Apparent ileal digestible tryptophan requirements of 22- to 42-day-old broiler chicks

Using apparent ileal digestible (AID) amino acids (AA) for feed formulation is considered preferable to formulating diets based on a total AA basis. However, data on AID tryptophan (Trp) requirements of broiler chicks are scarce. This study was conducted to determine the AID Trp requirements for broiler chicks from 22 to 42 d of age. A total of 1,008 (504 male and 504 female) 22-day-old broilers (Cobb 500) were randomly allocated within sex to 7 treatments (8 replicates with 9 birds per cage). The birds were fed diets based on corn, soybean meal, cotton seed meal, and corn gluten meal, which were supplemented with 0, 0.018, 0.036, 0.054, 0.072, 0.09, or 0.108% crystalline L-Trp (basal diet contained 0.144% Trp) in order to supply 0.144, 0.162, 0.1180, 0.1198, 0.216, 0.234, or 0.252% designed total Trp. Performance, meat quality, and fat deposition were evaluated at the end of the 21 d trial. The AID of AA was measured using the indicator method. There were no significant differences in the AID of AA between male and female broilers. The performance of female broilers was not affected by Trp level. However, weight gain and feed intake of male broilers showed a quadratic response to Trp levels (P < 0.05). The AID Trp requirement of males was estimated to be 0.14% using feed intake and 0.15% using weight gain based on a quadratic regression. The AID Trp levels quadratically affected abdominal fat content and drip loss of male broilers, as well as pH 45 of female broilers, and optimized abdominal fat and drip loss for male broilers was AID Trp of 0.13 to 0.15%. This study suggests that for male broilers 22 to 42 d of age, the AID Trp requirement is approximately 0.15%, based on performance and meat quality. For female broilers, based on the general lack of response to AID Trp levels, we conclude that the requirement is at or no higher than 0.11% AID Trp.