Association of the Leu(127) variant of the bovine growth hormone (bGH) gene with increased yield of milk, fat, and protein in Australian Holstein-Friesians

The occurrence of the Leu(127)/Val(127) variants of the bovine growth hormo ne (bGH) gene and their effect on milk production traits was investigated i n Australian Holstein-Friesian cattle. Animals were genotyped for the Leu(1 27)/Val(127) variants, with RFLP methodology, using PCR and AluI digestion of PCR products (AluI-RFLP). Alleles Leu(127) and Val(127) occurred with fr equencies of 82% and 18%, respectively. The quantitative effect of this pol ymorphic site on milk-production traits was estimated from lactation data a nd test-day data. Results from the 2 data sets consistently showed that the Leu(127) allele is associated with higher production of milk, fat, and pro tein and is dominant to Val(127). The average effects of the gene substitut ion are 95 L for milk yield, 7 kg for fat yield, and 3 kg for protein yield per lactation. This locus may be directly responsible for quantitative var iation or it may be a marker for a closely linked quantitative trait locus (QTL) for milk-production traits in Australian dairy cattle. In either case , it will be useful as an aid to selection for improvement of milk producti on traits. As the Leu(127) allele is dominant, selection of AI sires homozy gous for the Leu(127) allele (Leu(127)/Leu(127)) will result in maximum ben efit without the need for genotyping cows.