THE AFL will trial four field umpires in the NAB Challenge, with the extra umpire possible in future premiership seasons. 

AFL umpires coach Hayden Kennedy believes increasing field umpire numbers from the three used in the premiership season will lead to more accurate decisions. 

The four-umpire system will be tested in the pre-season competition starting on February 26. 

"One of the difficulties with congestion nowadays is we can't get all the angles covered," Kennedy said on Tuesday in Adelaide. 

"So the four-umpire system would allow us another umpire close to the play with which we can get a different angle, so therefore hopefully improve our decision accuracy.

"We will get some vision of it, see how good it is, get some feedback from the umps and see where it leads us. 

"It won't come in this (premiership) year, but it's something to look at in coming years." 

The trial comes amid two interpretation changes from last season - umpires will police the prior opportunity law tighter and be stricter on blocking in marking contests. 

Kennedy said umpires would focus on three cues deemed as prior opportunity: if a player had the ball when steadily balanced; if a player chose to take on an opponent when in possession of the ball, and if a player had a chance to dispose of the ball. 

Umpires were also being ordered to greater scrutinise blocking in marking contests, with two penalised cues - a player moving off his line to block an opponent, and using his arms to shepherd or block. 

"Prior opportunity, it's only a minor interpretation change," Kennedy said. 

"The blocking in marking is probably a little bit harder because we believe there are some risks involved - are the players acting and other types of things like that ... but I think we'll be pretty good with it." 

Last season, umpires averaged 9.1 errors a game, with Kennedy setting a target of 8.5 mistakes a game this season. 

He said umpires wanted to keep the centre bounce, despite some pundits calling for umpires to throw up the ball.