(Reuters) - Revenge may be the motive for the killings in South Carolina of more than 300,000 commercial chickens worth about $1.7 million over the past two weeks, authorities said on Monday.

Birds have been found dead of unnatural causes in 16 chicken houses at six farms that grow chickens for Pilgrims Pride Corp, the largest poultry producer in the United States, which laid off some 60 people right before the killings began, Clarendon County Sheriff Randy Garrett said.

The company has a processing plant in Sumter, South Carolina.

About 325,000 chickens have been found dead at the farms since mid-February, Garrett said. One farmer, W.L. Coker, lost the birds in eight chicken houses, or about 160,000 birds, he said.

Authorities are searching for killers with a deep working knowledge of raising chickens, Garrett said, adding that he believed the deaths of the chickens are related to the layoffs.

Vandals bypassed alarms systems and raised or lowered temperature in the chicken houses, killing them, Garrett said.

"Depending on the age of the birds, they knew whether to jack the heat up or jack the heat off," Garrett said.

Young birds need more heat, and older ones need less, he said.

"They had all that knowledge of the farms and how many weeks growth the chickens were," Garrett said.

Garrett said his office has contacted U.S. Department of Agriculture authorities and state police.

Pilgrim's Pride said in a statement that it was fully cooperating with authorities.

"These unfortunate, yet apparently deliberate acts show a blatant disregard for the welfare of the chickens and the livelihood of the family farmers involved," the statement said.

The acts were also condemned by state officials overseeing South Carolina's agriculture industry.

"Farming presents its own unique challenges without something like this happening," South Carolina Agriculture Commissioner Hugh Weathers said in a statement.