U.S. Agriculture Department officials ordered seed dealers yesterday to determine how much of the U.S. corn seed supply contains Starlink, the genetically engineered corn variety that led to huge recalls of food and corn crops last year. Starlink, which is produced by Aventis for animal feed, has not been approved for human consumption. The agriculture department requested that the American Seed Trade Association quantify the amount of tainted seed by mid-day today. Officials said that they expected the amount to be small — less than 5 percent — and that precautions were already in place to prevent farmers from actually planting the seed.