ENGLEWOOD, Colo. -- The 49ers' started slowly but finished strong on both sides of the ball in their opening practice against the Broncos. One of the possible explanations -- the altitude.
The 49ers' players said they felt the mile-high thinness in the air as soon as they started going through individual drills. Both the offense and the defense struggled early. Broncos' backup quarterback Brock Osweiller was particularly effective. Meanwhile on the other practice field, the 49ers quarterbacks were struggling to hit their targets downfield or mostly settling for dump-off passes.
That changed late in the session when, on the right-hand field, Colin Kaepernick hit tight end Garrett Celek on a long pass down the center of the field. It was the sort of seam pattern 49ers fans typically see Vernon Davis running. Celek quietly has been having a solid training camp and he has the team's only offensive touchdown in two preseason games.
"I thought it was huge because the play before he had had jumped offsides," Kaepernick said. "He came back and made a huge play for our offense. So I think mentally, just the focus he had to come back and make a big play like that was big for us."
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On the other side of the complex the 49ers' starting defense had a string of big plays, starting with safety Antoine Bethea's sideline interception of Peyton Manning. Bethea, who spent years with Manning in Indianapolis, said that perhaps Manning thought the 49ers were in a different look. When Bethea came down with the ball, there was a big cheer from the 49ers' sideline.
The next few plays went like this:
* Cornerback Marcus Cromartie knocks away a pass from Osweiller.
* Linebacker Nick Moody knocks down a pass from Osweiller
* Safety Eric Reid intercepts a pass over the middle from Osweiller
* Linebacker Michael Wilhoite knocks away a pass from Osweiller
* Cromartie knocks down a pass from Osweiller
The 49ers continue to juggle their starting cornerbacks. The top two on Wednesday were Tramaine Brock and Dontae Johnson. On nickel snaps, Brock moved inside and Kenneth Acker played on the outside. Wilhoite and NaVorro Bowman were the starting inside linebackers.
The offensive line was the same as Tuesday with Marcus Martin playing center and rookie Ian Silberman lined up at right guard. Silberman was knocked to the ground several times early in the session.
From left to right, the second-team offensive line was: Patrick Miller, Brandon Thomas, Joe Looney, Andrew Tiller and Trent Brown.
Tight end Vance McDonald had a walking boot over his sprained left ankle. His injury is not thought to be severe. Safety Craig Dahl (back) and linebacker Philip Wheeler (muscle strain) also did not practice.
Bruce Miller did not practice and was not seen on the sideline. He is not injured. In June, he pleaded no contest to misdemeanor disturbing the peace, which stemmed from an argument with his former fiancée. Miller’s case is being reviewed by the NFL and his absence Wednesday likely was related to that review. Trey Millard handled all the snaps at fullback on Wednesday.
With few fullbacks and not as many tight ends as usual, the 49ers used a lot of three-receiver sets. The top three are: Anquan Boldin, Torrey Smith and Quinton Patton. Bruce Ellington was back at practice while Kendall Hunter took more snaps than any other running back.
Second-round pick Jaquiski Tartt has played well in each of the preseason games. He was on the field in some three-safety sets on Wednesday.