49ers 8/21 practice: Bosa vs. Williams, Armstead returns, defense looks strong

The San Francisco 49ers moved their Friday practice up an hour due to the poor air quality resulting from the major fires burning around the Bay Area.

"Yeah, we have some tool that tells us the number of what the air quality is," head coach Kyle Shanahan told reporters on Thursday. "So, if it gets to 200, there's no choice. 200, we're not allowed to go out there and I think it was at times [on Wednesday] ... If the number gets to 200, and we're basically at the mercy of how the winds go, then we're not allowed to be out there."

The 49ers likely felt that number could spike above 200 on Friday, and moved practice up to 9:10 a.m. PT.

Below are some of Friday's practice notes from various media sources. It was the team's sixth practice of training camp. San Francisco's next practice is scheduled for Saturday at 10:15 a.m. PT.

There has been much discussion surrounding the training camp battle between defensive end Nick Bosa and offensive tackle Trent Williams. General manager John Lynch on Friday likened the battle to a pay-per-view match.

Bosa has often struggled to beat Williams, so he moved to the other side to go against right tackle Mike McGlinchey. "He beat Mike McGlinchey on the first rep. McGlinchey evened the score on the next snap. When Bosa switched back over, he had more success vs. Williams," wrote Matt Maiocco. (NBC Sports Bay Area)

Defensive lineman Arik Armstead returned to practice after missing time with some back tightness. (NBC Sports Bay Area)

New center Hroniss Grasu had a couple of strong blocks along the interior of the offensive line but still has a lot of learning on which to catch up. (The Athletic)

New 49ers safety Johnathan Cyprien, in his second practice with the 49ers, intercepted a Nick Mullens pass intended for wide receiver Jauan Jennings, who never turned to look for the football. (NBC Sports Bay Area)

Tight end George Kittle, who is pretty darn good, made some big catches from quarterback Jimmy Garoppolo, including a 25-yard pass and reception. (NBC Sports Bay Area, NBC Sports Bay Area)

Cornerback Richard Sherman intercepted a Garoppolo overthrow intended for wide receiver Kendrick Bourne. (NBC Sports Bay Area)

Solid outings from defensive players Kerry Hyder, Kentavius Street, Kwon Alexander, Jaquaski Tartt and Tarvarius Moore. (NBC Sports Bay Area)

Running back Tevin Coleman left practice early, due to the air quality. Defensive lineman Jaryd Jones-Smith also left early, before team drills. (KNBR, NBC Sports Bay Area)

Garoppolo had a nice pass to wide receiver Trent Taylor during team drills. (Bay Area News Group)

Wide receiver Deebo Samuel was seen sprinting and getting some other work in on a side field. "Perhaps his most extensive workouts we've seen during practice so far," wrote Chris Biderman. (Sacramento Bee)

"Jaquiski Tartt had a really nice PBU against Trent Taylor — he was really able to deftly maneuver his frame, which is much bigger than Taylor's and can actually be a coverage disadvantage in closed quarters," wrote David Lombardi. (The Athletic)

Rookie wide receiver Brandon Aiyuk "was stellar." He had a lot of snaps with both the first- and second-team offense. (KNBR, NBC Sports Bay Area)

Good practices for defensive linemen Solomon Thomas and Kentavius Street. "Looked like Thomas had a would-be sack of Jimmy Garoppolo early in 11s and played well in 1-1s. Street has been explosive off the ball and a handful for #49ers interior linemen," wrote Biderman. (Sacramento Bee)

"LB Kwon Alexander was very active against the run, meeting runners at or behind line of scrimmage a few times," wrote Nick Wagoner. (ESPN)