Relative effectiveness in genetic gain from genomic selection of candidate dams versus genomic selection of their progeny.

The ability to genomically predict the genetic merit of individuals coupled with the widespread availability of sex-sorted semen is causing a paradigm shift in global animal breeding. Many dairy producers wish to minimize the number of surplus dairy-bred calves born and, in turn, maximize the proportion of beef-on-dairy matings. The aim could be to select the number of candidate dams to be just sufficient to generate ample dairy-bred replacement progeny; using this strategy, minimal scope exists for selection within the resulting progeny cohort. However, the strategy of applying most of the selection pressure on the dam (and sire) ignores the contribution of the random segregation of genetic material during gametogenesis. The hypothesis of this study was that too strict selection within the candidate dam population for mating to dairy-sire semen with minimal opportunity for female progeny selection would affect genetic gain relative to generating surplus female progeny from which selection could then be exercised. Deterministic approaches were developed, complemented with simulations, where different intensity of selection in the dam and female progeny cohorts were imposed. Relative to the selection of the top ranking 10%, 20%, 30%, 40%, and 50% of candidate dams based on their genomic evaluation with 60% reliability, no preselection of dams coupled with genomically selecting the top ranking 10% to 50% of the female progeny born (also reliability of 60%) resulted in female progeny that were expected to, on average, have a true breeding value 0.58, 0.46, 0.38, 0.32, and 0.26 genetic SD higher. Benefits also existed from combining genomic selection within the cohort of candidate dams followed by genomic selection within the resulting female progeny. In most cases, herds will seek to generate ∼30% of their herd size as replacement heifers; based on the population parameters used, in such a scenario there is a clear benefit from selecting the top 60% of the candidate dams with the further gains thereafter being relatively small. This study provides the necessary equations to help producers make herd breeding decisions; the actual relative difference among different scenarios depends on the used population parameters.