Association of genetic polymorphism in GH gene with milk production traits in Beijing Holstein cows.

Associations were analysed between polymorphisms of the growth hormone gene (GH-MspI) (localized in intron 3) and milk production traits of Beijing Holstein cows (a total of 543 cows). Polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) method was used for identification of various genotypes. Frequencies of genotypes were 0.77, 0.21 and 0.02 for A/A, A/B and B/B, respectively. The frequency of the GH A allele is 0.875. The results of the least squares means show that in all three lactations, the GH A/A cows yielded more milk (P less than 0.01 for lactation I and P less than 0.05 for lactations II and III), whereas A/B cows showed higher milk fat content than A/A individuals (P less than 0.05 for lactations I and II, and P less than 0.01 for lactation III). The A/A cows yielded more fat than A/B individuals (P less than 0.01 only in lactation I). The A/A cows yielded more milk protein than A/B individuals (P less than 0.01 for lactations I, II, and III). The A/A cows produced milk of higher protein content than of A/B individuals (P less than 0.05 only in lactation II).