THE AFL won't introduce a fourth field umpire as a permanent fixture during season 2016, despite several successful trials last year. 

Further experimentation is set to be undertaken this season, with a view to introducing a fourth field umpire to officiate every game in 2017.

After canvassing all 18 clubs, the AFL has confirmed it will trial an additional umpire for every game during the split rounds (13, 14 and 15) of this year's home and away season, but not in the NAB Challenge.

AFL umpires boss Wayne Campbell told AFL.com.au those 18 games during the split rounds will give the League vital information on whether the system could be permanently introduced next year.

"We're confident the system works," Campbell said.

"We ran it past the clubs and they're comfortable with us doing it.  They can certainly see the benefits of doing it.

"It'll be good to have a look at it under real match conditions with a full complement of umpires and see what happens.

"They're obviously really important games so we wouldn't trial something where we thought it would be detrimental to anyone."

In order for the four-umpire system to become a reality next year, the umpiring department will need to expand its roster.

There are currently 32 AFL field umpires on the list but a minimum of 36 would be required for a full season.

"We think that ideally the list size should be 38 but it might take a year or two to get to that," Campbell said.

"For [four-umpires to go ahead], we need the error rate to be lower than what it would be with three umpires. We need more money and we need enough umpires."

Last year the League trialed an additional field umpire during selected games in the final four rounds of the 2015 home and away season, following a pre-season trial during the NAB Challenge.

The trial was deemed a success, with the error count reduced.

The AFL also found that while the umpires ran about the same distance per game, their high intensity running was reduced by about 15 per cent, leaving the men in green feeling less fatigued after games.

Campbell said the addition of an extra umpire could ultimately lengthen their careers.

"Our aims are to reduce the error rate, but also reduce the amount of running required, thereby allowing for older umpires to stay in the game longer," he said.