It has long been common knowledge in farming that antibiotics can help cause animals to grow fatter faster. Time is money, particularly in the food industry, and for many years ranchers used antibiotics not just for treating diseases but also for promoting growth so that animals would be ready for the slaughterhouse sooner. (Mr. Lewis says his grass-fed steers require 27 months to get to market without antibiotics, more than twice as long as it takes cows pumped full of antibiotics.)

In early 2017, the Food and Drug Administration enacted rules banning the use of human antibiotics purely for growth promotion in animals and requiring ranchers to get a prescription from a veterinarian for antibiotics that once could be purchased over the counter. The F.D.A. enacted the restrictions out of growing concern about the breeding of drug-resistant bacteria from antibiotic overuse. Those resistant bacterial strains can be transferred to humans by contact with animals or raw meat and possibly through the consumption of undercooked meat.

The growing resistance of bacteria to antibiotics causes some 23,000 American deaths a year and $34 billion in financial losses annually, according to the Centers for Disease Control and Prevention. The C.D.C. also estimates that more than 400,000 United States residents become ill with infections caused by antibiotic-resistant food-borne bacteria every year, with about one in five resistant infections caused by germs from food and animals.