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

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.