β-Casein Polymorphism in Serbian Holstein-Friesian and Busha Cattle and Its Association with Milk Production Traits.

Interest in β-casein polymorphism and A2 milk production has increased considerably in recent years because of their potential effects on milk quality and dairy cattle breeding. The aim of this study was to determine the distribution of β-casein A1/A2 genotypes in Holstein-Friesian (HF) cattle and indigenous Busha cattle from Serbia and to evaluate the association between β-casein genotypes and milk production traits in HF cows. The study included 334 HF cows, 58 HF bulls, and 64 Busha cows. Genotyping was performed using the ACRS-PCR method, followed by restriction fragment analysis. Milk production data for HF cows included milk yield, milk protein content, and milk fat content during the first and second lactation. Statistical analysis included determination of genotype and allele frequencies, testing of Hardy-Weinberg equilibrium, and evaluation of associations between genotypes and milk production traits. In HF cows, genotype frequencies were 14.07% for A1A1, 60.18% for A1A2, and 25.75% for A2A2, while allele frequencies were 0.442 for A1 and 0.558 for A2. Similar genotype and allele distributions were observed in HF bulls, where the A2 allele frequency (0.560) was slightly higher than the A1 allele frequency (0.440). In Busha cows, a pronounced predominance of the A2 allele (0.719) and A2A2 genotype (56.25%) was detected. HF cows carrying the A2A2 genotype were associated with significantly higher milk yield, as well as higher milk protein and milk fat content, compared to cows with other genotypes (p < 0.05; p < 0.01). Milk production traits were generally higher during the second lactation, regardless of genotype. HF cows carrying the A2A2 genotype generally showed higher milk yield and more favorable milk composition traits. In addition, the high frequency of the A2 allele in Busha cattle indicates that this indigenous breed may represent a valuable genetic resource for the conservation and utilization of the A2 allele in future breeding programs.