Genetic parameters for growth and feed efficiency in weaner versus yearling Charolais bulls

Data on young Charolais bulls were used to evaluate the effect of age of da m, and provide genetic parameter estimates for growth and feed efficiency t raits in weaners compared with yearlings. Records on 510 performance test b ulls born from 1985 to 1989 and 792 progeny test bulls born from 1988 to 19 97 were used, plus available pedigree information (total n = 5493). After w eaning (approximately 225 days of age), progeny test bulls were housed in b arns and tested until 15 months for all bulls and 19 months of age for half of the bulls. From the records, the test period was artificially partition ed into a weaner and a yearling test. Both tests were for 140 days, with th e weaner test commencing at approximately 274 days of age (6-8 weeks after weaning), and the yearling test commencing at approximately 430 days of age . For each test (weaner or yearling), average daily gain, daily feed intake , feed conversion ratio (feed intake per unit weight gain), and residual fe ed intake (feed intake net of the expected requirements for growth and main tenance) were calculated. Liveweights at 12 and 18 months of age were also studied. Age of dam effect was significant (P < 0.05) for weight at 12 and 18 months of age, and for all weaner traits except residual feed intake. All the tra its studied were moderately heritable (0.20-0.46) except for feed conversio n ratio in yearlings (0.10). Genetic correlations for the same traits measu red in weaners and later in yearlings were less than unity (0.42-0.95), wit h the 3 highest correlations obtained for weight (0.95+/-0.03), feed intake (0.90+/-0.07), and residual feed intake (0.75+/-0.12). Therefore, for gene tic improvement purposes, traits measured in weaners should be considered d ifferent from those measured in yearlings, except for weight and feed intak e. However, the medium to very high genetic correlations between weaner and yearling tests for most of the traits studied indicate that selection base d on weaners will have a positive correlated response in yearlings and vice versa.