Investigation of A2 allele frequency in Taiwanese Holstein cattle using genetic testing

β-Casein accounts for approximately 30% of total milk protein, with the A1 and A2 variants being the most common. A1 β-casein may release β-casomorphin-7 (BCM-7) during digestion, which is associated with adverse health effects, whereas A2 β-casein does not. This study investigated the A2 allele frequency and its potential influence on milk performance in Holstein cattle in Taiwan. A total of 1,050 cows from five herds were genotyped using the GeneSeek 50K SNP chip. The A2 allele frequency ranged from 0.58 to 0.75 among herds, with an average of 0.66. Hardy-Weinberg equilibrium tests indicated no significant deviation within herds. Pedigree validation confirmed Mendelian inheritance of A1 and A2 alleles. Furthermore, 348 cows with complete lactation records were evaluated for 305-day mature equivalent milk and fat yields across genotypes. Although cows with the A1/A1 genotype showed numerically higher milk and fat yields, one-way ANOVA and Tukey's HSD tests revealed no statistically significant differences. These results suggest that while A2 allele selection is increasing in Taiwan, β-casein genotype does not significantly influence milk or fat production in the studied population.