Pioneer Hi-Bred International Inc. and DuPont Co. said they are in talks about a combination, which could expand their alliance beyond DuPont's 20% stake in the world's largest seed corn company. Wilmington, Del.-based DuPont, the biggest U.S. chemical company, paid $1.7 billion in 1997 for its interest in Pioneer. Based on Pioneer's current share price, it could cost DuPont about $6 billion to buy the rest of the Des Moines-based company. "The next announcement will probably be either that the companies agree to merge or the companies break up talks altogether," said Tom Burnett, research director of Merger Insight, which tracks mergers and acquisitions. Burnett said Pioneer would fetch at least $40 a share. There are no assurances any transaction will occur between DuPont and Pioneer, the companies said in a statement. Pioneer's agreement with DuPont includes a standstill provision that prohibits DuPont from increasing its ownership without Pioneer's consent, according to a Securities and Exchange Commission filing. On the NYSE, DuPont shares fell 38 cents to close at $57.75; Pioneer gained $9.81 to close at $34.31.