and 3, chicks were tube-fed at 1.5 or 1.9 maintenance level with isoenergetic (13.1 kJ ME/g(-1) DM) semisynthetic diets of different protein quality. In Experiment 2, high protein diets (20% crude protein) based on casein (C), lupin (L), soya bean (SB), faba bean (FB), field pea (FP), vetch (V) or bitter vetch (B), as the only source of protein, were used. In Experiment 3, low protein diets (12% CP) contained SB, FP, V or B. In both experiments, a N-free diet was also given. Samples of blood were taken for AA analysis between 3 and 4 h after a meal. In Experiment 1, a post-prandial increase in aminoacidemia was found; AA levels in plasma rose quickly, reaching a peak at 1 h, remained constant until 5 h and then declined, without reaching the initial values after 6 h. Irrespective of the protein quality or the protein level of the diets, the plasma AA ratio technique was in reasonable agreement with the Protein Score procedures in identifying the two most limiting amino acids for mast diets assayed. Methionine was found to be either the first or second limiting PLA for all diets, with the exception of diet C in Experiment 2 (tryptophan and histidine, respectively).