Association of genetic variants of bovine growth hormone with milk production traits in Holstein cattle

Bovine growth hormone (GH) is a single polypeptide secreted by the anterior pituitary gland. Its function is influenced by a variety of factors such as GH-receptor. The GH gene consists of five exons separated by four intervening sequences (WOYCHICK et al. 1982; GORDON et al. 1983) and plays a major role in growth, lactation and mammary gland development. There are several genetic variants of the bovine GH gene that may influence milk production traits in dairy cattle (BECKMANN et al. 1986; HOJ et al. 1993; BURTON et al. 1994; SCHLEE et al. 1994a). Allelic variation at amino acid position 127 of bovine GH was implicated in the variation of milk production (LUCY et al. 1993). Injection of dairy cows with recombinant bovine GH containing valine (V) instead of leucine (L) at the L/V locus increased milk yield (EPPARD et al. 1992), suggesting that dairy cows carrying the V allele would produce more milk. The results are not, however, conclusive (LUCY et al. 1993; SCHLEE et al. 1994a). There is another DNA polymorphism with two known alleles (designated as C and D) within the intron 3 of bovine GH gene. These alleles can be distinguished by MspI restriction enzyme digestion of PCR amplified DNA product (ZHANG et al. 1993). An increased frequency of a specific haplotype consisting of MspI locus and an insertion/deletion polymorphism in the 3'-region of the GH gene, has been associated with increased milk fat in Norwegian Red cattle (HOJ et al 1993). The objectives of this study were to estimate the allelic frequencies at the LN and C/D loci of the bovine GH gene and to investigate the relationship of these two loci to the estimated transmitting abilities (ETA) of milk, fat and protein yields in Canadian Holstein bulls. The establishment of an accurate relationship between the bovine GH gene and production traits could enhance the selection of superior dairy cattle and, in the long term, replace the injection of growth hormone to increase milk yield.