Most dairy producers aim for a replacement heifer to give birth to her first calf, and thus join the milking herd, on her second birthday. As the cow's gestation period is a little over 9 months this means the cow must be inseminated by the age of 15 months. Because the breeding process is inefficient, most producers aim to first breed their heifers between 12 and 14 months. Before a heifer can be bred she must reach sexual maturity and attain the proper body condition to successfully bear a calf. Puberty in cattle depends largely on weight among other factors. Holstein heifers reach puberty at an average body weight between 550 and 650 lbs. Smaller breeds of cattle, such as Jerseys, usually reach puberty earlier at a lighter weight. Under typical nutritional conditions, Holstein heifers will reach puberty at the age 9–10 months. Proper body condition for breeding is also largely judged by weight. At about 800lbs Holstein heifers will normally be able to carry a healthy calf and give birth with relative ease. In this way, the heifers will be able to give birth and join the milking herd before their second birthday.