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Beef Heifer Replacement Considerations Related to Breed and Biological Type
The reproductive fitness of females is a major driver of profitability, and fertility considerations should be given high priority when selecting the best replacement heifers. Selection for replacement heifers is based on their readiness and ability to conceive in the proposed breeding season which places indirect selection on dam fertility, as fertile cows tend to conceive early and generate early-born heifers that are more likely to be selected as replacements. Age at puberty is variable among breeds and biological types (British<Continental<Bos indicus-influenced), and is moderately heritable. The choice of breed(s) and breeding system play an important role in overall ranch profitability and before making a commitment to any breed or breeding system, the logistics, costs (including opportunity costs), benefits, and feasibility should be objectively evaluated for a given set of environmental, resource, management and marketing constraints. Expected progeny difference (EPD) genetic merit estimates for heifer pregnancy (HP), stayability (STAY), and scrotal circumference (SC) are available for some breeds and have all been positively associated with female fertility. Heterosis (also known as hybrid vigor) occurs when the performance of the crossbred progeny for a specific trait is greater than the average of their parents. Heterosis effects are significant and important for low heritability fitness and survival traits such as longevity, lifetime production, and reproduction rate. Improvements in cow-calf production due to heterosis result from both the improved maternal performance of the crossbred cow and individual performance of the crossbred calf. Complementarity results from crossing breeds of different but complementary biological types. This occurs when specialized sire and dam breeds are used in terminal systems to optimize performance levels. Properly designed crossbreeding systems based on heterosis and complementarity will generally out-produce those based on straightbreeding in productivity, but the challenge is to manage the program to produce progeny that meet market specifications and acceptance. The combination of artificial insemination (AI) and gender-selected or “sexed” semen offers the opportunity to rethink the logistics and economics associated with different breeding systems. Gender-selected semen provides the opportunity to develop novel breeding scenarios, and avoid some of the logistical problems associated with the various cross breeding systems. Emerging reproductive and genomic technologies offer exciting possibilities for innovative approaches to heifer selection and breeding program design, but as with all new technologies enthusiasm needs to be tempered with a realistic evaluation of the costs and expected benefits.
Keywords: Breed, Heterosis, Complementarity