Lysine requirements of young broiler chickens are affected by level of dietary crude protein

Growth and body nitrogen and fat deposition in response to dietary lysine were assessed in chicks to 21 d of age when fed dietary crude protein (CP) at 170, 210, 250 or 290 g kg-1 diet. Response variables were evaluated using linear and quadratic regression, and linear-plateau models. In all cases, a better fit was observed with quadratic regression and linear-plateau models. Weight gain, F:G, and body nitrogen and fat deposition plateaued as dietary lysine was increased regardless of CP. Lysine requirements for weight gain to 21 d were significantly different when comparing 170 vs. 210 and 210 vs. 250 g CP kg-1 diets, although no differences were found when 250 and 290 g CP kg-1 were fed. Feed efficiency and body nitrogen responded in a similar manner to diet CP and lysine levels. Dietary lysine requirement increased as CP increased (170 vs. 210, and 210 to 250 g kg-1), although lysine requirement was unaffected when 250 or 290 g CP kg-1 diets are used. The requirements for lysine at low CP levels are better expressed as a percentage of CP. However, when dietary CP intake no longer limits growth, lysine needs should be specified as a diet percentage.