Nutritional management of replacement beef heifers: a review.

Systems to produce replacement heifers to calve initially at 2 yr of age and at 12-mo intervals thereafter require integrated forage-animal management plans. The process of developing heifers as replacements must begin during the cow-calf production phase. Age and weight at puberty are affected by several factors, including breed of livestock. Generally, breeds of a larger size at maturity are older and heavier when reaching puberty. Heifers that are kept as replacements must be weaned at relatively heavy weights in comparison to their expected mature body size without becoming obese. Excessive adipose tissue in heifers during the preweaning phase may result in reduced subsequent performance of these animals. Several management techniques were reviewed that can be used both pre- and postweaning, including using anthelmintics, anabolic growth implants, and ionophores. Anabolic growth implants can increase heifer rates of gain and increase pelvic size. In addition, there are indications that these implants may alter mammary gland development, which may subsequently increase milk production. Ionophores were shown to increase growth rates, particularly in grazing heifers but more importantly to decrease age at puberty. Age and weight at puberty have been shown to be affected by level of nutrition. It is preferable for heifers to reach puberty one or two estrous cycles before the breeding season, and the breeding season for heifers should begin > or = 21 d before the breeding season of the mature cow herd. This time difference allows for the generally longer postpartum interval for primiparous cows than for multiparous cows. Because of the limited opportunities (every 12 mo) for cows to calve, optimum efficiency must be pursued.