A Comparison of Two Approaches forDetermining the Optimum Dietary Amino Acid Ratios of Fast-growing Broilers

摘要

Two approaches using the amino acid (AA) dilution method, the comparative slaughter and nitrogen (N) balance methods, were used in the present study to re-evaluate the assumptions of ideal ratios between the essential amino acids (EAA): lysine (Lys),methionine + cystine (Met+Cys), threonine (Thr), tryptophan (Trp), arginine (Arg), valine (Val), isoleucine (He), leucine (Leu), phenylalanine + tyrosine (Phe+Tyr), glycine + serine (Gly+Ser) and histidine (His) for growing broilers of the Cobb 500 genotype over three periods (I: 6 to 21 days, II: 22 to 37 days, and III: 38 to 53 days). In each trial 120 male chickens were housed in metabolic cages for assessment of individual N balance and AA efficiency. An AA-balanced diet (BD) was formulated according to the recommendations of Brazilian tables for the ideal protein in growing broilers. The diets with different limiting AAs were created by dilution of the BD with maize starch to achieve 0.70 of the AA level in BD and supplemented with crystalline AAs, except for the AA under study. The AA-diluted diets led to significant impairment of protein utilization and indicated the valid limiting position of these AAs. Also, at the start and end of the trial a group of birds with the mean body weight of eachreplicate was killed with no blood loss to determine N deposition using the comparative slaughter technique. The mean values of the optimum ratios of the 11 tested EAAs determined by comparative slaughter in the three periods are: Lys 100, Met+Cys 65, Thr 66, Trp 17, Arg 108, Val 79, He 61, Leu 122, Phe+Tyr 128, Gly+Ser 155 and His 41. Based on observed AA efficiency data, the optimum ratios determined by N balance are: Lys 100, Met+Cys 72, Thr 65, Trp 17, Arg 106, Val 76, He 67, Leu 107, Phe+Tyr 115, Gly+Ser 137 and His 35. There are some differences among the results obtained by the two methods (the broken line model and the Goettingen approach) and they will be discussed in accordance with the key assumptions.