Three workers at Canada's largest dairy farm have pleaded guilty to animal cruelty charges following a hidden camera investigation that uncovered shocking cattle abuse.

Travis Keefer, Chris Vandyke and Jamie Visser were all charged after the activist group Mercy for Animals sent a member to work with them undercover at Chilliwack Cattle Sales three years ago.

Court records showed all three entered guilty pleas to six counts each of causing an animal to continue to be in distress and one count of taking, injuring or destroying a bird or its egg.

They are expected to be sentenced early next month, Mercy for Animals said.

The employees' guilty pleas came four months after the company that owns Chilliwack Cattle Sales and one of its directors pleaded guilty to charges of causing an animal to continue to be in distress.

A total of 20 charges were laid against the company and seven of its employees following Mercy for Animals’ investigation. The case was the first time a B.C. company was held accountable for acts of cruelty on a farm, the SPCA said at the time.

The undercover employee wore a hidden camera after being hired at the farm in 2014. Video appeared to show employees kicking and punching cows, hitting them with canes and ripping out the hair from their tails. One cow was lifted by a chain around her neck using a tractor, the court saw during the first trial. An employee can also be seen attaching milking equipment to the testicles of bulls at the farm.

In some of the footage, employees can be heard cheering and laughing in the background. Others can be seen watching without intervening.

"The wheels of justice are finally turning for these tortured animals," said Mercy for Animals' Canadian vice president Krista Hiddema of Thursday's pleas.

"Only the most sadistic acts of cruelty are being prosecuted, however. It is obvious the dairy industry is incapable of self-regulation. Until the Dairy Code of Practice has the force of law in every province, animal abuse and neglect will run rampant in the Canadian dairy industry."

With files from The Canadian Press