Assessment of genomic selection for introgression of polledness into Holstein Friesian cattle by simulation

Naturally hornless cattle are called polled. The possibility to introgress the allele responsible (P) for polledness in a cattle population that is more intensively selected for other traits is investigated in this paper. Gene introgression, generally carried out by several steps of backcrossing and selection, takes a long time and may lead to unacceptable genetic loss in other traits or inbreeding. The main objective of the current study was to evaluate the use of genomic selection to speed up the introgression of a target allele in a conventional dairy cattle breeding scheme with overlapping generations. A cattle population and a breeding program were simulated and run over a 12 year selection period. Assuming that the polled population was inferior for overall genetic merit , two selection strategies were evaluated: i) selection on conventional BLUP-EBV (CBLUP); ii) selection on genomic EBV (GEBV) obtained with the genomic relationship matrix used in BLUP (GBLUP). Both selection strategies were applied with (PSEL) and without (NOPSEL) selection for the single polled locus (P). The overall level of genetic merit, the P allele frequency and the inbreeding level (F) in the new born animals were monitored each year, as well as the average genetic gain per year of selection (ΔG). The overall genetic level of new born animals was higher for GBLUP compared to CBLUP, with an average ΔG/year of 8.34% (GBLUP) against 7.49% (CBLUP). The PSEL scenario reduced genetic gain, but P allele frequency increased from 0.130 to 0.415 (CBLUP) and from 0.128 to 0.440 (GBLUP) for PSEL, after 12 years of selection. No substantial changes in allele frequency were recorded for NOPSEL scenarios, both for CBLUP and GBLUP breeding schemes. The overall inbreeding rates for GBLUP were 0.28%/y (NOPSEL) and 0.30%/y (PSEL) and for CBLUP 0.52%/y (NOPSEL) and 0.44%/y (PSEL). In conclusion, application of GS to gene introgression helped to speed up the process of introgression of a gene while simultaneously increasing the genetic gain and reducing the inbreeding rate.