WASHINGTON — The Food and Drug Administration called on drug companies to help limit the use of antibiotics given to farm animals, a decades-old practice that scientists say has contributed to a surge in dangerous, drug-resistant bacteria.

Antibiotics are mixed with animal feed to help livestock, pigs and chickens put on weight and stay healthy in crowded barns. Scientists have warned that this routine use leads to the growth of antibiotic-resistant germs that can be passed to humans.

The FDA has struggled for decades with how to tackle the problem because the powerful agriculture industry says the drugs are a key part of modern meat production.

Under the new FDA guidelines, the agency recommends antibiotics be used “judiciously,” or only when necessary to keep animals healthy. The agency also wants to require a veterinarian to prescribe the drugs. They can currently be purchased over-the-counter by farmers.

The draft recommendations by the FDA are not binding, and the agency is asking for drug manufacturers’ cooperation to put the limits in place. Drug companies would need to adjust the labeling of their antibiotics to remove so-called production uses of the drugs. Production uses include increased weight gain and accelerated growth, which helps farmers save money by reducing feed costs. The FDA hopes drugmakers will phase out recommendations for those uses within three years.

But the voluntary approach was met with skepticism by some public health advocates, who said they do not trust the drug industry to voluntarily restrict its own products.

“This is not an issue where trust should be the measure. This is an issue where the measure is whether or not the FDA has fulfilled its authority of protecting public health,” said Richard Wood, chairman of the Keep Antibiotics Working coalition, in a statement.

FDA officials said a formal ban would have required individual hearings for each drug, which could take decades.