From AolNews:

Authorities investigating the deaths of 200 cows in Wisconsin have come up with an unlikely culprit: the sweet potato.

The cows were found dead in a Stockton pasture two weeks ago. Locals were left scratching their heads about what caused the mass die-off. Investigators from the University of Wisconsin have determined that the animals were killed by a poison found in spoiled sweet potatoes that were part of the cattle's feed.

"It is likely that a mycotoxin from moldy sweet potato was a major factor in the disease and deaths of these steers," said Peter Vanderloo, associate director of the Wisconsin Veterinary Diagnostic Laboratory.

Mystery of 200 Dead Cows in Wisconsin Solved