Optimal methionine: cystine ratio for performance and nitrogen balance in growing chickens fed low protein diet

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Abstract
This study aimed to determine the optimal methionine:cystine (Met:Cys) ratio in diets with reduced crude protein (CP). Two trials were carried throughout14 to 28 days. In trial 1, 1200 broiler chicks were distributed under a random design of ten treatments in a 2 × 5 factors of variability arrangement with two protein levels (21.5 and 19.5 % CP) and five digestible Met:Cys ratios (42:58, 46:54, 50:50, 54:46, and 58:42). Each treatment consisted of six replicates each, with 20 birds per experimental unit. In trial 2, 36 birds were selected and randomly allocated in six treatments with three replications each to carry out a nitrogen (N) balance study. The treatments were designed in a 2 × 3 factorial arrangement with two levels of protein (21.5 and 19.5 % CP) and three Met:Cys ratios (42:58, 50:50, and 58:42). Statistical analyses were performed as two-way analysis of variance (ANOVA) with interactions and main effects compared by Tukey's test. All statistical procedures were performed using the PROC GLM, and PROC NLIN was used to fit the quadratic broken line. No interaction was observed for weight gain, but there was a significant effect of the Met:Cys p < 0.05. There was a significant interaction between Met:Cys and CP level for feed intake (FI) and feed conversion rate p < 0.05. The CP level did not affect N retention (NR) p > 0.05 but significantly affected N intake (Nin), excretion (Nexc), and utilization (kN) efficiency. Therefore, an average Met:Cys ratio of 52:48 promoted a better performance response, improved in kN, and even CP reduction.

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