The tampering has ruined about 44,000 gallons of milk worth about $49,000 to farmers, state police Lt. John Hibsch said.
Authorities have no suspects in the 14 cases under investigation since the fall, Hibsch said. The most recent cases were reported late last month.
Authorities said none of the tainted milk made it to store shelves or into milk products like cheese because milk is tested for contaminants before being unloaded from the trucks that take it to processing plants.
Only someone allergic to antibiotics like penicillin would be at risk if exposed to the tainted milk, Hibsch said.
Investigators said they are looking for suspects in any number of places, from animal rights groups opposed to dairy farm practices to disgruntled farmers or employees.
Dairy farming is a $116 million industry in Wyoming County, New York's largest dairy producer, where 11 of the cases were reported.
With no obvious motive for the apparent sabotage and no claims of responsibility, anxiety among farmers is high.
"Who it is we don't know. If we knew why, we'd at least know what direction to look," said farmer Mark McCormick, whose 200-head Mar-Dan Dairy Farm in Wyoming County was among the latest targets. "That's one of the frustrating parts. We don't have a clue."
McCormick lost more than 4,000 gallons of milk and estimated his financial loss at $6,200 to $6,500.
"It's going to be very hard to overcome," said McCormick, who said that with milk prices set by the government, raising his rates to make up the loss is not an option. As it is, he said, he is paid only slightly more for his milk than his father was 20 years ago.
New York Farm Bureau spokesman Chris LaRoe said there have been isolated cases of angry employees tampering with milk tanks in the past, but "we've never seen it this widespread before."
The introduction of bovine growth hormone in the early 1990s touched off protests by health activists and others around the country, but the Farm Bureau said New York has not seen such demonstrations for several years.
The artificial hormone is injected in cows to increase the amount of milk they produce. Critics of such biotechnology products contend that too little is known about their health and environmental effects.
The Farm Bureau is urging farmers to limit access to storage tanks, but that is difficult. Dairy farms, in general, are easily accessible because of the need to keep barns housing hundreds of animals open for ventilation. The milking areas containing the tanks may have open walls to enable cows to be moved in and out.
Investigators initially had to determine whether a mix-up by a farmer or an employee had caused the contamination. Most dairy farms store antibiotics on the premises to treat ill cows or those that are about to give birth.
It would take only a minute amount of antibiotics to contaminate a load of milk, authorities said. A vial dumped into one storage tank would contaminate not only that tank, but an entire truckload. That is because in dairy farming, collection trucks make numerous stops on a single run and the milk from each farm is mixed together along the way.
By Carolyn Thompson