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Tryptophan Requirement of Growing Chicks as Affected by Dietary Protein Level

University of Illinois¹ at Urbana-Champaign Campus, Urbana 61801

Abstract

A 6-day growth assay (8 to 14 days posthatching) was conducted with 480 chicks to determine the dietary tryptophan requirement at five levels of dietary crude protein. A crystalline amino acid diet devoid of tryptophan was fed at 8.7, 11.6, 14.5, 17.4 and 20.3% protein (N x 6.25). At each of these protein levels L-tryptophan was fed at 0.414, 0.621, 0.828, 1.034, 1.241 and 1.448% of the dietary protein. Using maximum weight gain as the criterion of adequacy, the tryptophan requirement (% of diet) was calculated by the method of least squares to be 0.077, 0.101, 0.126, 0.153 and 0.175% at 8.7, 11.6, 14.5, 17.4 and 20.3% dietary protein, respectively. This relationship was described by the linear regression equation Y=0.0024+0.0086X (Y=dietary tryptophan as % of the diet, X=% dietary crude protein) which indicates that the requirement for tryptophan is a direct function of dietary protein. The requirement expressed as percent of dietary crude protein was thus constant at 0.87% at all five protein levels. These results do not support the generally-accepted concept that the requirement for an essential amino acid increases curvilinearly as a percent of the diet and decreases linearly as a percent of the dietary protein with increasing protein levels.

At the lowest level of tryptophan (0.414% of the protein) increasing the protein level produced little or no improvement in gain, suggesting that the efficacy of a protein source extremely deficient in tryptophan is not enhanced by feeding more of the deficient protein.

¹ Department of Animal Science.