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This study was carried out to determine the effect of varying levels of methionine supplementation in diet of broiler chickens on their immunological response to Newcastle disease, growth performance, organ characteristics and haematological parameters. A total of two hundred birds were raised from day-old to six (6) weeks. They were distributed into four treatments A, B, C and D with five replicates per treatment, each replicate having 10 birds. At both starter and finisher phases of the experiment, there were four experimental diets A, B, C and D. Diet A was the control diet and contained a standard of 0.38% methionine, diet B was the diet containing 0.5% methionine as permitted by National Research Council (NRC) standards while diets C and D contained 0.63% and 0.75% methionine supplementary levels, respectively. By the third week of the experiment, some of the general observations made included leg weakness which later progressed to complete paralysis in about 40% of the birds placed on diet C and D. Mortality occurred in 70% of the paralysed birds. The immunological responses after vaccination with Newcastle disease vaccine intra-ocular (NDV i/o) revealed that chickens fed with 0.5% methionine (treatment B) recorded the highest antibody titre value (log 27) while treatment D (fed with 0.75% methionine) had the lowest mean titre value (log25). Thereafter, administration of NDV LaSota elicited immune responses with antibody titre values being highest in treatment B (log29) and least in both treatments A and D (log27). The performance characteristics such as the final body weight (FBW), weight gain (TWG), feed intake (TFI) and feed conversion ratio (FCR) showed significant (p<0.05) differences among the various dietary treatments, with birds on NRC requirement methionine based diet having significantly (p<0.05) higher FBW, TWG, FCR and TFI than those fed the rest test diets throughout the experimental period. Relative weight of the organs liver and gizzard were also significantly (P< 0.05) different between treatment groups. Among the haematological variables only the red blood cell count (RBC), mean corpuscular haemoglobin (MCH) and mean corpuscular volume (MCV) values were significantly different (p<0.05) among treatment groups in the starter phase of the experiment and erythrocyte sedimentation rate (ESR), basophil and eosinophil in the finisher phase. It was concluded that methionine supplementation at this high inclusion levels is detrimental to the immune response, growth performance, nutrient utilization and organ characteristics of broiler chickens.
Keywords: Methionine, immunity, Newcastle disease, performance, vaccinations, antibody titre, growth, supplementation