A feeding trial was conducted to determine the bioefficacy of herbal methionine (HM) compared to synthetic methionine (SM) in the diets of domestic laying hens. The herbal methionine (Meth-o-Tas®) was supplied by Intas Pharmaceutical Limited, India. The HM and SM were added to a standard diet at 0.5 and 1.0 kg per ton and fed to 144 laying birds, 30 weeks of age, housed in 3 birds per cage unit. The birds were divided into 4 dietary treatments of 36 birds each and each treatment group was replicated 4 times with 9 birds per replicate. The trial lasted for 112 days and was divided into 4 periods of 28 days each. At the end of the feeding trial, 2 birds per replicate, representing 8 birds per treatment were sacrificed for the determinations of haematological and plasma biochemical profiles, liver weight and abdominal fat pad. Layers fed on diets supplemented with HM produced less (P ≤ 0.01) eggs, had lower egg mass output (P ≤ 0.01) and final body weight (P ≤ 0.05) and poorer feed conversion efficiency (P ≤ 0.05) than those layers fed on diets supplemented with SM. Feed intake was not affected (P ≥ 0.05) by dietary methionine source. Exterior and interior egg quality characteristics such as egg weight, shell thickness, albumen weight and albumen height decreased (P ≤ 0.05) with dietary supplementation of HM. Dietary supplementation of HM reduced (P ≤ 0.05) abdominal fat pad. Total protein and albumin decreased (P ≤ 0.05) with dietary supplementation of HM. Plasma and liver total cholesterol and triglyceride concentrations were lowered (P ≥ 0.05) in birds fed on diets supplemented with HM. The activities of plasma and liver alanine transferase (ALT), aspartate transferase (AST) and alkaline phosphatase (ALP) were not altered (P ≥ 0.05) by HM supplementation. All haematological variables determined were not affected by HM supplementation. Under the conditions of this study, it was concluded that herbal methionine (Meth-o-Tas®) is not an effective substitute for synthetic methionine for optimum production performance.Keywords: Herbal methionine, synthetic methionine, performance, laying hen
