COMPARISON OF TWO MATHEMATICAL MODELS IN FITTING LACTATION CURVES FOR PURELINE AND CROSSLINE DAIRY COWS

Abstract

Weekly milk yield of 2066 first, 1407 second, and 755 third lactation pureline and crossline cows was used to study environmental and genetic effects on the coefficients of the lactation curves derived by modified gamma and inverse polynomial functions. The inverse polynomial function provided a better fit than the modified gamma function based on comparison of R2 values. Coefficients of the two models were analyzed to evaluate environmental and genetic effects on the shape of the lactation curve. The model included station, year of calving, month of calving, age at calving, days open, line of sire, line of dam, interaction of line of sire with line of dam, and sires within line of sire. In addition, coefficients of the lactation curve were analyzed by another model which included station, year of calving, month of calving, age at calving, days open, breed additive, maternal, and heterosis effects. Effects of station, year of calving, month of calving, and days open were mostly significant (P < 0.05); however the effect of age at calving was not significant on the coefficients of the lactation curves. Significant (P < 0.05) line of sire and line of dam effects on the level of initial yield suggest that genetic improvement of this trait could be achieved through selection. Breed additive and maternal effects were mostly nonsignificant on the coefficients of the lactation curves indicating shape of the lactation curve could not be changed by selection. There was very little evidence of nonadditive genetic variation associated with the coefficients of lactation curves. Key words: Lactation curves, pureline, crossline, dairy cows