The U.S. Department of Agriculture revealed the new information from its investigation Thursday night and also said the cow was 10 years and seven months old. It came from a dairy farm in Tulare County, the nation's No. 1 dairy-producing county.
Routine testing at a transfer facility showed the dead cow destined for a rendering plant had mad cow disease, or bovine spongiform encephalopathy.
It's the fourth case of BSE ever discovered in the U.S. and the first since 2006.
U.S. health officials say there is no risk to the food supply. The California cow was never destined for the meat market, and it developed BSE from a random mutation, not tainted feed.