ISSN: 0041-3216 (Online), 0041-3216 (Print)
Volume 92 Number 2
Comparison between mathematical functions for describing lactation curve of Ardi goats in Saudi Arabia. (139)
A total of 5170 weekly average milk records were collected during the period from May 2011 till May 2013, from 127 Ardi does, a native breed of the Kingdom of Saudi Arabia. The objectives were to describe the shape of lactation curve of milk production by different functions, and to compare the results obtained from fitting these models. Four mathematical models were fitted and compared, namely the linear (LW) and nonlinear (NLW) forms of the Wood, the monophasic (MONO) and the diphasic (DI) functions. Comparisons of these models were carried out based on root mean square error (RMSE), Akaike Information Criteria (AIC), Bayesian Information Criteria (BIC) and Durbin-Watson statistic (DW). The results indicated that the DI fitted the data better than the other models, as judged by lower mean square prediction error. The predicted milk yield estimated by DI model was markedly closer in magnitude to the actual milk yield than those predicted by the other models. Correlation coefficients between actual milk yield and predicted milk yield calculated by DI function were higher than the corresponding coefficients estimated by the other functions. The estimates of DW statistic indicated that the residuals obtained from fitting the LW, NLW and MONO functions were positively and highly correlated. The corresponding values obtained by DI function were better. The AIC and BIC values obtained by fitting DI function were larger than those estimated from fitting the other functions. In conclusion, DI function would be the model of choice for describing lactation curve of Ardi goats. The ranking of the models should be taken with caution because of the small size of the data set used in this study.
Keywords: lactation curves, Ardi goats, milk yield, Saudi Arabia