LEAD: U.S. Agriculture Sec'y Johanns to resign, expected to run for Senate

(EDS: ADDING DETAILS, BUSH QUOTES)

U.S. President George W. Bush said Thursday that Agriculture Secretary Mike Johanns will resign in an apparent move to run for the U.S. Senate.

Bush announced Johanns' departure at a White House ceremony. ''I couldn't have asked for a better secretary of agriculture,'' he said.

Johanns, who has been agriculture secretary since January 2005, is expected to seek the Nebraska Senate seat being vacated at the end of 2008 by retiring Republican Sen. Chuck Hagel.

Bush voiced support for Johanns' anticipated Senate bid, saying, ''He would make an outstanding member of the United States Senate.''

Johanns is known to have been deeply involved in negotiations with Japan over Tokyo's restrictions on American beef imports due to concerns over mad cow disease.

Negotiations over lifting the restrictions have recently been complicated by paralysis in Japan's scandal-tainted farm ministry.

Bush praised Johanns' efforts on the issue, saying, ''He worked endlessly to open up foreign markets for American beef.''

Deputy Agriculture Secretary Charles Conner will serve as acting agriculture secretary, Bush said.