Either a baby bull or a baby cow is called a calf. If the baby bull is castrated prior to puberty, he is called a steer.
Both sexes of cattle share the same names when they are very young. Once weaned, they are referred to as "weaners" or "feeders." At a year old, they are called "yearlings." Female cattle are differentiated at this point as "heifers" (if they have not borne a calf) or "cows" (once they have given birth). Castrated males are steers at first, but later may be referred to as bullocks or oxen, depending on the regional dialect.