WASHINGTON — The head of the Southern California slaughterhouse that produced 143 million pounds of recalled beef acknowledged yesterday that cows too sick to stand at his plant were apparently forced into the nation’s food supply in violation of federal rules.

The president of Westland/Hallmark Meat Co., Steve Mendell, made the admission after a congressional panel forced him to watch gruesome undercover video of abuses at his slaughterhouse. Mr. Mendell watched red-faced and grim, sometimes resting his head on his hand, as cows were dragged by chains, sprayed in the nostrils with water, shocked, and harshly prodded with forklifts to get them into the box where they would be slaughtered.

Afterward Mr. Mendell briefly bowed his head, then backed away from claims he had made in his prepared testimony, delivered under oath, that no ill cows from his plant had entered the food supply.

So-called downer cattle have been largely barred from the food supply since a mad cow disease scare in 2003 because they pose a higher risk for that disease and other illnesses, partly because they often wallow in feces.

The panel’s chairman, Rep. Bart Stupak, a Democrat of Michigan, asked Mr. Mendell whether it was logical to conclude from the videos that at least two downer cows had entered the nation’s food supply.

“That would be logical, yes, sir,” Mr. Mendell said.