ADDITIVE AND NONADDITIVE GENETIC-EFFECTS OF GROWTH-CURVE PARAMETERS OF HOLSTEIN, AYRSHIRE AND CROSSBRED FEMALES

Crossbreeding parameters (line additive, dominance and additive x addi tive epistatic effects for individual and for maternal performance) on growth traits of females from a crossbreeding experiment between Hols tein-based (HS) and Ayrshire-based (AS) lines were estimated by indivi dual animal models, incorporating all known additive genetic relations hips among animals, through restricted maximum likelihood and mixed-mo del methodologies. The growth traits [asymptotic weight (A), rate para meter (k), inflection parameter (m), average lifetime absolute growth rate (AGR), average lifetime absolute maturing rate (AMR) and average lifetime relative growth rate (RGR)] were estimated by fitting the Ric hards function to the observed growth curve of 3076 individuals. The s tatistical model included the random effect of the animal breeding val ue and the fixed effects of genetic group and station-year-season of b irth. Results indicated that the HS exceeded (P < 0.001) the AS in add itive effects for individual performance for both A and AGR. The HS ex ceeded (P < 0.05) the AS in additive effects for maternal performance for A. Individual heterosis was positive for A (P < 0.001) and for AGR (P < 0.01). Maternal heterosis was negative for A (P < 0.05) and posi tive for AMR (P < 0.05). Total heterosis (TH) had positive effects on both AGR and AMR (P < 0.05). Heterosis retained in advanced crossbred generations was not significant (P > 0.05) for any of the studied trai ts. The results suggest that crossbreeding designed to exploit TH can alter the shape of the growth curve of dairy cattle.