WASHINGTON, March 2 (Reuters) - The U.S. Agriculture Department and seed industry officials refused to disclose on Friday an estimate of how much U.S. seed corn for spring planting was contaminated with an unapproved biotech variety.

Corn futures on the Chicago Board of Trade fell on Friday amid concerns over the U.S. seed corn supply containing residue of the Cry9C protein of StarLink, a variety engineered to repel pests.

Corn futures for May delivery closed down 1-1/4 cents lower at $2.22-3/4 per bushel.

Government and industry officials met on Thursday and confirmed that traces of the protein were found in various kinds of seed corn by companies preparing to deliver planting supplies to farmers.

In response, the American Seed Trade Association swiftly conducted a survey of 200 member companies to pinpoint the amount of StarLink contamination in seed corn.

Angela Dansby, spokeswoman for the trade group, said the survey information was collected and submitted to the U.S. Agriculture Department on Friday. She refused to disclose any information about the results.

The USDA also refused to comment.

USDA spokesman Kevin Herglotz said the department would issue a statement on the results early next week.

Some industry sources have speculated that the amount of seed contamination was possibly around five percent of all U.S. seed corn supplies. But the issue is a key one for Japan, the biggest buyer of U.S. corn, and for American farmers, who blame StarLink for depressing corn prices and exports last year.

Aventis SA the maker of StarLink, said it had no comment on the seed survey. A spokeswoman for the Franco-German firm repeated that it was cooperating with the USDA and industry to keep the unapproved StarLink corn off the market.

American farmers, foodmakers and grain handlers are especially nervous this year after the StarLink contamination last year triggered a recall of more than 300 kinds of taco shells, corn snacks and other foods from grocery shelves.

The Environmental Protection Agency approved StarLink in 1998 for animal feed only, because of lingering concerns about whether it might cause allergies in people.

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