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Data from topcross cows (n = 468) from six breeds of sire (Angus, Brahman, Hereford, Pinzgauer, Sahiwal, Tarentaise) and two breeds of dam (Angus and Hereford) of Cycle III of the Germplasm Evaluation (GPE) program at the U.S. Meat Animal Research Center (MARC) comprising cow weight (CW, n = 9,012), height (CH, n = 9,010), and condition score (CS, n = 8,991) recorded in four seasons per year from 2 to 6 yr of age were used to estimate breed-group differences. The mixed models included cow age, season of measurement and their interactions, year of birth, pregnancy-lactation code (PL), and breed group as fixed effects for CW and CS. Analyses of weight adjusted for condition score included CS as a linear covariate. Model for CH excluded PL. Random effects were additive genetic and permanent environmental effects. Differences among breed-groups were significant for all traits for different ages and were maintained across ages, with few interchanges in ranking through maturity. Cows with Sahiwal sires were lightest (392 to 479 kg), whereas Hereford- Angus (HA) reciprocal-cross cows were shortest (119 to 123 cm) at each age. Cows with Brahman sires were heaviest and tallest among breed groups at all ages, exceeding HA cows by 19 to 24 kg and 9 to 10 cm, respectively. Cows with Pinzgauer and Tarentaise sires were intermediate for weight and height and interchanged ranking across ages. Differences in weight due to differences in condition seemed to be of small magnitude because adjustment for condition score did not affect rankings of breed groups across ages. Important changes for mature size of cows can be achieved by breed substitution with the breeds of sires used in this study.