Limitations of dietary isoleucine and valine in broiler chick diets

The purpose of this study was to understand the needs and relationship of Ile and Val, the most likely candidates to become fourth limiting amino acids in practical broiler formulas. Broiler chicks were fed a diet adequate in all nutrients that served as positive control (PC). A second diet served as negative control (NC), was formulated to resemble the nutrient amounts of the PC, and was supplemented with various crystalline amino acids but not with l-Ile and l-Val, and therefore, no nutrient minimums were given to Ile or Val in the formula. The other treatments fed were as follows: NC plus 0.15% Ile, NC plus 0.15% Val, NC plus 0.075% Ile and 0.075% Val, and NC plus 0.15% Ile and 0.15% Val. Diets were fed in crumble form from placement until 21 d of age. A total of 1,080 Ross x Ross 708 males chicks were randomly allotted into 72 floor pens (15 chicks/pen), and each treatment was replicated 12 times. Individual supplementation with Val, but not Ile, to the NC diet resulted in BW gain of chicks equal to those fed the PC diet (P<0.005). Feed conversion values of chicks supplemented with Val or Ile, or both, resulted in an improvement, but for this variable it would appear that Val and Ile were both equally needed (P<0.001). Responses observed for plasma concentrations of total protein and albumin suggest that Val may be warranted before Ile (P<0.02). Overall responses indicate that Val was the fourth limiting amino acid under these dietary conditions but may reach a point where Ile becomes co-limiting judging by the response observed with feed conversion.