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.
There's been a spate of mass animal deaths in recent weeks, from fish in Maryland and Arkansas to birds in Louisiana and South Dakota.
The farmer who owned the cows had thought they might have fallen victim to disease such as infectious bovine rhinotracheitis, according to The Wisconsin Rapids Tribune. Vanderloo and his team ruled that out.
"None of the major respiratory pathogens of cattle were identified in the samples provided to the lab," said Vanderloo.
He also explained that the toxic sweet potatoes were not in the human food supply chain, so there was no threat to people.
The U.S. Department of Agriculture acknowledged last week that it was behind the deaths of thousands of starlings in South Dakota. The USDA said it put out poisoned bait after a farmer complained the birds were defecating in his animal feed.
The thousands of red-winged blackbirds that dropped out of the sky in Arkansas on Dec. 31 apparently died of blunt force trauma. Investigators say loud noises prompted the birds to take a rare night flight, and that they likely slammed into objects such as trees and houses.