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 da ys (Experiments 1 and 2) and 28 days (Experiment 3). In Experiment 1 c hicks 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 punctur e for amino acid (AA) analysis at hourly intervals between 0 and 6 h a fter 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) semisynthe tic diets of different protein quality. In Experiment 2, high protein diets (20% crude protein) based on casein (C), lupin (L), soya bean (S B), 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 di ets (12% CP) contained SB, FP, V or B. In both experiments, a N-free d iet was also given. Samples of blood were taken for AA analysis betwee n 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 r eaching the initial values after 6 h. Irrespective of the protein qual ity or the protein level of the diets, the plasma AA ratio technique w as in reasonable agreement with the Protein Score procedures in identi fying the two most limiting amino acids for mast diets assayed. Methio nine was found to be either the first or second limiting PLA for all d iets, with the exception of diet C in Experiment 2 (tryptophan and his tidine, respectively).