Performance through weaning of 543 calves raised on 3-yr-old and older Hereford dams was studied. Calf breed groups were straightbred Hereford, 50% Angus-50% Hereford, 25% Simmental-75% Hereford and 50% Simmental-50% Hereford. Breed group was a significant source of variation for gestation length, birth weight, calving difficulty, late survival (from 24 h to weaning), percentage of calves weaned per cow calving, 180-d weight, 180-d withers height and 180-d weight:height ratio. Breed group differences were not significant for early survival or weaning condition score and approached significance (P less than .10) for net kilograms weaned (zero if no calf was weaned and 180-d weight if a calf was weaned). Fifty percent Simmental calves had the heaviest birth weight (41.4 kg), most calving difficulty (1.19 score), heaviest 180-d weight (200.5 kg) and tallest 180-d height (101.3 cm). Fifty percent Simmental calves also had the longest gestation length (287.5 d), lowest late survival rate (95%) and largest weight:height ratio (2.00), although they did not differ significantly in the first two traits from 25% Simmentals and in the latter trait from 50% Angus. Herefords differed (P less than .05) from 50% Simmentals in all traits for which breed group was significant except for percentage of calves weaned. Fifty percent Angus and 25% Simmental calves were similar to Herefords for many traits.(ABSTRACT TRUNCATED AT 250 WORDS)