Wary of head trauma, San Francisco 49ers linebacker Chris Borland is leaving football. And not even the promise of NFL stardom and piles of money could change his mind.

So now, after one stellar rookie season, he is retiring at age 24.

The 49ers announced his decision Monday night, without offering specifics. But Borland told ESPN's "Outside the Lines" earlier in the day he wants to do "what's best for my health."

Borland had a team-leading 108 tackles as a rookie, emerging as a punishing defender. He also had a sack and two interceptions.

"From what I've researched and what I've experienced, I don't think it's worth the risk," Borland said in the interview. "I feel largely the same, as sharp as I've ever been. For me, it's wanting to be proactive. I'm concerned that if you wait till you have symptoms, it's too late."

San Francisco General Manager Trent Baalke said the team was surprised by his move and called Borland a "consummate professional."

The NFL, like the 49ers, said it respects Borland's decision, adding that "playing any sport is a personal decision." The league stressed that "football has never been safer," noting progress with rule changes, tackling techniques, equipment, protocols and medical care.

"Concussions in NFL games were down 25 percent last year, continuing a three-year downward trend," Jeff Miller, NFL senior vice president of health and safety, said in a statement Tuesday.

Chargers add receiver Steve Johnson

The Chargers agreed to terms of a three-year contract with free agent receiver Stevie Johnson. He became the first player in Buffalo Bills history to have three straight 1,000-yard seasons, from 2010-12. but his production fell off the last two seasons. In 2013, his last with Buffalo, he had 52 catches in 12 games, for 597 yards and three scores. Last year with San Francisco, he caught 35 passes for 435 yards and three touchdowns.

... Tight end Charles Clay signed a five-year, $38 million offer sheet with more than $20 million guaranteed with the Bills. Since he is the Dolphins' transition player, the team has five days to match the contract or they'll lose him for nothing.

iditarod

Dallas Seavey leads his dad

Defending champion Dallas Seavey led his father, Mitch Seavey, who was in second place, Tuesday afternoon in the Iditarod Dog Sled race. Aaron Burmeister, 39, was in third place.

Dallas Seavey was the first musher into the checkpoint at White Mountain, where mushers and their dogs must take a mandatory eight-hour break. Seavey can begin the last 77 miles of the race to Nome at 6:10 p.m. Tuesday. If his three-hour lead holds up, that could put him into Nome early Wednesday morning.

But at the checkpoint, Seavey told Anchorage television station KTUU, "Anyone remember last year?"

Jeff King, a four-time champion, was in a similar position last year with a big lead. He was 25 miles from the finish line when a sudden snowstorm with unrelenting winds blew him out of the race.

"This is ominously too familiar," Seavey said. "A team out in front with a lot of speed was guaranteed to win it, and they didn't."

around the horn

College football: Three football players from Morgan State were stabbed when two groups of people got into a fight outside a dining hall on the Baltimore campus. The players were taken to hospitals and their injuries were not considered to be life-threatening, police said.

NBA: Jack Haley, who played nine seasons in the NBA including a reserve role on the Chicago Bulls' 1996 championship team, has died. He was 51.

Tennis: Struggling with her serve and a rash of errors, Serena Williams needed three sets to get by Sloane Stephens 6-7 (3), 6-2, 6-2 in the fourth round at the BNP Paribas Open.

ASSOCIATED PRESS