Weaning weights of 7 771 Hereford (18 herds) and 16 666 Angus calves (37 herds), from 9 years' records of the Beef Cattle Weight Gain Performance Recording Scheme, were analysed within breeds by least squares to investigate the main effects of herd, year of birth, age-of-dam group, and sex, with age at weaning as a covariate, and the first-order interactions among herd, year, age-of-dam group, and sex effects. Only the herd #xD7; year interaction for Hereford, and the herd #xD7; year and herd #xD7; sex interactions for Angus, were important. Within sub-class linear regression coefficients of weight on age at weaning for Hereford and Angus calves were 0.70 #xB1; 0.01 and 0.62 #xB1; 0.01 kg/day, respectively. Hereford and Angus males were heavier (P 0.01) than females at weaning by 29.8 and 25.6 kg respectively. Weights of Hereford calves out of dams 2, 3, 4, 5, and 10 years of age and older deviated from those of calves out of mature dams (6 to 9 years) by 33.3, 17.6, 8.7, 3.8, and 2.2 kg, respectively, and of Angus calves by 22.7, 15.2, 7.9, 5.4, and 1.4 kg. Age-of-dams effects were also estimated by best linear unbiased procedures from similar numbers of calves; effects so estimated generally differed little from those derived by least squares. Additive and multiplicative applications of the linear regression coefficients of weight on age for each breed, to adjust for age at weaning, reduced the dependence of adjusted weight on age more effectively than the procedure used in the National Beef Recording Service (NBRS). Apart from factors for Angus calves out of 3-year-old dams, adjustments determined for age-of-dam effects by least squares and by best linear unbiased estimation did not differ significantly from adjustments used in NBRS. Variances within age-of-dam groups indicated that additive, rather than multiplicative, adjustment factors should be used for each breed. Multiplicative adjustment factors for sex were more appropriate in equalising variances within sex groups.
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