CARLTON coach Mick Malthouse has slammed the AFL's plan to trial four field umpires during the NAB Challenge, saying it will cause confusion and empower too many people on the field.

The use of a fourth on-field umpire has been ruled out for the 2015 home-and-away season, but the proposed system could be introduced in future years if the pre-season trial is a success. 

Malthouse, who expressed his frustration with inconsistent decisions and interpretations, said it was an unnecessary adjustment to the game. 

"We know the NAB Challenge is about experimentation, but I sincerely hope we don't have four senior umpires out there in regular games in the future," the coach told Adelaide radio station 5AA.

"I just think we've got enough coverage … we're over-empowering too many people on the ground.

"All it's going to do is add more confusion. Four umpires, I think we're just going over the top." 

AFL umpires coach Hayden Kennedy said introducing a fourth field umpire could lead to more accurate decisions by covering more angles of the play. 

Malthouse, however, said three well-positioned umpires were capable of covering all the angles required to make the right call.