Meta-analysis of the relative efficiency of methionine-hydroxy-analogue-free-acid compared with DL-methionine in broilers using nonlinear mixed models

The purpose of this paper was to perform a meta-analysis to compare the biological efficiency of DL-methionine with methionine-hydroxy-analogue-free-acid in broiler chickens. A database was developed which contained dose-response studies of these 2 methionine sources. Criteria for entry into the database were defined before the meta-analysis was initiated. Data from 46 dose-response experiments, extracted from a total of 27 peer-reviewed papers, were used for the analysis with the Statistical Analysis System. Initially, the NLIN procedure was applied to fit an exponential model of the form y = alpha + beta*[1-exp (-gamma* dose)] + e. Thereafter, meta-analysis was conducted by means of nonlinear mixed models, which were fitted by a full maximum likelihood method as implemented in the NLMIXED procedure. The nonlinear model used for the analysis allowed for separate plateaus or different efficiencies of the methionine sources. Mixed modeling was applied to account for heterogeneity among the studies in all regression parameters for both sources of methionine via random effects. Statistical hypotheses were tested by the asymptotic Wald test. In addition, potential co-variables were tested for inclusion as linear regressors for the nonlinear model parameters. In conclusion, the null hypothesis of equal plateaus of the 2 methionine sources was not rejected. The analyses of the response variables average daily gain (ADG) and gain to feed ratio (GF) showed a highly significant difference between the tested methionine sources. The covariate age at start of experiment significantly affected the intercept term a for the response variables ADG and GF, respectively. The meta-analysis showed that biological efficiencies of DL-methionine-hydroxy-analogue-free- acid were 81 and 79% of the values for DL-methionine, on an equimolar basis, for ADG and GF, respectively.