Wilmington, Del. -- E.I. du Pont de Nemours & Co. agreed Monday to buy Pioneer Hi-Bred International in a deal valued at $7.7 billion. The Des Moines, Iowa-based seed company is to operate as a wholly owned subsidiary of Wilmington-based DuPont. The deal is expected to be completed this summer. ``By bringing together DuPont's ag businesses and Pioneer's seed business, we are creating an agricultural enterprise with $5 billion in annual sales, which will be focused on growth,'' said Charles Holliday Jr., DuPont chairman-CEO. Pioneer has 5,000 employees worldwide. DuPont has 92,000 employees in 70 countries.