Inside a West Tennessee barn, the horses are whipped and beaten. Trainers drag them by their heads. Some are kicked. Chemicals are dripped on their ankles which are then bound tightly with plastic wrap.

It was all done in an effort to accentuate the well-known, high leg kick of the Tennessee Walking Horse, animal rights advocates say. And, it all unfolds on an undercover video.

The Humane Society of the United States released the footage Thursday to urge the horse industry, the U.S. Department of Agriculture and Congress to crack down on "soring." The practice is illegal under federal and state law.

It involves using chemicals on the horses' ankles as a way to force them, because of pain, to lift their legs higher when they walk. The walk, known as the "Big Lick," is prized in Walking Horse competitions.

PepsiCo quickly moved to withdraw its sponsorship of the Tennessee Walking Horse National Celebration as a result of the video, a spokesman confirmed Thursday. The video first aired Wednesday on ABC News' Nightline. The walking horse show runs this year from Aug. 22 to Sept. 1 in Shelbyville, Tenn.

The celebration's CEO, Doyle Meadows, issued a statement Thursday saying he hopes PepsiCo one day returns as a sponsor. "The celebration has worked extremely hard over recent years to gain the trust of our corporate partners, and we would do nothing to destroy that relationship," Meadows said.

Soring has been illegal under the federal Horse Protection Act since 1970. But Keith Dane, director of equine protection for the Humane Society, said it has "continued unabated for the past 42 years."

The video, shot last year by an undercover humane society investigator, shows trainers at a Collierville, Tenn., farm applying the chemicals to the horses.

The Humane Society filmed the video in a barn owned and operated by nationally known trainer Jackie McConnell. The organization gave the video to federal prosecutors last year, and McConnell and three others were charged in a 52-count indictment unsealed in early March.

Papers filed this week in federal court in Chattanooga, state that McConnell intends to plead guilty to one count of the indictment. His attorney could not be reached for comment.

Dane praised Bill Killian, the U.S. Attorney for the Eastern District of Tennessee, for pursuing soring cases. Under federal law, it is illegal to transport and show sored horses. In addition to McConnell, federal prosecutors brought charges against Lewisburg, Tenn., trainer Barney Davis. Davis pleaded guilty and was sentenced to a year in prison and fined $4,000 for soring.

Regulation is limited

Tennessee Walking Horses already have a natural, high gait and soring is unnecessary, said Dane, a licensed horse judge.

The U.S. Department of Agriculture's Animal and Plant Health Inspection Service is charged with enforcing the Horse Protection Act. Violations of the act could mean up to two years in prison and a $5,000 fine. But USDA veterinarians can only make it to a small number of the more than 450 shows held across the nation each year.

Because of the lack of federal funding, the department relies on a form of industry self-regulation.

But a 2010 report from the USDA's Office of Inspector General found that the self-regulation is "not adequate to ensure that these animals are not being abused."

Under the system, horse industry organizations hire inspectors. Because of that conflict of interest, inspectors do not always examine horses according to the requirements of the Horse Protection Act. Many in the industry, the report concluded, do not believe abuse is a problem and resent federal oversight.

The Humane Society on Thursday called for abolishing self-regulation and urged Congress to step in and close loopholes in the Horse Protection Act.

"We want to see all of these violators brought up on charges," Dane said.

U.S. Department of Agriculture spokesman Dave Sacks said Thursday he could not comment on the undercover video because the case is still pending against McConnell.

But he said the department has made strides to improve oversight and reduce soring. For instance, the department's veterinarians went to 80 horse shows in 2011, up from 50 the year before, Sacks said.

The department also is proposing changes to the Horse Protection Act.

Sacks said one proposal would allow the department to train and license the inspectors hired by horse industry organizations. Another change would require horse industry organizations to enforce the USDA's minimum penalties for soring.

State Rep. Janis Sontany, a Nashville Democrat who has sponsored several bills to increase penalties for animal cruelty, called the practice "disgusting."

"Nothing will ever happen until you get a district attorney who is willing to prosecute," she said.

Industry reacts

Two leading walking horse industry groups Thursday condemned the practice of soring in the wake of the video's release.

"The walking horse holds an inherent natural gait that has been in existence for nearly 100 years," Marty Irby, president of the Tennessee Walking Horse Breeders' and Exhibitors' Association said in a prepared statement. "(The association) adopted a zero tolerance policy in regards to soring a number of years ago and has recently challenged every member to adopt a zero-tolerance policy themselves."

Stephen Mullins, a veterinarian and president of SHOW, one of the horse industry groups that helps regulate the industry, called the scenes on the videotape "delporable."