Replacement heifer development programs generally begin at weaning as the heifers begin preparation for their initial breeding season. However, decisions relating to use of hormonal growth-promoting implant programs ( see Growth Promotants and Production Enhancers Growth Promotants and Production Enhancers ) for replacements must be made beginning at the working of calves at 2–3 mo of age. Reimplanting heifers intended for breeding at weaning, or after 6 mo of age, is not recommended. Heifers that calve at 22–24 mo of age have increased lifetime production compared with heifers that calve first at 30–36 mo of age.
Selection of replacement heifer prospects begins at weaning, when heifers are typically 6–8 mo old, and is based on birth date, genetics, frame score, disposition, weaning weight and ratio, and dam production records. Selection of heifers born early in the calving season leads to replacement heifers that are older and heavier at the start of the breeding season. Heifers should be evaluated for structural soundness, and unacceptable conformation should disqualify a heifer as a replacement prospect. Age-adjusted frame scores can be used to estimate the mature size of prospects. These scores provide an objective method of selecting replacements to maintain a consistent cow size suitable for the environment and feed resources.
Once potential replacement heifers are selected, a nutrition and vaccination program should be instituted in preparation for breeding. If heifers are to calve at 22–23 mo of age, they must be bred at 13–14 mo of age. For optimal fertility, heifers should weigh ~53%−65% of their mature body weight by this time. The ration must be balanced to provide the required rate of gain to meet the target goal of 53%–65% of mature body weight in the time available. Specific requirements vary with the weight and breed of the heifers and the amount of time available before breeding. (Also see Nutrition: Beef Cattle Nutrition: Beef Cattle .)
Considerable research on the most cost-effective target weight for breeding heifers has shown that it is almost always below the previous recommendation of 65% of mature body weight. Instead of a goal of the maximum number of heifers conceiving, a better goal may be to have a slightly lower percentage of heifers bred at a cost that is significantly lower than developing heifers to breed at 65% of mature body weight.
The vaccination program for replacement heifers should provide optimal protection from reproductive diseases and should include vaccination for infectious bovine rhinotracheitis (IBR) and bovine viral diarrhea (BVD) types 1 and 2. Depending on the local disease risk, vaccinations for Brucella, Leptospira, Trichomonas, and Campylobacter may also be indicated. Brucella vaccination is performed according to state or regional regulations. Modified-live vaccines for IBR and BVD give the broadest immunity to strain differences and should be given twice to ensure a high level of immunity. Some evidence suggests that modified-live BVD and IBR vaccines may transiently infect the ovary and cause decreased fertility. For this reason, vaccination should be done ≥1 mo before breeding. Vaccination of heifers with Trichomonas vaccine increases calving rate and decreases duration of infection in infected herds but does not prevent infection. A vaccine for trichomoniasis may be useful in infected herds or in herds at high risk of infection, but it may not be economic in low-risk herds.
Heifers should be adequately developed at the time of first breeding, and their management during gestation must ensure their continued growth. Pregnant heifers should be separated from the main cow herd at the time of pregnancy testing and maintained separately until reentry into the breeding herd as animals move to pasture. Undernutrition during pregnancy in first-calf heifers can lead to an increased incidence of dystocia because of lack of weight and size, weakness at the time of parturition, insufficient colostrum, weak calves at birth, and a high incidence of prolonged postpartum anestrus, which leads to a high percentage of nonpregnant animals that will need to be culled. Thus, pregnant heifers should be fed and managed separately from cows and should be on a higher plane of nutrition than cows. Such management may slightly increase calf birth weight but will not increase dystocia scores if heifers calve in BCS 6–7. Heifers are ideally bred 2–3 wk before cows, and they should always be bred for a shorter period of time (~42 days) to ensure that those with the highest inherent fertility are retained in the herd. This also ensures that any increase in the postpartum anestrus period after their first, perhaps difficult, calving does not compromise their chances for cycling and rebreeding with the main cow herd.