LOS ANGELES — The Humane Society of the United States has released undercover video footage shot at two of the nation’s largest egg farms showing workers slamming chickens into metal bins and dead birds littering cages — the latest salvo in an escalating war between the food sector and the country’s leading animal-rights organizations.

At stake, both sides said, is regulating how livestock are treated and how Americans’ food is produced.

Since California voters passed Proposition 2 in 2008, Humane Society officials have ramped up their political campaigns to alter state laws regarding animal welfare. They’re reaching out to young people, including a presentation at last month’s National 4-H Conference in Washington, where they encouraged teenage future farmers to treat livestock with respect.

The organization has also been buying chunks of stock in publicly traded food companies, in part to be able to introduce shareholder resolutions and pressure company executives to alter their purchasing decisions.

The strategy has worked. Companies including Wendy’s, Sonic Corp. and the parent company of the IHOP and Applebee’s restaurant chains all have started shifting to using cage-free eggs, according to Humane Society officials. Wal-Mart, the nation’s largest grocery, said in February that the eggs sold under its store label were now cage-free.

“”Companies generally don’t want graphic information about how their suppliers abuse animals for their products sent to every investor and the media,”” saidPaul Shapiro, head of the group’s Factory Farming campaign.

But the farmers are fighting back. In recent months, agribusiness lobbyists and farm groups have bombarded companies sympathetic to the Humane Society with letters asking them to halt donations to the group.

“”HSUS seeks to remove meat from our dinner tables, leather goods from our closets, animals from zoos and circuses and eventually — pets from our families,”” Kansas Farm Bureau PresidentSteve Baccuswrote in a letter to Bank of America posted on the bureau’s Web site. The Humane Society, he wrote, is “”a powerful, well-funded activist organization pursuing what most reasonable observers would consider an extreme anti-animal agenda.””

In Missouri, livestock farmers were so worried that a Humane Society-backed ballot initiative to ban puppy mills could lead to attacks on livestock interests that they spearheaded their own initiative that “”affirms the rights of Missourians to raise animals in a humane manner.””

And in Ohio, a state animal welfare standards board was created in an effort by farm interest groups and lawmakers to block future ballot measures that might impact livestock production operations. But that hasn’t deterred the Humane Society. The group is collecting signatures to put a bill before voters this November that’s similar to the one passed in California.

“”They are huge. They are influential, and they are our biggest concern on the impact of our ability to domestically produce food,”” saidKay Johnson Smith, executive vice president of the Animal Agriculture Alliance, a trade group that represents livestock farmers and ranchers.

The egg farm footage released Wednesday was shot surreptitiously over the past two months inside Iowa plants owned by Rose Acre Farms and Rembrandt Enterprises. It was taken by a Humane Society volunteer, who then landed work at hen operations in four Iowa cities.

Among other things, the video footage showed chickens crammed into cages so crowded that the animals couldn’t move and their talons couldn’t touch the floor; chickens held in battery cages above manure pits that allegedly hadn’t been regularly cleaned; and a worker stuffing birds into a euthanizing chamber with such force that the thunk of the animals’ heads hitting the metal exterior could be heard.

One worker, whose face has been blacked out on the footage, told the undercover videographer that disease had killed many birds at one location: “”It was nothing to pull 5,000 out of there a day,”” the worker is filmed as saying. “”And that’s all we did for about two weeks straight, is just pull dead birds.””

Tony Wesner, executive vice president of Rose Acre Farms in Seymour, Ind., said Wednesday morning the company “”doesn’t condone inhumane treatment”” of its livestock. “”Anyone violating our standards would be immediately terminated,”” Wesner said.

Officials for Rembrandt Enterprises, based in Rembrandt, Iowa, could not be reached for comment Wednesday.

Similar clandestine videos of farm operations have led to national changes in food-safety policy. An undercover video of a Chino, Calif.-based slaughterhouse, where workers were shown using chains and forklifts to drag cows too injured or ill to stand, led to the largest beef recall in the country’s history, and prompted the federal government — eager to prevent mad cow disease and other contaminates from entering the nation’s food supply — to permanently ban the slaughter of such cattle.

“”We’re not asking for an end to the confinements of animals in buildings. We’re asking they not be crammed into cages and crates barely larger than their bodies,”” said Humane Society PresidentWayne Pacelleduring a news conference in Des Moines on Wednesday.

That’s a message Pacelle has been pushing far more aggressively since California’s Prevention of Farm Animal Cruelty Act was signed into law. The measure, which passed by more than 63 percent of the vote, banned small, confining crates or cages for veal calves, egg-laying hens and pregnant sows. Farmers have until January 2015 to phase out their existing structures and build new facilities.

The bill’s success created a ripple effect, putting pressuring on other states to pass similar reforms.

Pacelle said the Humane Society and Farm Sanctuary — the biggest farm-animal-rights groups in the U.S., which co-sponsored California’s Proposition 2 — first started introducing ballot measures for farm animals in 2000. Their first shot: pig farmers in Florida. They won in November 2002, when the first ban on gestation crates was passed. After that, Pacelle and the Humane Society moved onward. To date, seven states have passed laws banning various animal confinement systems.