ASSOCIATION OF SOMATOTROPIN (BST) GENE POLYMORPHISM AT THE 5TH EXON WITH SELECTION FOR MILK-YIELD IN HOLSTEIN COWS

A selection project produced control cows from continuous matings with breed average bulls for predicted transmitting ability for milk (PTA- milk) in 1964 and select cows from matings to four of the highest PTA- milk bulls each year since 1964. Blood samples were collected in 1992 when milk yield difference of select and control line cows exceeded 3, 800 kg of milk/305-day lactation. Genomic DNA from control (n = 49) an d select (n = 101) cows was analyzed for the presence of variants asso ciated with amino acid position 127 (leucine, AluI[+]; valine, AluI[-] ) of bovine somatotropin (bST). Amplification of a 428 base-pair fragm ent of the bST gene from individual cows, subsequent restriction enzym e (AluI) digestion, and separation of resulting fragments indicated th ree genotypes AluI(+/+), AluI(+/-), and AluI(-/-) in 110, 39, and 1 an imal(s), respectively. Gene frequencies of leucine(127) and valine(127 ) alleles were similar for control (0.867, 0.133) and select (0.861, 0 .139) animals. United States Department of Agriculture-PTA values were compared between the two genotypes, AluI(+/+) and AluI(+/-). Estimate d breeding value for milk (EBV-milk) and average yield deviation for m ilk (AYD-milk) were not associated with genotype for control animals. However, presence of the valine allele was correlated with decreased E BV-milk (P = 0.03) and AYD-milk (P = 0.16) in select animals and accou nted for a decrease of approximately 170 kg of EBV-milk and 240 kg of AYD-milk.