PARADISE VALLEY, Ariz. -- Baseball's replay age has finally dawned, thanks to Thursday's unanimous approval by owners of what commissioner Bud Selig called a "historic" expansion of replay to correct missed calls.

The new system, which will go into effect this season, will give managers most of the power to trigger reviews, by providing them with one challenge per game, along with a second potential challenge if their first is upheld.

Only after a manager has used up all of his challenges, and only from the seventh inning on, would umpires be authorized to initiate a review on their own.

For the first time, calls at first base, at the plate and on the bases will be reviewable. There will be limited exceptions, including the fabled "neighborhood play" at second base. But MLB executive Tony La Russa, one of the architects of the new system, estimated that almost 90 percent of all potential calls are now reviewable.

Disputed home runs will be reviewed under existing rules and do not need to be formally challenged.

Baseball officials paved the way for Thursday's vote by negotiating late deals with the Major League Baseball Players Association and with the Major League Umpires Association. Sources said an agreement with the players' union wasn't finalized until Wednesday night.

"The Players look forward to the expanded use of replay this season, and they will monitor closely its effects on the game before negotiating over its use in future seasons," MLBPA executive director Tony Clark said in a statement.

Meanwhile, MLB alleviated a key concern of the umpires by agreeing to hire two additional umpiring crews (a total of eight new umpires), and staffing the replay center in New York through a rotation of current umpire crews instead of with former umpires and umpiring supervisors.

"For some, the discussions regarding expanded replay appeared to move too slowly, too deliberately. But there were technical and operational challenges that needed to be addressed, and that took time," World Umpires Association representative Brian Lam said in a statement.