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Eighteen cattle likely died of selenium poisoning near a southeastern Idaho phosphate mine, the latest livestock deaths in a region rich in phosphates where a legacy of pollution has killed horses and hundreds of sheep since the 1990s.

The cattle died around Aug. 5 near the defunct Lanes Creek Mine. Fertilizer-maker J.R. Simplot Co. controls the mineral rights here, but says it's never actively mined the site.

The dead cattle's livers showed high levels of selenium, a naturally occurring mineral unearthed with phosphate ore that contaminates groundwater and plants near mines if it's not properly contained. Their deaths come as J.R. Simplot and Monsanto Co., which makes Roundup herbicide from phosphate mined nearby, are seeking to dig new mines and public scrutiny is focused on the region.

"This is the first known incident of cattle loss to selenium in the phosphate patch, and true to form probably comes at a very inopportune time for Simplot," said Douglas Tanner, regional environmental manager at the Idaho Department of Environmental Quality, in a letter obtained Friday by The Associated Press in a state public records request.

Tanner also raised concerns Simplot didn't adequately inform the public.

"Upon questioning would Simplot do a press release on the incident it was stated no, they would not," Tanner wrote Tuesday to DEQ Director Toni Hardesty. "My personal opinion is this is very short sighted and hints of covering up the issue."

Hardesty overruled him.

"Absent any discernible threat to human health or any environmental threat, it appears that any sort of press release would be speculative and premature," she wrote back Wednesday. "I believe Simplot has acted responsibly in providing early notice to us."

Mining at Lanes Creek began in 1978 by the Alumet Co., a partnership that included National Steel Corp., which is no longer in business after a 2002 bankruptcy. Ore from the mine was processed by the J.R. Simplot Co. in the 1980s before digging was suspended in the late 1980s. The mine was never officially closed.

A Simplot spokesman in Boise, David Cuoio, said his company leases mineral rights from the Bear Lake Grazing Association, but hasn't extracted ore and isn't responsible for the poisonings. Simplot did some reclamation work -- such as planting vegetation -- in 1998 on the mine's waste dump.

"This area is not near any of our mining operations," Cuoio said. "Simplot has never mined in this area."

Dan Keetch of the Bear Lake Grazing Association, which has since relocated survivors of the 500-steer herd grazing near the mine, didn't immediately return a phone call. It wasn't clear who owned the cattle.

Cattle have long been believed less susceptible than sheep to acute selenium poisoning, in part because they generally avoid plants known to accumulate the toxic mineral. But U.S. Department of Agriculture researchers who analyzed the dead steers' livers say the likely culprit was purple aster, found growing on Lane Creek's waste dump.

Cattle have grazed at Lanes Creek for years, but a combination of factors may have contributed to deaths this summer, including heavy rains that boosted plant growth. The grazing association also put its cattle on the site later than normal.

The USDA's Agricultural Research Service poisonous plant laboratory in Logan, Utah, took blood samples from 36 cattle that survived, and purchased the 10 steers with the highest selenium levels for further monitoring.

The Greater Yellowstone Coalition, a group pushing Monsanto, J.R. Simplot and Canada's Agrium Inc., to clean up leaking mines before digging new ones, said these latest cattle deaths illustrate that mining companies haven't done enough to address past pollution.

Marv Hoyt, the group's director in Idaho Falls, said last week's cattle deaths are the latest sign that mining managers and government officials have underestimated selenium's impacts.

"We know this stuff kills sheep, horses, water fowl, fish," Hoyt said. "Now we know it kills cows. One of our concerns as an organization is, nobody is really paying attention to how this stuff affects deer, elk, badgers, grouse -- a whole array of other species. I don't think anybody knows that answer."

Bruce Olenick, the Pocatello regional administrator for the Department of Environmental Quality, said the Lanes Creek Mine has been on his agency's list of sites meriting further scrutiny, but hasn't been at the top because unlike other old open-pit phosphate mines it isn't contributing to water pollution in the Blackfoot River, one of 15 streams in the region where selenium exceeds legal state levels.

Following the cattle deaths, that's likely to change.

"It's one of the next ones we're going to resurrect," Olenick said.