Growth Responses of Broiler Chicks to Different Levels of Threonine and Tryptophan in Chemically-Defined Diets

In Experiment 1 three semi-purified diets were formulated using isolated soy protein and crystalline amino acids (AA) to contain three profiles of essential amino acids (EAA) based on NRC (1994; Diet 1), Illinois Ideal Chick Protein (IICP; Diet 2) and Blair et al. (1977; Diet 3), respectively, and were fed to male broiler chicks from 0 to 14d of age. Mean body weight gain at 2 w of age was 132.7, 145.0 and 192.8g (P<0.05) and mean daily feed intake 169.4, 173.9 and 240.4g (P<0.05), respectively, for the NRC, Illinois and PRC diets. Mortality with Diet 3 was significantly higher (P<0.05) than with the other two diets, due mainly to an increased incidence of gizzard impaction. In Expt. 2, three levels (90%, 100% and 110% of NRC, 1994) of threonine (Thr) and tryptophan (Trp) were arranged in a 3×3 factorial manner in a modified Diet 3 to give nine purified diets containing crystalline AA as the only source of AA. Broiler chicks were fed a commercial diet during the first w of age and experimental diets were fed from 1 to 3w of age. Weight gain was significantly lower (P<0.05) at the highest levels of both AA (110% Thr and 110% Trp) suggesting that that the diets became imbalanced at high levels of both Thr and Trp. The results of Experiment 2 suggest that dietary levels of Thr and Trp should be targeted at 65% of lysine (equivalent to 0.63% of digestible Thr) and 16% of lysine (equivalent to 0.16% of digestible Trp), respectively, for starting broilers. The results of this study suggest that the requirements for digestible Thr and Trp are no more than 7.2 and 1.8g/kg diet, respectively.