Submitted to: British Poultry Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/26/2005
Publication Date: 8/1/2005
Citation: Corzo, A., Kidd, M.T., Thaxton, J.P., Kerr, B.J. 2005. Effect of dietary tryptophan on growth and stress responses of male broiler chicks. British Poultry Science. 46:478-484. Interpretive Summary: Tryptophan concentration in vertebrate organisms is the lowest of all the amino acids, yet it is an essential amino acid for structural proteins, affects lipid and vitamin metabolism, and is linked to the biosynthesis of various hormones. Practical diets composed largely of vegetable proteins typically results in valine, arginine, and tryptophan being limiting near the same degree after that of methionine, lysine, threonine and isoleucine. The specific degree of tryptophan limitation depending on the inclusion levels of the various vegetable ingredients. Although feed grate tryptophan is currently available for supplementation into dietary formulations, the exact level of tryptophan required in the diet for optimal production is poorly understood due to its lower order of limitation. Experimentation indicated that birds fed a low protein basal diet required approximately 0.21% dietary tryptophan to support maximum growth and feed conversion for chicks from 0 to 20 days of age. Consequently, dietary tryptophan should be considered as a limiting nutrient in early chick nutrition, especially if dietary protein is low. As environmental issues with nitrogen losses from poultry operations becomes more pressing and as the availability of crystalline amino acids becomes more economically viable, understanding tryptophan limitations in low crude protein diets and its concentration in feeding programs for optimal production and minimal nitrogen excretion, is paramount to the broiler industry. Research results described in this report provides nutritionists at universities, feed companies, and broiler production facilities vital data on the importance of tryptophan in dietary formulations.
Technical Abstract: The need for dietary Trp and its effect on stress of broiler chicks from 0 to 20 d of age was evaluated. Ross x Ross 508 male chicks were used in three studies: two diet-validation studies and a dose-response study. The first stydy compared the titration diet (corn-soybean meal-gelatin byproduct based diet) against two corn-soybean meal control diets (marginal and high in dietary lysine). Day-old chicks were randomly distributed across a closed-curtain sided house (24 floor pens; 13 chicks/pen), and fed the experimental diets. Growth and feed intake were determined at 20 d of age. The control diets had higher BW (P < 0.001) and feed intake (P < 0.001) than the titration diet; however, feed conversion of the chicks was similar among treatments. A second study estimated the Trp needs for growth of chicks from 0 to 20 d of age. The second study utilized five corn-soybean meal based diets with inclusion levels of gelatin rangeingfrom 0 to 10%. Although there was no effect on feed consumption of geed:gain when gelatin was increased in the diet, body weight gain decreased in a linear manner, apparently due to a bulk-density problem with increasing levels of gelatin. The titration diet was fed to chicks (49 floor pens, 13 chcks/pen; 7 replicates/trt) providing 0.13% total Trp to which 0.02% increments of L-Trp were supplemented to 0.25% of diet at the expense of a filler. Using regression analysis (95% of minimum or maximum response), it was determined that chicks optimized BW, feed intake and feed conversion at 0.21, 0.20, and 0.22% dietary Trp, respectively. Blood plasma free Trp displayed a sigmoidal response, in agreement with live performance needs. Plasma cholesterol, heterocyte/lymphocyte ratio, high/low density lipoprotein ratio and corticosterone were unaffected by dietary Trp, suggesting that no stress effect was imposed with Trp deficiency. Plasma glucose increased in a linear manner with dietary Trp, perhaps as a result of gluconeogenesis rather than physiological stress. Present results are in agreement with current NRC (1994) recommendations of 0.20% total Trp.