(bST; also known as bovine growth hormone or BGH) — A naturally occurring protein that can be produced as a synthetic compound that causes cows to increase the efficiency of milk production per unit of feed consumed.

Also called bovine growth hormone, bST is a naturally occurring protein that has been genetically engineered as a synthetic compound (now manufactured in large quantities and commercially available to farmers) that causes cows to increase the efficiency of milk production per unit of feed consumed. Its use has caused public controversy, and some states require retail dairy product labels to identify the use of synthetic bST.

A natural protein produced in the pituitary glands of all cattle, bST helps adult cows produce milk. Marketed by Monsanto as Posilac bovine somatotropin.