Undercover videos produced by animal rights groups are fueling a debate over the need for new laws to regulate the treatment of American dairy cows. Farmers say it keeps dairy cattle clean, opponents say it's cruel and pointless.

The graphic videos include one made inside a huge New York dairy operation where cows never go outside, have the ends of their tails cut off in painful procedures without anesthesia, and are seen being abused by one employee who hits a cow over the head with a wrench when it refuses to move.

An investigator for the group Mercy for Animals worked at the New York dairy farm, Willet Dairy, one of the largest in the state, for two months as a mechanic.

"These animals are really treated as little more than milk-producing machines," said Nathan Runkle, executive director of Mercy for Animals. "The overall environment at this facility was really a culture of cruelty and neglect."

Portions of the video will be played in reports to be aired this evening on ABC World News with Diane Sawyer and later on Nightline. Check your local listings for times.