The study was carried out to estimate the covariance components and genetic parameters associated with growth traits of crossbred cattle using random regression models as well as to examine their trend over trajectories. The study encompassed a total of 13,839 body weight records taken across different months from birth up to 540 days of age on 902 F1 HFB (Holstein Friesian Boran) crossbred growing calves born at Holetta Agricultural Research Center (HARC) between 1978 and 2003. Random Regression Models (RRM) were fitted to cubic Legendre polynomials (LP) of age at recording for additive genetic and permanent environmental effects of the animals and linear LP of age for their maternal genetic effects. The findings indicated that the additive direct heritability ranged between 0.16 and 0.30 and increased from birth until the age of 120 and then decreased from the age of 150 - 300 days and slightly increased again until the age of 450 days followed by slight decrease at the end of the trajectory. Maternal heritability patterns showed a decreasing trend from birth (0.07) to the age of 270 days (0.002) and continued declining to value closer to zero at the end of growth period (540 days). Genetic correlation estimates were higher between consecutive weight records as compared to records that were far apart from each other. The correlation estimates ranged from 0.03 to unity (1.00). Selection on crossbred dairy cattle can be done at weaning age for better genetic improvement of body weight traits.
