BELLINGHAM, Wash. (AP) — Organic farmers in Whatcom County say that a herbicide contamination in the manure and compost they obtain from non-organic farms is causing severe crop damage.

The Bellingham Herald reports that farmers are linking the herbicide aminopyralid. They say cows ingest the herbicide through grass or silage tainted with it. The herbicide passes through the cows digestive system unchanged.

Farmer Kirk Hayes says the herbicide caused a loss of about $40,000 in the last two months from crop being damaged.

"It's killed off most of our potato crop, our salad crops," said Hayes, who sells his crops to local co-ops. "We've contaminated about seven and a half acres, it looks like."

Aminopyralid was approved for use in the U.S. in 2005. It's produced by Dow AgroSciences, a subsidiary of Dow Chemical Co. Farmers use it for weed control.

Clayton Burrows, director of the nonprofit Growing Washington, says soil tests this year convinced him that the herbicide is linked to crop damage. The nonprofit operates Alm Hill Gardens and other organic farms in the area and he estimates losses in the hundreds of thousands.

The herbicide is not believed to pose a threat to humans or animals.

The contamination also poses a problem for dairies, which need places to get rid of cow manure. Many ship the manure to organic farms.

"There's a lot of dairy waste that needs to get off of dairy farms," said Colleen Burrows, integrated pest management coordinator at the Washington State University Whatcom County Extension office. "We need to have a place where that excess fertilizer ... can go."

George Boggs, executive director of the Whatcom Conservation District, said the contamination was almost certainly a mistake, rather than an intentional violation of the rules.

Dow spokesman Garry Hamlin said the company has not confirmed aminopyrali caused damage in Whatcom County, but he said the herbicide has caused similar problems elsewhere in the country.

"At present, we're providing technical expertise and working closely with state regulatory authorities, and we are also in the process of meeting with concerned growers to get the facts and - if our product is, in fact, present in the compost - to find out the specifics of how and where label directions have not been followed and determine how to put a stop to it," Hamlin said in an e-mail.

Copyright 2010 The Associated Press.