HARVARD, Ill. — Although experts determined that botulism is behind the deaths of about half a farmer's dairy herd, they have not determined how it ended up on his farm.

Lee Wenzel's cows started dying in September. By the end of October, 65 animals had died or had to be put down.

Experts from Illinois, New York, Pennsylvania and Wisconsin descended on the farm to find the cause. Tests on feed, tissue samples and animal autopsies ruled out nitrate poisoning, mad cow disease, foot-and-mouth disease, even bioterrorism.

Experts concluded that the killer was botulism, a rare AND severe illness caused by a bacterium that lives in soil and untreated water. Conditions under which it can grow — low oxygen and a lot of nutrients — are found in feed silos.

Although tests on the soil and the feed did not positively identify the toxin produced by the botulism bacteria, 75 tons of Wenzel's feed had to be buried on his farm.

Jeoff Stevens, Wenzel's veterinarian, said that it's possible that botulism spores might have been brought into the silo from Wenzel's fields.

The state Department of Agriculture quarantined Wenzel's farm and Wenzel stopped delivering milk to his buyer. He could only watch as the animals grew weaker and weaker.

"If crying would have helped, I would have bawled my eyes out," said Wenzel, who has lived on the farm for all of his 60 years and took it over from his father in 1971.

Colleen O'Keefe, manager of the animal protection division of the Illinois Department of Agriculture and the acting state veterinarian, downplayed the chances that such an incident might happen again.

"It was an extraordinarily horrible situation for Mr. Wenzel," O'Keefe said. "But in reality, it's a freak situation. Like getting hit by lightning."

Wenzel is trying to rebound. Insurance will cover most of the $115,000 loss in cattle. Neighbors and friends are donating feed and cattle to help make up the rest, and 325 people recently showed up for a spaghetti dinner to raise money for him.