Strait, Brooks & Dunn and Rascal Flatts were vying for the entertainer of the year award along with Tim McGraw and two-time winner Kenny Chesney.
Strait, the top nominee with eight, was also in the running for best male vocalist, top album and best single. Brooks & Dunn, the academy's top trophy winners with 22 between them, had seven nominations this year including top vocal duo -- a category they have dominated since 1991. Stiff competition this year included Sugarland, Montgomery Gentry, Big & Rich and the Wreckers.
"We're going to lose it sooner or later. We did it before," Ronnie Dunn said. The Dixie Chicks won in 1998 and 1999 when the category was briefly expanded to duo or group.
Rascal Flatts was up for best vocal group, best album and best single, as well as entertainer of the year.
Underwood, who parlayed her 2005 win on "American Idol" into the debut album "Some Hearts," which has sold more than 5 million copies, had five nominations and Big & Rich tallied four.
Underwood, whose dark hit "Before He Cheats" dominated the video awards at last month's fan-voted Country Music Television Music Awards, was up for best female vocalist, album of the year, single of the year, song of the year and video of the year at the ACMs, where winners are picked by the 4,500-member academy.
The entertainment-driven program, hosted by Reba McEntire, was slated to hand out just a dozen awards. The show was to air live on CBS Television from the MGM Grand hotel-casino at 8 p.m. EDT, and on tape at 8 p.m. PDT.
Along with the top nominees, performers were to include Martina McBride, Josh Turner, Taylor Swift, Toby Keith, Sugarland, and Brad Paisley.
Brooks & Dunn earlier were named recipients of the Home Depot Humanitarian Award, joining past honorees McEntire, Vince Gill, Lonestar, McBride and Neal McCoy. The group will be honored with two playgrounds to be built in cities of their choice.
"It's a pretty cool thing. I told them we're a duo, so they should give us two. They didn't blink an eye," Dunn said. He is looking for a playground spot in Nashville. Partner Kix Brooks has chosen Shreveport, La.
Other artists whose awards were announced before Tuesday's show include: Producer Dann Huff; bass, Glenn Worf; drums, Eddie Bayers; fiddle, Aubrey Haynie; guitar, Brent Mason; keyboard, John Hobbs; specialty instrument (fiddle, mandolin), Stuart Duncan; steel guitar, Michael Johnson.