The objectives of this study were to determine the importance of the effects of age of dam (AoD), sex (S) and age of calf (AC), month of birth (M) and the interaction between age of dam and sex (AoD*S) on birth weight (BW) and 205 day adjusted gain from birth to weaning (AWG) of Angus cattle and to estimate correction factors for these effects. Data from 40,915 calves, of which 12,706 were weighed at birth, controlled by the Beef Cattle Improvement Program (PROMEBO) between 1974 and 1997, were used. The data were analyzed using PROC MIXED of SAS, by a model including the effects of calving season (Fall and Spring), contemporary group, M, AoD, S, AC (linear and quadratic), AoD*S and the random effect of sire. All the fixed effects were nested within calving season. Using least square means of BW and AWG, regression equations for AoD*S and M were estimated. The inclusion of contemporary group in the statistical model does not eliminate the effect of seasonality during the year, so correction factors for Julian birth date are presented. The use of average daily gain is not enough to eliminate the effect of AC on AWG. Deviations of linearity of this effect should be considered on genetic evaluations. Different correction factors for AoD and AC are proposed for calves born in the fall and in the spring, since the magnitude of these effects depends on calving season.
age of dam; age of calf; birth weight; month of birth; preweaning gain; sex of calf
