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An important part of any production system is the reproductive performance and costs associated with developing replacement heifers. The cost of developing replacement heifers has a tremendous impact on the profitability and sustainability of cow-calf operations, and reproductive failure costs the U.S. beef and dairy industries approximately $1 billion annually (Bellows et al., 2002). Heifers need to calve by 24 months of age to achieve maximum life-time productivity (Patterson et al., 1992), and heifers that lose a pregnancy or conceive late in the breeding season are likely to not have enough time to rebreed during a defined breeding season. In addition, heifers that calve earlier with their first calf are more likely to bred back as two year olds and remain calving at the beginning of the calving season. This is important to overall profitability since age of calf at weaning is the single largest factor that affects weaning weight. Analysis of 3700 calves at the USDA- Meat Animal Research Center indicated that for each day of age after the beginning of the breeding season that a calf is born 2.4 pounds of weaning weight is lost (personnel communication R. Cushman). Therefore, to improve the efficiency of production, it is important to develop heifers that will bred early and calving at the beginning of the calving season.