The alternatives are to buy better cows or breed better cows, the latter being the more common method. To produce replacements within the herd most successfully we must manage the herd to produce the maximum number of heifers, reduce involuntary cow losses, and choose parents wisely.
Nature places limitations on the number of replacements.
A cow has her first calf at 2 years of age, with a gestation period of 280 days, and the need for a 52-day dry period limits us to one calf per year. Half of the calves will be bulls, and these facts combine to produce a long-generation interval. A long-generation interval does not necessarily restrict progress, but it certainly slows down the rate. Management, too, plays a role. This limits the number of replacements available and the number of unprofitable cows that can be culled.