DES MOINES, Iowa — Ventilation systems at poultry barns affected by bird flu would be shut off as a means of suffocating entire flocks if other methods of euthanasia cannot be completed within 24 hours, according to a new federal bird flu control policy released Friday.

The practice, which immediately drew criticism from animal rights groups, is part of a new set of policies the U.S. Department of Agriculture developed in response to the H5N2 virus, which swept through 15 states and killed 48 million birds, mostly chickens and turkeys.

The policies are designed to help farms more quickly keep the virus from spreading, to other farms by air, equipment, humans and other animals. USDA officials have said that teams hired to euthanize birds in Iowa and Minnesota fell behind on destroying infected birds this spring, due to the size of flocks — many with millions of birds. In some cases birds suffered and died from disease before they were euthanized.

The previous policy called for the most immediate but humane method of euthanasia. A common method used for turkeys includes pumping oxygen-depleting, water-based foam into the barns and corralling the birds at one end. In chicken facilities where cages are stacked vertically, the birds were removed from their cages and placed inside a chamber filled with carbon dioxide gas.

The new euthanasia policy also initiates a 24-hour “stamping-out” policy, after which the USDA is authorized to approve ventilation shutdown, but only after federal and state officials and the producer agree that no other method would meet the 24-hour deadline.

In this method, the heat is turned up in the barns, which are sealed, and ventilators shut down. Within 30 to 40 minutes, the birds suffocate and die from heat stress.

“It’s something we do not do lightly and we want to make sure all other options are exhausted,” said Dr. T.J. Myers, associate deputy administrator for veterinary services at USDA’s Animal and Plant Health Inspection Service. “There are humane questions here, but the other side of that humane coin is if we can prevent four, five or six additional flocks from becoming positive then that is something that we really want to try and do.”

The Human Society of the United States immediately condemned the policy.

“Shutting down ventilation systems in these operations essentially bakes the birds to death over a period of timewhich can last hours and involves intense suffering,” Chief Veterinary Officer Michael Blackwell said.