RALSTON, Iowa — Piglets hop, scurry and squeal their way to the far corner of the pen, eyeing an approaching human. “It shows that they’re healthy animals,” Craig Rowles, the owner of a large pork farm here, said with pride.

Mr. Rowles says he keeps his pigs fit by feeding them antibiotics for weeks after weaning, to ward off possible illness in that vulnerable period. And for months after that, he administers an antibiotic that promotes faster growth with less feed.

Dispensing antibiotics to healthy animals is routine on the large, concentrated farms that now dominate American agriculture. But the practice is increasingly condemned by medical experts who say it contributes to a growing scourge of modern medicine: the emergence of antibiotic-resistant bacteria, including dangerous E. coli strains that account for millions of bladder infections each year, as well as resistant types of salmonella and other microbes.

Now, after decades of debate, the Food and Drug Administration appears poised to issue its strongest guidelines on animal antibiotics yet, intended to reduce what it calls a clear risk to human health. The guidelines, which are voluntary and will not have the binding force of regulations, would end farm uses of the drugs simply to promote faster animal growth and call for tighter oversight by veterinarians.