Tryptophan requirement of 22 to 34-week-old caged-laying hens

Tryptophan, the third or fourth limiting amino acid for laying hens, is important in protein synthesis and as a precursor in the serotonin and kynerunine pathways. An experiment was conducted to evaluate the digestible tryptophan requirement of first cycle laying hens from 22 to 34 weeks of age. A total of 252 Hy-Line W36 laying hens were selected at 16 weeks of age and allocated by weight (P = 0.90) to seven dietary treatments resulting in 12 replicate cages of three birds for each treatment. A Trp deficient basal diet was formulated using corn, corn gluten meal and soybean meal for each of the dietary phases and supplemented with synthetic L-Trp to generate diets that provided 105, 119, 133, 147, 162, 176, and 190 mg digestible Trp on a daily basis over the egg producing phase of production, respectively. Over the pullet phase, hens were fed complete diets that contained increasing amounts of corn gluten meal to gradually transition to experimental diets first offered at 16 weeks of age. Over all dietary phases, birds were provided a controlled amount of feed based on expected feed intake of birds in commercial management conditions. Hen-housed egg production (HHEP), feed intake, egg weights, egg mass (EM), feed efficiency (FE) and mortality data were collected on a daily basis and calculated over two-week periods to correspond with diet changes. Linear and quadratic broken-line, and quadratic polynomial models were used to estimate digestible Trp requirements based on HHEP, EM and FE. Overall, HHEP, EM and FE ranged from 88.3 to 94.2%, 46.7 to 50.0 g/d, and 510 to 540 g/kg, respectively, with increasing dietary Trp resulting in increased responses until a plateau was reached. Digestible Trp requirements were estimated to be 136.6, 183.3 and 192.2 mg/d for HHEP; 133.0, 180.0 and 183.1 for EM and 132.5, 177.4 and 173.0 for FE using linear broken-line, quadratic broken-line and quadratic polynomial analysis, respectively. Linear broken-line analysis of digestible Trp requirements resulted in lower estimates than other models, and HHEP, EM and FE resulted in similar estimated digestible Trp requirements in 22 to 34-week-old laying hens.