Optimal replacement value of betaine for dietary methionine in broilers from 22 to 42 days of age

Betaine acts as a methyl donor and has been proposed as a potential substitute for methionine (Met) to reduce feed costs. This study aimed to investigate the optimal replacement value of betaine for dietary Met in broilers aged 22 to 42 days. A total of 1,440 male broilers were randomly divided into 6 treatments (6 replicates/pen). The positive control (PC) received a standard diet (0.53% Met), while the negative control (NC) received a low-Met diet (0.33% Met) with 2000 mg/kg betaine. Four additional groups received the NC diet supplemented with 0.05%, 0.10%, 0.15%, or 0.20% Met. Data were analyzed using one-way ANOVA and three regression models. Results indicated that Met deficiency significantly impaired growth performance: final body weight (BW) decreased from 1696 g (PC) to 1604 g (NC) (P < 0.001), while the feed-to-gain ratio (F/G) increased from 1.87 to 2.01. No significant differences were observed in slaughter performance (P > 0.05). Regarding metabolism, the NC group exhibited significantly elevated serum homocysteine (19.42 vs. 7.94 μmol/L) and hepatic ALT activity (11.77 vs. 4.35 U/g prot), alongside reduced hepatic BHMT activity (17.47 vs. 103.14 ng/mg prot, P < 0.001). Furthermore, Met deficiency downregulated the mRNA expression of genes related to one-carbon metabolism and intestinal tight junctions (relative expression ∼0.20 vs. 1.00 in PC, P < 0.05). Supplementation with 0.10% Met restored these parameters to levels statistically equivalent to the PC group. One-slope straight broken line and two-slope straight broken line regression analysis of the F/G ratio (R2 = 0.8598) determined the optimal replacement dose to be 0.1474% Met, corresponding to a 27.81% substitution efficiency.