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Effects of Tryptophan to Large Neutral Amino Acid Ratios and Overall Amino Acid Levels on Utilization of Diets Low in Crude Protein by Broilers

J. Si^*, C. A. Fritts^*, P. W. Waldroup^* and D. J. Burnham

^* Poultry Science Department, University of Arkansas, Fayetteville, Arkansas 72701
Ajinomoto Heartland LLC, Chicago, Illinois 60631

Correspondence: P. W. Waldroup, E-mail: Waldroup{at}uark.edu

It has been suggested that inadequate ratios of Trp to large neutral amino acids (LNAA; sum of Ile, Leu, Val, Phe, and Tyr) may inhibit feed intake of animals fed low CP diets. This study was conducted to examine this relationship as a potential contributor to the reduced performance of broilers fed low concentrations of CP. Using corn and soybean meal of known composition, diets were formulated to provide a minimum of 100 and 110% of NRC [1] amino acid requirements with 22, 20, 18, or 16% CP. The dietary electrolyte balance was maintained at a minimum of 250 mEq/kg. The Trp:LNAA ratio (Trp as percentage of LNAA) ranged from 4.14 to 4.72% in these diets; additional diets were supplemented with Trp to attain Trp:LNAA ratios of 5.0 to 6.0%. Each treatment plus a positive control 23% CP diet were fed to 6 replicate pens of 6 male chicks from 1 to 21 d. Performance of chicks fed the 22% CP diets, with either 100 or 110% of NRC amino acid recommendation, did not differ significantly from that of chicks fed the 23% CP positive control diet. A further reduction of CP to 20% or less resulted in significant reduction in BW and increased feed conversion ratio. Increasing minimum amino acids from 100 to 110% of NRC did not help overcome this reduction in performance and, in fact, may have contributed to reduced performance of chicks fed diets containing 16% CP. The ratio of Trp:LNAA had no significant effect on any of the parameters measured; therefore, the reduced performance at lower CP concentrations does not appear to be a deficiency of Trp or due to an imbalance among these amino acids.

Key Words: broiler • crude protein • tryptophan • large neutral amino acids • nitrogen

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