Now in a pushback led by the meat and poultry industries, state legislators across the country are introducing laws making it harder for animal welfare advocates to investigate cruelty and food safety cases.
Some bills make it illegal to take photographs at a farming operation. Others make it a crime for someone such as an animal welfare advocate to lie on an application to get a job at a plant.
Bills pending in California, Nebraska and Tennessee require that anyone collecting evidence of abuse turn it over to law enforcement within 24 to 48 hours -- which advocates say does not allow enough time to document illegal activity under federal humane handling and food safety laws.
"We believe that folks in the agriculture community and folks from some of the humane organizations share the same concerns about animal cruelty," said Mike Zimmerman, chief of staff for Assembly Member Jim Patterson, R-Fresno, whose bill was unveiled this week. "If there's abuse taking place, there is no sense in letting it continue so you can make a video."
Patterson's bill, sponsored by the California Cattlemen's Association, would make failing to turn over video of abuse to law enforcement within 48 hours an infraction punishable by a fine.
Critics say the bills are an effort to deny consumers the ability to know how their food is produced.
"The meat industry's mantra is always that these are isolated cases, but the purpose of these bills is to prevent any pattern of abuse from being documented," said Paul Shapiro, vice president of farm animal protection for the Humane Society of the United States, which conducted the California and Vermont investigations.
In Indiana, Arkansas and Pennsylvania it would be a crime to make videos at agricultural operations.
The goal of the proposed California law, industry representatives say, is to halt any abuses quickly and get video evidence to government regulators within two days, not to impede undercover investigations by animal welfare groups.
"The people doing this aren't cops so I wouldn't think it's their job to build a case. The goal for all of us is to reduce instances of animal abuse," said David Daley, a Cattlemen vice president and professor of agricultural science at California State University-Chico.
Formal opposition to the California bill comes from the ASPCA, the Teamsters, the HSUS and dozens of others. They say these attempts by the agriculture industry to stop investigations are a part of a nationwide agenda set by the American Legislative Exchange Council, a conservative think tank backed by business interests.
ALEC has labeled those who interfere with animal operations "terrorists," though a spokesman said he wishes now that the organization had called its legislation the "Freedom to Farm Act" rather than the "Animal and Ecological Terrorism Act."
"At the end of the day it's about personal property rights or the individual right to privacy," said spokesman Bill Meierling. "You wouldn't want me coming into your home with a hidden camera."
Animal welfare advocates say all of the focus on secrecy is energy misspent.
"I wish the cattlemen actually wanted to stop cruelty, not the documenting of cruelty," said HSUS California director Jennifer Fearing. "One could think of a thousand ways for them to actually stop cruelty rather than waiting for people to make videos and turn them over."
Animal welfare advocates say law enforcement agencies do not have the time or inclination to work complex animal abuse and food safety cases, and that federal USDA inspectors in slaughter plants have turned a blind eye to abuse.
When a USDA inspector at the Vermont plant was heard in 2009 coaching a plant worker on how to avoid being shut down, U.S. Agriculture Secretary Tom Vilsack weighed in, calling the conduct "inexcusable."
In reaction to concerns, the USDA has been working to improve enforcement of its humane handling regulations over the past two years, including establishing an ombudsman position that accepts reports of violations. Last year 24 new positions in the Food Safety Inspection Service were dedicated to humane handling, said a high-ranking food safety official not authorized to speak publicly.
That hasn't slowed investigations or the bills designed to stop them. The Arkansas bill goes further than the others and would prohibit anyone other than law enforcement from investigating animal cases.
Last year Iowa, a major egg-producing state, passed a bill making it illegal to deny being a member of an animal welfare organization on a farm job application. Utah passed one that outlaws photography.
Most of the sensational videos of abuse in recent years are shot by undercover operatives who surreptitiously apply and are hired by the meat processors for jobs within the facilities. One recorded last year by Compassion over Killing at Central Valley Meats in Hanford, Calif. showed a worker standing on a downed dairy cow's nostrils to suffocate it and others repeatedly shot in the head, prompting several fast-food hamburger to cancel contracts, at least temporarily.
Animal welfare groups say investigations take weeks because the operatives nose around only when they aren't performing the duties for which they were hired.
An HSUS investigator was in the Hallmark plant in Southern California for six weeks between October and November 2007, when the nonprofit turned over to the local district attorney evidence that included fraud in the federal school lunch program because animals too sick to walk were being slaughtered. In January 2008, HSUS released the video to force the DA to act. Two employees were convicted of cruelty charges.
Late last year, nine workers at a Wyoming pork processing facility were charged with animal cruelty after an HSUS video showed them kicking and tossing piglets and failing to euthanize a sow gravely injured by a worker while giving birth.
In 2009, HSUS spent 21 days in the Vermont slaughterhouse where male calves born to dairy cows were killed for veal.
"Believe me our investigators would like to be out of there as soon as possible. They're stoic, they're courageous, but they are not enjoying their work at all," said Mary Beth Sweetland, director of investigations for HSUS.