Influence of A1/A2 allelic variants of the CSN2 gene on milk composition and production in Holstein cows from Nariño, Colombia

This study aimed to determine the allelic frequency and the effects of the A1 and A2 variants of the CSN2 gene on milk production and quality in Holstein cows fromfive municipalities in Nariño, Colombia. Productive and compositional milk datawere collected from 200 cows across 10 herds located in Pupiales, Pasto, Gualmatán,Guachucal, and Túquerres. The A1 and A2 variants of the CSN2 gene were identified through allele-specific PCR using DNA extracted from blood samples. Associationsbetween genotypes and milk production and composition traits were assessed using analysis of variance (ANOVA). The results showed allele frequencies of 0.46 for A1 and 0.54 for A2, and genotype frequencies of 0.23 (A1A1), 0.46 (A1A2), and 0.31 (A2A2). Observed heterozygosity (Ho) was 0.46, expected heterozygosity (He) was 0.47, andthe polymorphic information content (PIC) was 0.37. Cows with the A1A1 genotype produced significantly more milk and total solids than A2A2 cows (p < 0.05). The A1 allele was associated with significantly higher yields of milk, fat, and protein (p < 0.05). A discriminant analysis revealed differentiation by municipality, suggesting that environmental factors influence variability in milk production and composition. It is concluded that the A1 allele is present at a moderately high frequency and is associated with enhanced productive traits in Holstein cows in the Department of Nariño. Therefore, the use of A2A2 genotype bulls with high genetic merit for productive traits is recommended, with the additional aim of promoting potential health benefits for milk consumers.