Genetic impacts of using female-sorted semen in commercial and nucleus herds

This study was designed to investigate the genetic effects of using sorted semen in a dairy cattle population. Progress was monitored in elite and commercial animals over 20 yr of selection. To study the genetic impact of using sorted semen in commercial herds, a scenario was evaluated in which female-sorted semen was available to commercial herds. Second, to study the genetic impact of using sorted semen in nucleus herds, scenarios were simulated in which female-sorted semen was used only in a centralized nucleus herd, in which multiple ovulation and embryo transfer (MOET) took place. Because of the additional advantage of marker-assisted selection when sorted semen was used in nucleus herds, a second scenario was simulated in which both sorted semen and marker-assisted selection were implemented. In the scenario in which female-sorted semen was used in commercial herds, a large genetic advantage was observed early in commercial cows. The average superiority in first-lactation cows exceeded 30% in yr 11, relative to a base scheme with regular semen, but continued to decrease until it reached 9% in yr 20. The increased selection intensity in commercial cows contributed to the genetic merit of future cows (cow-to-cow contribution), but the contribution of the nucleus grew over time and gradually marginalized the cow-to-cow contribution. The genetic advantage of gender control in MOET schemes was minimal except when marker-assisted selection was also available. Two factors that affected the contribution of marker-assisted selection were studied: 1) within- vs. across-family selection of donors, and 2) the number of loci in the quantitative trait locus component. Schemes that selected donors regardless of their family structure were superior, and the quantitative trait locus component with more loci increased the effectiveness of sorted semen. Finally, we studied a reduced MOET scheme in which the number of harvested females was reduced from 42 to 25/yr. The reduced scheme in combination with female-sorted semen was not found to be genetically inferior to the large scheme in combination with regular semen.