Effect of dietary crude protein level on the lysine requirements of young broiler chickens
Growth response and production parameters were evaluated in broiler chickens from 1 to 21 d-old and muscle (pectoralis major) protein turnover in chicks at 21 days of age fed dietary crude protein (CP) levels from 170 to 290 g CP/kg diet with graded dietary lysine levels. Weight gain, feed conversion (F:G), carcass nitrogen and fat deposition improved as dietary crude protein and lysine levels were increased. However, response variables reached a plateau to an optimum lysine level, with no further improvement as lysine was increased. Feed intake (FI) decreased dramatically as lysine was increased in chicks fed low CP diets. The same effect was not found for diets at 250 and 290 g CP/kg. Lysine requirements of chicks for weight gain to 21 d of age increased (P < 0.05) as dietary CP was increased from 170 to 210 and from 210 to 250 g CP/kg diet. However, no differences (P > 0.05) in the requirements of lysine were found when comparing 250 vs 290 g CP/kg. Lysine requirements for F:G showed similar pattern as weight gain. Muscle protein turnover was affected by dietary levels of CP and lysine. Dietary lysine levels affected (P < 0.05) fractional synthesis rate (FSR, %/d) of muscle at 170 and 210 g CP/kg diet. Linear regression analysis showed that FSR (%/d) tends to increase as lysine increases with diet CP levels of 250 and 290 g CP/kg. Breast muscle protein deposition (AGR, mg/d) plateaued with 1.22% dietary lysine at CP levels of 170, 210 and 290 g/kg diet. The most efficient scenario for muscle protein deposition in chicks to 21 days of age was with diet CP containing 210 g/kg and lysine at 1.22%. This is based on ASR/AGR and reduced ABR (mg/d). These data suggest that lysine needs of the young broiler are influenced by level of dietary crude protein. Lysine requirement of chicks increased when diet CP increased from 170 to 250 g CP/kg diet. However, no differences in lysine needs were found when CP was higher than 250 g/kg.