Running the ball isn’t a luxury for Oklahoma, it’s a necessity. No matter how well quarterback Sam Bradford delivers the ball to a talented stable of receivers, the Sooners’ offense begins on the ground.

Through the first four games, that was no problem. OU averaged 252 rushing yards a game in lopsided wins over North Texas, Miami, Utah State and Tulsa.

Each carry brought an average of 5.7 yards and the dual threat gave the impression the offense was an unstoppable juggernaut. But something has changed since Big 12 Conference play began. The rushing yards have slowed dramatically.

Against Colorado, Texas and Missouri, the yards per carry have dropped to 4 and the Sooners haven’t rushed for 200 yards in any of the conference games.

The low point was reached last week against Missouri. OU rushed for 118 yards, but over half those yards (76) came in the fourth quarter.

When the final period began, the Sooners had just 42 yards on 22 carries.

“We just kind of got it going more in the fourth quarter,” center Jon Cooper said. “We changed up a little bit in the scheme and things started going a little more downhill. The running backs kept going hard like they have the whole year.”

Chris Brown, who rushed for 50 of his game-high 67 yards in the fourth quarter, said running the ball late in the game is usually easier.

“It takes a little while to get adjusted and it’s usually a lot longer than we want it to be,” Brown said. “Defenses come out in different stuff, stuff we didn’t see in practice. It’s a matter of being patient, sticking with it and waiting on things to open up.”

OU has been patient in its last three games. But that patience is wearing thin.

The coaching staff may very well be out of it if the fourth-ranked Sooners (6-1, 2-1 Big 12) get off to another slow start when they face Iowa State (1-6, 0-3) at Jack Trice Stadium in Ames Saturday.

“We’re more talented and should be more effective in the running game than we have the last couple weeks, credit to good defenses the last three weeks,” offensive coordinator Kevin Wilson said. “But we can and will and need to perform better. That’s our challenge — to be more consistent early in the game. It’s great that we finished good, but we can start a little bit better.”

Some of that blame falls on the offensive line. The highly-touted group hasn’t been as dominant as it was the first four weeks.

But there’s plenty of blame to go around.

“I think the tight ends and fullbacks enter into that as well,” OU coach Bob Stoops said. “You get on the edges, if those guys aren’t doing their job you have no chance. So it’s all of them together. At times we’re really not far off, but it doesn’t take much to be far off and you’re not as productive as you need to be.”