Associations of BoLA-DRB3 genotypes with breeding values for milk production traits in Russian dairy cattle population

Breeding for genetically resistant cattle is an attractive approach to eradicate infectious disease in livestock. Bovine leukaemia is one of the most common infectious diseases of dairy cattle, which causes significant economic losses. Numerous studies have demonstrated that polymorphisms in the DRB3 gene at the bovine leukocyte antigen ( BoLA ) locus are associated with cow resistance to persistent lymphocytosis (PL), a clinical sign of leukaemia. Alleles associated with PL susceptibility are generally prevalent in dairy cattle populations and may reflect their association with milk productivity. Our objective was to evaluate the associations between genotypes at the BoLA-DRB3 locus and milk production traits in dairy cattle in Russia. In total, 171 Holstein sires were genotyped for DRB3 using the PCR-RFLP technique. Detected BoLA-DRB3 alleles were assigned to three categories according to their PL susceptibility. The DRB3.2*08, DRB3.2*16, DRB3.2*22 and DRB3.2*24 alleles were defined as susceptible (S), whereas alleles DRB3.2*11, DRB3.2*23 and DRB3.2*28 were identified as resistant (R), and the remaining alleles were denoted as neutral (N). We calculated the estimated breeding values (EBV) for milk production traits, including milk yield (MY), fat yield (FY) and protein yield (PY), using genotype categories at the BoLA-DRB3 locus as a fixed effect. We detected fifteen alleles, forty-nine genotypes and six genotype categories for the BoLA-DRB3 gene. The PL-susceptibility alleles were prevalent in Russian dairy sires (ranging from 0.0877 for BoLA-DRB3.2*08 to 0.2135 for BoLA-DRB3.2*22 ), assuming 0.6258. The bulls carrying the SS genotype category were characterized by significantly higher EBVs for MY and PY compared to the other genotype categories: 89.3 kg for MY (p < 0.001 vs. NN, NR, SN, p < 0.01 vs. SR, p < 0.05 vs. RR), 2.15 kg for FY (p < 0.001 vs. NN, NR, p < 0.05 vs. SN); 4.81 kg for PY (p < 0.001 vs. all other genotype categories). Our findings indicate that high frequencies of PL-susceptibility alleles in BoLA-DRB3 may be a result of their association with milk production traits in dairy cattle in Russia. Therefore, to prevent the development of clinical forms of bovine leukaemia in dairy herds in Russia, the impact of the genotypes at the BoLA-DRB3 gene locus on milk production traits must be considered.