Narrator: This is Science Today. Bovine growth hormone, a genetically engineered hormone that makes dairy cows produce more milk than they would naturally, has been available to farmers since 1994. Bill Liebhardt, an expert in sustainable agriculture at the University of California, Davis, says farmers and scientists are still debating its merits.

Liebhardt: What some people will say is that we'll be able to produce the same amount of milk with less number of cows, and therefore they'll say that will have an environmental benefit. Which is true. But what will happen is that you'll have these cows concentrated in a smaller and smaller area.

Narrator: In other words, says Liebhardt, there will be fewer, larger dairy farms -- and each farm will be a concentrated source of animal waste.

Liebhardt: And it will, in a sense, instead of spreading the animals over the countryside where waste can be handled easier, you're moving in the opposite direction.

Narrator: Besides the waste problem, many consumer groups and farmers still question the safety of bovine growth hormone for humans and cows. For Science Today, I'm Steve Tokar.