The chicken industry is pushing back against a growing campaign to move away from breeding larger birds that bulk up quickly, saying the “slow-growth” movement would use more energy, cost shoppers more money and possibly result in less protein on people’s plates.

If even a third of the nation’s $48 billion poultry industry switched to more svelte chickens, the national flock would need to grow by 1.5 million birds to keep up with current consumption rates, according to an economic analysis released by the National Chicken Council, a not-for-profit trade association.

That increase would require an additional 5.1 billion gallons of water and 7.6 million acres to raise the additional feed those chickens would need, according to the report.

“It comes with trade-offs, and that’s what we want to get out there,” said Tom Super, spokesman for the council. “Before it becomes a domino issue like the cage-free eggs, we want our customers to have all of the information on hand while they’re making these decisions.”

In the past year, pressure has increased from consumers, retailers and food service companies to transform practices at the nation’s factory farms, including the use of antibiotics, housing chickens in crowded cages and failing to stun birds properly before slaughter.

Last March, Whole Foods Market announced it would require its suppliers to switch back to slower-maturing breeds and to improve farm conditions by 2024 as part of the Global Animal Partnership program the retailer created to push for more humane treatment of farm animals. In November, food-service companies Aramark and Compass Group agreed to follow the same standards.

“It is really unsustainable to have business practices that cause so much suffering, are so unhealthy, that consumers are appalled and reject the product when they’re educated about what’s happening,” said Daisy Freund, director of farm animal welfare for the American Society for the Prevention of Cruelty to Animals, which has pushed the slow-growth movement.

In the 1920s, the average U.S. domesticated chicken was a relatively scrawny bird — weighing a mere 2½ pounds at slaughter, about 115 days after it was born. It now averages about 6 pounds and takes only 49 days to reach “market maturity,” according to the council.

Most of that transformation has occurred since 1965, when Americans ate an average of 28 pounds of chicken each annually, according to the U.S. Department of Agriculture. Just since 1990, the average broiler has put on 2 pounds, Super said, and Americans now eat an average about 91 pounds of chicken each every year, according to the USDA.

Part of that growth spurt came from routinely feeding animals small doses of antibiotics that change their gut microbiome, improving immunity and enhancing their metabolism. But most of the growth came from simple animal husbandry advances.