Comparison of some Non-Linear Statistical Models to Describe the Growth Curve of Mazandaran Native Chickens

Growth curves are an approach to describe the growth over a specific period of time, so it would be useful to use nonlinear growth functions to measure and predict weight at different times of an animal's life and calculate production profitability. Growth curves actually summarize the relationship between age and weight into a limited number of interpretable parameters. The aim of this study was to determine the best nonlinear model describing the growth curve in Mazandaran native chickens. To achieve this goal, body weight data on different ages of native chickens (hatch, 8 weeks, 12 weeks and sexual maturity) for the first 21 generations collected at the Mazandaran Indigenous Breeding Center were used. Nonlinear models of Gompertz, logistics; Brody, Verhulst, Richards and VonBertalanffy were tested to describe the growth curve. Fitting of nonlinear models was performed using nonlinear least squares (NLIN) procedure of SAS software. To determine the most appropriate growth function and comparison between models, the coefficient of explanation (R2), corrected coefficient of explanation, Bayesian information criterion (BIC), Akaike information criterion (AIC) and root mean square error (RMSE) were used. Gompertz model with the lowest values for MSE, RMSE, AIC and BIC statistics was detected as the best model describing growth curve and Richards’s model as the most inappropriate model describing growth in Mazandaran native chicken. Correlation between parameters obtained using the best nonlinear model (Gompertz) showed that the correlation between parameter and b was positive (72%) and the correlation between and k was negative (86%). Also correlation between parameters b and k was negative (95%). The value of coefficient of explanation and adjusted coefficient of explanation for different functions varied between 0.92 to 0.93. In general, the results showed that the Gompertz model was the best function describing the growth curve in Mazandaran native chickens that could be used in herd management to predict the future weight of birds.