Narrator: This is Science Today. Bovine growth hormone is a genetically engineered hormone. When you give it to a cow, the cow produces more milk than she would naturally.

Liebhardt: It was approved by the FDA, although there is still considerable resistance to its use by a number of farm and consumer organizations.

Narrator: Bill Liebhardt, a sustainable agriculture expert at the University of California, Davis, says even though the government approves its use, many farmers are against bovine growth hormone for one simple reason: there's a milk surplus in the U.S.

Liebhardt: And so in order to try and reduce the surplus, the government actually had a dairy herd buyout program where they bought out whole herds of dairy cattle and disposed of them, they became hamburger basicallly, as an effort to reduce milk production.

Narrator: Given the milk surplus, Liebhardt says many farmers and consumers question the need for a substance that increases milk production even more. And consumer groups are still concerned about its effects on the human body. For Science Today, I'm Steve Tokar.