Long-term changes in the frequency of beta-casein, kappa-casein and beta-lactoglobulin alleles in Polish Holstein-Friesian dairy cattle

. The purpose of the study was to analyse changes in the frequency of genotypes and alleles of beta-casein ( CASB ), kappa-casein ( CSN3 ) and beta-lactoglobulin ( LGB ) loci among bulls (used for insemination in 1991–2021) and cows (milked in 2014 and 2021). The analysis included 1812 bulls and 2628 cows of the Polish Holstein-Friesian breed. Genotypes of three milk protein loci were identified using PCR-RFLP/ACRS method (bulls) or the Illumina Bovine MDv2 Chip (cows). For bulls used in insemination between 1991 and 2021, the frequency of the desirable CSN3 allele B and allele A2 at the CASB locus increased by 44% and 8%, respectively. In cows, the frequency of allele B of CSN3 and allele A2 of CASB increased over the past seven years by 11% and 7%, respectively. The frequency of LGB allele B decreased in cows from 47% to 41%. An increase in the frequency of the rarest allele E at the CSN3 locus in bulls was also observed (from 2% to 7%). The number of animals carrying the most desirable combination of CASB and CSN3 genotypes (A2A2 and BB) reached 4.25% and 2.8% in cows and bulls, respectively. The current results showed a slow but marked increase in the desirable alleles at casein gene loci which should improve milk quality and cheese-processing properties. This positive effect was attenuated by the simultaneous decrease in the frequency of LGB allele B.