Technical Abstract: A 2-yr study was conducted to evaluate the interactions of castration, dietary CP, and feeding length on growth and carcass characteristics of male cattle that vary in expression of muscular hypertrophy. Crossbred cows were bred by AI to Hereford, Limousin, or Piedmontese bulls, which represented genotypes with normal, moderate, and hyper muscularity, respectively, but with similar mature weights. Male calves were either castrated at 30 to 60 d of age or left intact. After weaning each year, 131 male calves were placed into pens having individual electronic feeding gates. Calves were fed growing diets until reaching 386 kg BW and then switched to finishing diets for 90 or 132 d. There were many interactions among sire breed, gender, and feeding length on carcass composition. Bulls were more efficient than steers in producing live weight gain. Length of finish accounted for a larger source of variance than gender for weight characteristics, whereas gender was the larger source of variation for carcass composition. Level or degradability of dietary CP influenced rate of gain from weaning to 386 kg but had no effect on growth during the finishing phase or on carcass characteristics. Differences in carcass characteristics among calves of different sire breeds indicate differences in mode of expression of muscularity in various breeds of cattle and that cattle of these breeds may benefit from different management strategies.