Broiler responses to reduced-protein diets supplemented with valine, isoleucine, glycine, and glutamic acid

Reduced performance is eventually observed when broilers are fed low-protein diets, even when industrial amino acids (AA) are provided to reach requirement levels. The reason for this detrimental effect has not been precisely determined, but it is possible that nonessential AA become limiting below a certain level of protein in the diet. The objective of this study was to evaluate the effects of Val, Ile, Gly, and Glu supplementation, individually or combined, in broiler diets with gradual reductions in protein. A total of 2,016 Ross x Ross 308 male broilers were fed 8 treatments with 9 replications (28 birds per pen) during a 42-d production period. overall care of birds complied with animal welfare directives from the Universidade Federal do Rio Grande do Sul. The control diet (treatment 1) was formulated without restricting CP, maintaining a Gly + Ser: Lys ratio of 155 or 158%. The other treatments, when not supplemented with L-Glu, had a CP content dependent on the fixed ratios of AA to Lys, generating diets with lower CP contents than the control diet. Treatment 2 was formulated with L-Val; treatment 3 with L-Val and Gly; treatment 4 with L-Val + L-Glu; treatment 5 with L-Val + Gly + L-Glu; treatment 6 with L-Val + L-Ile; treatment 7 with L-Val + L-Ile + Gly; and treatment 8 with L-Val + L-Ile + Gly + L-Glu. Dietary CP varied with feeding phase and treatment and reached 2.5% as the greatest reduction obtained. Supplementation of Gly and Glu improved BW gain and feed conversion. Benefits attributable to Gly supplementation were observed mainly in the early phases of growth. Conversely, broilers fed diets supplemented with Glu exhibited advantages in growth throughout experimentation. Addition of Ile, Glu, and Gly increased breast meat yield compared with broilers fed diets supplemented with only L-Val. Supplementation of Ile improved breast meat yield. Good broiler growth and meat yield can be obtained with Gly and Glu supplementation in reduced-CP diets, which implies a need for nitrogen for the synthesis of nonessential AA.