Head's fighting 101 came after Queensland skipper Usman Khawaja selflessly enforced the follow-on in Adelaide on day three of play.

Khawaja's decision may backfire on a personal level as he's locked in a duel with Head for Australia's vacant No.5 slot for the Ashes opener against England.

But the follow-on has all but guaranteed the Bulls another Shield win, though Khawaja may not get another hit: chasing just 88 to win, Queensland are 0-5 at stumps.

SA, despite Head's heroics, were bowled out for 284 in their second innings with Bulls paceman Gurinder Sandhu claiming 6-47.

Sandhu, after collecting his best first-class figures and first five-wicket haul since his debut Shield game for NSW eight years ago, praised Khawaja's follow-on decision.

"He's a captain that puts his team first, that is amazing and probably a reason he is the captain," Sandhu said.

"He's not a selfish person at all.

"It (the follow-on) was what the team wanted and he must have felt the same."

Both Khawaja (four) and Head (eight) failed in the first innings at Karen Rolton Oval.

Head made amends in his second dig, reaching his second ton of the Shield season from 146 balls laced with 11 fours.

But the SA skipper, who also cracked 230 in a one-day match last month, fell just three balls later.

Head's dismissal ended the Redbacks' resistance, with Bulls paceman Gurinder Sandhu claiming 6-47, his best first-class figures.

Ashes outsider Alex Carey was among Sandhu's victims, out for three when caught at bat-pad but replays showed the ball struck only his thigh pad.

SA's first dig ended earlier Thursday when all out for just 102, with Bulls spinner Matt Kuhnemann also collecting career-best figures of his 5-25 which featured the scalps of Head and Carey (two) on Wednesday.

Batting first, Queensland posted 299 with Test star Marnus Labuschagne top-scoring with 110.

Australian Associated Press