Dow Chemical Co., the world's fifth-largest chemical company, said it's in talks to buy the 32% of Mycogen Corp. it doesn't already own. Mycogen is an agriculture biotechnology company with annual sales of about $211 million. Dow owns 68% of the San Diego-based company through its crop protection unit, Indianapolis-based Dow Agrosciences, which has annual sales of about $2 billion. Dow is bound by a 1996 agreement with Mycogen not to raise its stake beyond 68% until February, so the companies must agree to drop that restriction. Shares of Midland, Mich.-based Dow Chemical fell 81 cents to close at $92.75 on the NYSE; Mycogen's rose 31 cents to close at $23.81 on Nasdaq.