Federal officials in the US are asking drup companies to discontinue the use of some antibiotics meant for farm animals. Antibiotics are medicines used to fight infections caused by bacteria. Farmers around the world often use the drugs to make healthy animals grow faster and eat less feed.

Farmers put antibiotics in the food and water they give cows, pigs and birds such as chickens. Critics say feeding antibiotics to farm animals lets bacteria develop resistance to such drugs. They say this makes the drugs less effective when they are used to fight diseases in humans.

Last year, the Food and Drug Administration asked the drug companies to voluntarily change the directions for antibiotics sold for use in farm animals. Farmers will be required to stop using the drugs to make animals bigger.

The FDA also wants to require medical supervision when antibiotics are used in farm animals for health needs. Some activists dispute the agency's decision to request, rather than order, the manufacturers to cooperate. Some large drug companies have already agreed to make the changes.

Dimitri Drekonja is a doctor with the Infectious Diseases Society of America. He supports the FDA's decision to ask for voluntary changes, but he is not sure that all drug companies will cooperate. The National Pork Producers Council says hog farmers plan to change their use of antibiotics. The group says its members will limit the use of antibiotics to when they are medically necessary. But it says the changes mean farmers will have more sick animals.