bovine somatotropin

Hormone that increases an injected cow's milk yield by 10–40%. It is a protein naturally occurring in milk and breaks down within the human digestive tract into harmless amino acids. However, doubts have arisen as to whether such a degree of protein addition could in the long term be guaranteed harmless either to cattle or to humans.

Although no evidence of adverse side effects to consumers has been found, BST was banned in Europe in 1993 until the year 2000. In the USA genetically engineered BST has been in use since February 1994; in Vermont a law requiring milk containing BST to be labelled as such was passed in September 1995. In January 1999, Canada announced a ban on BST. The reasons given for the ban included a 25% increase in the likelihood of mastitis in cows given BST; an 18% increase in infertility; and double the risk of becoming lame.