Evaluating growth response of broiler chickens fed diets supplemented with synthetic DL-methionine or DL-hydroxy methionine: a meta-analysis

Methionine (Met) is the first limiting amino acid in corn and soybean meal-based diets (containing L-Met) in broiler chickens, which are often supplemented with synthetic DL-Met or DL-Hydroxy Met (OH-Met). Our objective was to quantitatively assess the efficacy of synthetic Met sources and determine differences in growth rate of broilers fed at or below requirements in response to Met intake. A systematic literature search resulted in building a database containing 480 treatment means from 39 articles published between 1985 and 2019 globally. The database was divided into starter, grower, and finisher subsets based on the age of the broilers. For each subset, linear-plateau and quadratic-plateau models were fitted to determine Met or sulfur amino acid (SAA; Met + Cysteine) requirements using average daily gain as a response variable. For each phase, 4 new subsets were obtained by only retaining records with digestible Met or SAA intake at or below requirement by linear-plateau or quadratic-plateau models. Then, a linear model (without plateau) was fitted for all new subsets for each rearing phase using supplemental digestible synthetic Met or SAA intake (basal Met intake was subtracted from total Met intake) as independent variables. The basal diet was made of only raw materials without supplementation of any synthetic Met source. Finally, the models were extended to evaluate source of synthetic Met effects on the slope parameter. At all stages of model fitting, the inclusion of a random study effect was evaluated for each parameter. All models were fitted within a Bayesian framework, for which minimally informative priors were used. The best models, that is, the most accurate inclusion of random effects, were selected based on at least 10-point difference in leave-one-out cross-validation information criterion. Model selection criteria did not consistently favor either of the linear- and quadratic-plateau models to determine Met or SAA requirements across broiler growth phases. Extending models with covariates (e.g., dietary energy and amino acids) did not improve any model fit. Body weight gain response of broiler chickens to the 2 sources was not different when fed at or below Met requirements for any of the growth phases.