Optimal dietary methionine requirement of bullfrog Rana (Lithobates) catesbeiana

Abstract

A feeding experiment was conducted to determine dietary methionine requirement of bullfrog Rana (Lithobates) catesbeiana. Bullfrogs were fed six experimental diets containing 0.63%, 1.00%, 1.42%, 1.78%, 2.26% or 2.68% methionine for 8weeks. After the feeding experiments, growth performance, body composition, blood biochemistry and expression levels of hepatic genes were determined. The results showed that growth and feed utilization were significantly affected (P<0.05) by dietary methionine levels. Weight gain (WG) and protein efficiency ratio (PER) both increased with increasing level of dietary methionine from 0.63% to 1.42% and then decreased. While, the variation trend of feed conversion ratio (FCR) is just opposite with WG and PER. Crude lipid concentrations of whole body and muscle tissue were also significantly affected (P<0.05) by dietary methionine levels. However, there was no significant difference (P>0.05) in amino acid composition in muscle among methionine treatments. Triglyceride (TG) and cholesterol (CHO) levels, as well as aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities, were all significantly affected (P<0.05) by dietary methionine levels. TG and CHO levels decreased as dietary methionine increased from 0.63% to 1.78% and then increased with further increases in methionine. Serum AST and ALT activities in bullfrogs fed with 0.63% methionine diet were both significantly higher (P<0.05) than in bullfrogs fed other diets. Gene expressions in bullfrogs fed with 1.00%, 1.42% and 1.78% methionine diets was higher (P<0.05) than these of other groups. On the basis of WG and PER, the optimal level of dietary methionine for bullfrogs was estimated to be between 1.53% and 1.62% of diet using second-order polynomial regression analysis. The corresponding optimal level of total sulfur amino acids in the diet of bullfrogs is therefore between 1.79% and 1.88% of diet in the presence of 0.26% cystine. Statement of relevanceThe authors have declared that no competing interests exist.