Posilac is the genetically engineered bovine growth hormone, used in about one-third of the nation's dairy cows. This hormone, according to it's manufacturer, Monsanto, increases milk production by 10%. As a matter of fact, Monsanto has a web site for farmers with a calculator to demonstrate increased production. (The site can be found at www.make10.net.)

The FDA and Monsanto say that milk produced with this hormone (also known as recombinant bovine somatratropin (rBST)) is safe. Samuel Epstein, professor emeritus of environmental medicine at the University of Illinois at Chicago School of Public Health, strongly disagrees.

"In his new book "What’s in Your Milk" (Trafford Publishing, $24.95), he charges that rBGH milk can increase the risk of breast, colon and prostate cancer and is profitable to Monsanto while posing dangers with no benefits to consumers."

Got hormone-free milk? - Chicago Tribune