Beefing up livestock.

Breeders have made great strides in reducing the percentage of fat in cattle, pigs and sheep. But fearing these genetic approaches to leaner meat may be nearing their limit, livestock researchers have turned their attention to a class of experimental drugs known as repartitioning agents. Currently under federal review for use in animals intended for human consumption, these compounds could dramatically increase the proportion of its diet that an animal converts to protein instead of fat, according to Donald H. Beermann of Cornell University.

One type, known as beta-agonista, appears to mimic steroid hormones. Beermann showed recent data indicating that, compared with untreated animals receiving the same diet, supplemented lambs will lay down up to 40 percent more muscle -- edible meat -- in their hind legs. "Similar effects were obtained in cattle . . . and to a lesser extent in pigs," he notes. The drawback? Compounds and doses producing the greatest repartitioning from fat to protein often led to tougher cuts of meat.

Somatotropin, also known as growth hormone, has exhibited similar effects in growing livestock -- without causing a corresponding toughening of their meat, Beermann reports. For instance, he showed data from pigs indicating that low doses of the drug could increase muscle deposition by 28 to 38 percent, while reducing fat content in the edible cust from 37 to 78 percent. "That's better than we could achieve with 10 to 20 years of breeding changes," he concludes.