A2 milk is a niche product that is already in demand by consumers abroad, but in Slovenia we have only one supplier of A2 milk. Only cows that have two copies of the A2 gene for beta-casein can produce A2 milk. A2 beta-casein has a proline at position 67 that prevents the release of beta casomorphin 7 (BCM7) from A2 beta-casein. BCM7 is potentially known to be an opioid and has been linked to a risk factor for human health because it can affect opioid receptors in the nervous, hormonal, and immune systems. Traditional cattle breeds subjected to less intensive selection for production traits generally have a higher frequency of the A2 allele. The objective of this thesis was to compare different strategies for breeding animals for A2 milk production and to evaluate the consequences of each selection strategy. We obtained genotyping data from 965 brown cattle. Data included BV12 per milk yield, fat and protein contents in milk, somatic cell count, and total merit index. The fastest way to achieve herd status 100% A2 is to use semen from sires with the A2A2 genotype and genotyping all cows. The speed of replacement can be further increased by using sexed semen. We found that genotype had a significant effect on BV12 for protein content and on BV12 for total merit index. Animals with genotype A2A2 had higher BV12 for these two traits. The interaction between sex and genotype had no effect on the observed traits, but sex had a significant effect on BV12 for total merit index and on BV12 for milk yield. The use of sires with the A2A2 genotype may result in lower genetic gain because elite sires with the genotype A1A1 or A1A2 for beta casein was not used.
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