The Boise State football team’s top two tailbacks might as well be dinosaurs in the running backs room this season.

D.J. Harper, a sixth-year senior, and Drew Wright, a fifth-year senior, began practicing with the Broncos before most of their five fellow tailbacks started high school.

That gap has left a major question about the Broncos’ depth at tailback this season. The Broncos have landed three highly regarded recruits in the past two classes — and at least two of those guys will need to produce this year.

Through fall camp, it looked like they would.

“It’s good to see a lot of young guys being productive,” Harper said. “We’re really excited to have those guys.”

The tailback depth chart starts with Harper, who contributed a 100-yard rushing game at Fresno State as a true freshman in 2007. He missed most of the 2009 and 2010 seasons with knee injuries and is expected to become one of this team’s stars this year.

Wright, a former walk-on from Vallivue High, emerges from the shadows as the backup. He has rushed for 311 yards in his career.

But that’s not nearly enough depth to survive the rigors of a college football season. Last year against TCU, Doug Martin and Harper were injured and the Broncos put Wright in the game as they tried to preserve a late lead.

In 2009, Martin returned from defense to offense after Harper was injured.

In 2007, Harper and then-redshirt freshman Jeremy Avery were pressed into key roles by an injury to Ian Johnson.

Running backs coach Keith Bhonapha says he needs a minimum of two but ideally three tailbacks for each game. It might take four or five guys to get that done over the course of a season.

“Each year I start with seven guys and toward the end of the year I end up with three,” Bhonapha said. “That’s a hard math problem to answer.”

Redshirt freshman Jay Ajayi is the third certain contributor. He was considered a potential contributor last year as a true freshman from Plano, Texas, but started slow in fall camp. His first season became even more difficult when he tore an anterior cruciate ligament in practice and was not allowed to travel to the bowl game for disciplinary reasons.

Ajayi returned to contact practice this fall and seemed to get stronger as camp progressed. He led the team with 78 rushing yards in the final scrimmage.

“Jay had a good camp,” Bhonapha said. “He’s a strong, big guy — a strong runner. One of the main things was getting him back after his knee and making sure he has confidence in his knee when it’s time for him to run. He showed that.”

The next man in could be true freshman Jack Fields (El Paso, Texas) or Devan Demas (Houston). The other two tailbacks are true freshman walk-ons.

Coach Chris Petersen said he expects to play Fields or Demas but would prefer not to use both.

The Broncos were in a similar situation in 2007. They decided to use Harper and redshirt Martin — then nearly pulled Martin’s redshirt midway through the season.

“If everybody stays healthy, probably in an ideal world, yes,” Petersen said of redshirting Fields or Demas. “But that decision may not even come until game four.”

Bhonapha said the freshmen have accepted the challenge in front of them and found ways to get better daily. He sees a bright side to his unique roster — one that should pay off in 2013 or 2014.

“In some ways, it’s very exciting because you know you’re going to have those guys for the next four or five years,” he said. “Now it’s that foundation training where you’re trying to get these guys up to speed as fast as you can.”

He gets help in that regard from Harper and Wright, who have learned to lead and teach.

The students have impressed their mentors.

“They’re going to have to grow up, and they’ll do fine,” Wright said. “They’re catching on quick.”

Chadd Cripe: 377-6398, Twitter: @IDS_BroncoBeat

EDITOR’S NOTE: This is the eighth in a series of stories previewing the Boise State football team by position. Coming Sunday: quarterbacks.

Projected depth chart, tailbacks

D.J. Harper (5-9, 205, sr.): The sixth-year senior becomes one of the stars and leaders of the Broncos’ offense. He rushed for a career-high 557 yards and nine touchdowns last season.

Drew Wright (5-9, 203, sr.): The Vallivue High graduate’s reliability and experience make him an important piece of the rotation.

Jay Ajayi (6-0, 222, fr.): He rushed for 2,240 as a high school senior. Coming off a torn ACL, he should get stronger as the season progresses.

Jack Fields (5-9, 194, fr.) OR Devan Demas (5-8, 170, fr.): Fields rushed for 2,478 yards last year and Demas gained 2,114 all-purpose yards, both at Texas high schools.