WASHINGTON (Reuters) - German drug and crop chemical maker Bayer AG's bid to buy seed and chemical company Monsanto Co is on track to win U.S. antitrust approval by the end of May, unless there is a last-minute complication, a source familiar with the matter said on Thursday.

The transaction, if it closes as seems likely, will create a company commanding more than a quarter of the combined world market for seeds and pesticides in the fast-consolidating farm supplies industry.

(Reporting by Diane Bartz; Editing by Susan Heavey)