This study aimed to evaluate the effects of dietary supplemental methionine (Met) source and betaine (Bet) replacement for Met on performance and activity of mitochondrial respiratory chain enzymes (MRCE) in normal and heat-stressed broiler chickens. Total of 1,200 d-old Ross 308 chicks were allocated to two houses, each consisted of 12 treatments, 5 replicates of 10 birds each with 2×2×3×2 (temperature×Met source×Met level×Bet, respectively) split-plot factorial arrangement. Met level in the basal diets was 70% requirements (Req) that was increased to the requirement or 130% by supplemental DL- or L-Met. Bet was or was not substituted at the rate of 30% supplemental DL- or L-Met. Feed conversion ratio (FCR) in chicks fed 70% L-Met was lower than those fed 70% DL-Met diet during 1-10 d (P=0.04). Broilers fed diets containing requirement or 130% Met, regardless of its source showed higher weight gain (WG) than those received 70% Met diet during 11-42 d (P<0.001). Feed intake (FI) of broilers fed 130% Met diet was decreased compared to other two groups during 11-42 d (P<0.05). 130% Met requirement diet resulted in lower FCR comparing to other two groups during 11-42 d (P<0.001). Heat stressed birds grew less than those under normal condition (P<0.05). Broilers fed Req Met diet under normal temperature exhibited higher activities of complexes (Cox) I and III (P<0.05). Cox I activity in heat-stressed birds fed Bet+ diet was similar to those fed Bet- diet under normal temperature (P=0.046). It is concluded that performance and the activities of Cox I and III were increased as the level of Met increased. Bet replacement for 30% supplemental Met resulted in similar consequences comparing to non-Bet replacement diets on performance, but increased the activity of Cox III. L-Met was effective than DL-Met at the cellular level. High ambient temperature depressed performance and MRCE activity.

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