ABSTRACTThree experiments were conducted to determine tryptophan requirements of broiler chickens under tropical environment. Experiments 1 and 2 were conducted to determine the requirements for the cool and hot seasons respectively. The third experiment was conducted to determine the effect of diet formulations on the basis of four critical amino acid requirements (lysine, methionine, tryptophan and threonine) on the crude protein requirements of broiler starter and finisher chickens. The first and second experiments had 19 birds and 18 birds per replicate for the starter and finisher phases respectively. The minimum daily temperature was from 14oC - 20oC during the cool season while the maximum daily temperature is from 190C- 400C during the hot season. Diet 1 had insitu tryptophan content of 0.15%. Diets 2 to 5 contained graded levels of supplemental tryptophan at 0.04% intervals giving total dietary tryptophan levels of 0.15, 0.19, 0.23, 0.27 and 0.31- percent levels for the starter phase studies conducted during the cool and hot seasons. For the finisher phases, there were five experimental diets. Diet 1 had insitu tryptophan content of 0.13% while diets 2 to 5 contained graded levels of supplemental tryptophan at 0.04% intervals resulting in total dietary tryptophan levels of 0.13, 0.17, 0.21, 0.25 and 0.29- percent respectively in both cool and hot season experiments. A total of two hundred and eighty five day old broiler chicks were used for each of the starter phase experiments while two hundred and seventy chicks were used for the finisher experiments. Experiments 1 and 2 had five dietary treatments. Each experiment lasted from 0 to 4 weeks for the starter phases and 5 - 8 weeks for the finisher phases. For the third experiment, four iso-caloric starter diets were formulated with the dietary crude protein levels increasing by 1% intervals from the 20% CP to 23% CP while the finisher diets had the crude protein increasing by 1% intervals from 18% CP to 21%.CP. The three experiments were of completely randomized designs, each treatment had three replications. The third experiment had 16 birds and 15 birds per replicate for the starter and finisher phases respectively. The results for the cool season starter study showed that chicks fed starter diet containing 0.23% dietary tryptophan gave significantly (P<0.05) higher final body weight and average daily gain than those fed other dietary tryptophan levels. For the finisher phase, the birds fed 0.21% tryptophan diet performed significantly (P<0.05) better in terms of final weight, average daily gain and feed conversion ratio than all other treatments. Chickens fed diets containing supplemental tryptophan had better carcass values such as %breast, % thigh and % drumstick compared to chickens fed diets with no supplemental tryptophan in the diet. For experiment 2, conducted during hot season, starter chicks fed 0.23% total tryptophan diet gave better results in terms of final body weight, average daily gain and feed cost (N) /kg gain compared to chicks fed other tryptophan diets. The results for the finisher phase showed that birds fed 0.21% tryptophan diet performed significantly (P<0.05) better in terms of final weight, average daily gain and feed cost (N)/kg gain compared to other dietary tryptophan levels. Birds fed 0.21% total tryptophan diet also had significantly (P <0.05) better carcass weight, dressing percentage and % breast. It was also observed that broiler chickens fed diet high in essential amino acids (lysine, methionine, tryptophan and threonine) had slightly low abdominal fat and tissue fat. Results for the starter phase for the third experiment indicated 21% CP diet gave similar results as obtained with the higher crude protein diets in terms of the final weight, and average daily gain of chicks across treatments. The FCR also indicated similar results for the 21 and 23 % CP diets. For the finisher phase chicks fed 19 %CP or 20%CP diets had similar results for final weight, weight gain and average daily gain as those fed the 21% CP diet. Also the digestibility for ash and NFE were significantly better for the 21% CP diet. The 21% CP diet was however regarded to be the optimum crude protein requirement due to slightly better results for FCR and feed cost per kg gain. It was concluded that dietary tryptophan requirements during the cool and hot seasons for the starter phase was 0.23% and for the finisher phase was 0.21%. The crude protein of broiler starter diets can also be reduced from the current practice of 24 or 23 percent to 21% while the optimum for the finisher phase stood at 21% CP.
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