Association of β-casein gene polymorphism with milk production characteristics in Holstein cows

The aim of this study was to analyze the association between the β-CN <i>CSN2</i> gene polymorphism, energy, enzymatic, and nitrogenous metabolites, and production, composition (fat, protein, lactose, total solids, nonfat solids), and SCC in the milk of Holstein cows submitted to the same nutritional and environmental conditions. Eighteen cows (6 A1A1 genotype, 6 A1A2 genotype, and 6 A2A2 genotype) were selected and matched for parity, calving season, and days in lactation. For the statistical analysis, a fixed linear model was applied in which genotypes were considered as a fixed effect and the dependent variables included metabolites, milk production, milk composition, and SCC. We found no statistically significant differences associating the genotype for the β-CN <i>CSN2</i> gene with milk production and composition characteristics or with energy, enzymatic, and nitrogen metabolites of blood and milk, except for variable glucose concentration (mg/dL), in which only the A2A2 genotype presented the highest means (67.78a) in relation to the other genotypes A1A1 (58.28b) and A1A2 (55.69b), where the letter a corresponds to the value of the highest mean and the letter b corresponds to the value of the lowest mean. Under the conditions in which the experiment was developed, the highest averages were numerically observed for variables milk production (kg/d), milk composition and yield (%), and urea nitrogen concentration (%) in the blood, milk, and urine of cows carrying the heterozygous genotype (A1A2) compared with both homozygous genotypes (A1A1 and A2A2), and we found that polymorphism of A1 and A2 alleles does not alter milk composition or blood and urinary metabolites.