Application of growth models to describe the lactation curves for test-day milk production in Holstein cows

In order to describe the lactation curves of milk yield traits, six standard growth models (Brody, logistic, Gompertz, Schumacher, Von Bertalanffy and Morgan) were used. Data were 911,144 test-day records for unadjusted milk yield, 4% fat-corrected milk yield and energy-corrected milk yield from the first lactation of Iranian Holstein cows, which were collected on 834 dairy herds in the period from 2000 to 2011. Each model was fitted to monthly production records of dairy cows using the NLIN and MODEL procedures in SAS. The models were tested for goodness of fit using adjusted coefficient of determination, root means square error, Durbin-Watson statistic, Akaike's information criterion (AIC) and Bayesian information criterion (BIC). The Morgan model provided the best fit of the lactation curves due to the lower values of AIC and BIC than other models, but the Brody model did not fit the production data as well as the other equations. The results showed that the Morgan equation was able to estimate the time to the peak and peak yield more accurately than the other equations. Overall, evaluation of the different growth equations indicated the potential of the nonlinear functions for fitting monthly productive records of Holstein cows.