ROSEBURG, Ore. (AP) _ Country and western singer Merle Haggard canceled out of the Douglas County Fair two hours before he was to go on stage, claiming he suffered a sudden bout of dysentery.

But the explanation didn't satisfy officials at the fair, where Haggard was to perform Tuesday afternoon and evening.

''If he was sick, why didn't he call us yesterday,'' said Ed Dougherty, of Concert Services, which produces the fair's concerts.

Haggard had abruptly left the Clark County Fairgrounds near Vancouver, Wash., after doing his afternoon show on Monday, leaving an estimated 10,000 fans angry and confused over his failure to appear at the evening performance.

Clark County sheriff's department spokesman Gary Lucas said he was told Haggard, ''left the fairgrounds after some sort of misunderstanding with his wife.''

Dennis Bottomley, Haggard's director of business affairs, said the singer had come down with dysentery and returned to his home outside Redding, Calif., on Tuesday. Bottomley said he expected Haggard would recover in time to make a scheduled appearance Thursday in Hemet, Calif.

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ATLANTIC CITY, N.J. (AP) - After nearly 25 years in show business, Marie Osmond says she feels as though her career is just beginning as she prepares for six casino shows this weekend.

''I'm young, things are going well for me,'' the 26-year-old singer said in a telephone interview this week. ''It's like a whole new phase in my life.''

The new Marie Osmond is a divorced, single mother making plans to move away from her family home in Utah to Nashville, Tenn.

She will perform this weekend with brother Donny at the Claridge Casino Hotel, one of the first stops on a promotion tour for the new album, ''I Only Wanted You.''

It's been almost 10 years since the brother and sister duo had their weekly television show.

She describes her new album as a ''very positive, up approach to country music.''

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CARMEL-BY-THE-SEA, Calif. (AP) - Clint Eastwood has made a minor change in the movie ''Heartbreak Ridge,'' after Army veterans complained that the film showed a Marine as a hero in the Korean War battle.

Eastwood, an Army veteran, said Tuesday that he made the change after discussing the matter with Hal Barker, who is writing a book about Hearthbreak Ridge, one in a series of battles for control of hilltops.

''I said I didn't want to change the title, but I would change one of the points of reference in the film,'' said Eastwood, the mayor of this coastal town.

Eastwood, the producer, director and star of the movie, said the change amounted only to ''a couple of lines. With a minor correction, it satisfies the 23rd Infantry which fought so well in that battle.''

Some veterans of the 23rd Infantry Regiment of the 2nd Army had criticized the movie, saying that the battle was fought by the Army, not the Marines. In the film, Eastwood plays a Marine Corps sergeant who as a young man won a medal at Heartbreak Ridge. Eastwood has maintained that Marines did take part in the battle, under Army command. This position was backed up by Lt. Col. Fred Peck, the film's Marine Corps liaison.

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MIAMI (AP) - President Reagan's son Michael didn't finish the Miami-Nassau- Mi ami Searace, but his wasn't the only boat overcome by rough conditions.

In fact, only one boat completed the 370-mile trip to the Bahamian capital and back on Tuesday. General's Titan, the 63-foot Magnum Diesel driven by Ted Theodoli of Miami, was the winner and only finisher in the field of seven starters.

Theodoli, along with navigator Jim Wynne of Miami, made the trip in 7 hours and 28 minutes for an average speed of 48.52 mph, well below the course record speed of 76.7 mph set by Rocky Aoki in Behihana in the 1970s.