Evaluating genomic breeding programs for a small dairy cattle population with widespread use of private bulls.

Modern reproductive technology combined with genomic selection allows for great genetic gains for dairy cattle populations. However, for small, local breeds, the risk of excessive inbreeding, low selection intensity, and lack of resources makes it challenging to stay competitive with the larger international breeds. In the case of Icelandic dairy cattle, the continued use of private bulls for natural mating for an otherwise centralized breeding program adds to the challenge. The objectives of this study were to compare genomic breeding programs for a population mimicking the Icelandic population and evaluate how the use of private bulls combines with genomic selection schemes, multiple ovulation and embryo transfer (MOET) programs, and the use of sexed semen to influence genetic gain and rate of inbreeding. We simulated 80 alternative breeding schemes for a dairy cattle population of 18,000 cows and compared the genetic gain and rate of inbreeding. We found that a breeding program that used a combination of sexed semen and 50 donors for a MOET program gave the highest genetic gain, with a rate of inbreeding near or less than 1.0% per generation if 15 or more artificial insemination (AI) sires were selected each year regardless of the use of private bulls. Private bulls reduced the genetic gain in the population and genomic selection of the private bulls only partially offset the reduction. The use of sexed semen needs to be combined with a donor program or controlled to ensure that enough bull calves are available to be selected as AI sires. The findings suggest that both MOET and sexed semen can increase genetic gain in genomically selected small populations, but the cost of a MOET donor scheme, and the production of replacement sires must be considered. Optimum contribution selection should be used to control inbreeding rate.