Dee Dee Trotter won her first US outdoor title in a personal-best 49.64 seconds. South Carolina's Natasha Hastings was second in a college-record 49.84 and Mary Wineberg third at 50.24. Trotter came from third on the final turn to draw away.
Richards, who ran under 50 seconds nine times last year, finished at 50.68.
"I just lost my thinking," Richards said. "I thought I was already on the team and didn't fight for that third spot, so I'm really disappointed. But I still have the 200, and I'm going to come out and win that."
Richards advanced through the qualifying round of the 200 later in the day.
"It was really tough," Richards said. "My parents came down and they encouraged me and said that maybe the 400 wasn't for me to win this year. Maybe it's the 200. I tried to get my head back into it. I felt OK in the 200. I didn't feel great. I'm sure it was more mental than physical. Hopefully, I'll run really well tomorrow."
Mikele Barber ran the fastest women's 200 in 22.73.
Tyson Gay, running less than 24 hours after his 9.84 clocking in his 100-meter victory, was the leading 200 qualifier at 20.66 seconds into a brisk headwind. Wallace Spearmon, Xavier Carter, and Jeremy Wariner were among the others to advance to today's semifinals.
"It's a new day," Gay said. "I really didn't warm up long. I'm a little fatigued. I just wanted to get it done."
In the men's 400, Angelo Taylor ran side-by-side with LaShawn Merritt, edging ahead in the finish to win in 44.05 to Merritt's 44.06. Joanna Hayes, Olympic gold medalist in the 100-meter hurdles, experienced leg cramps and finished last in her semifinal heat. Ginny Powell went on to win the event. Amy Acuff won her sixth US outdoor championship in the high jump.
Reese Hoffa had the five best throws to win his first US outdoor shot put title with a best of 70 feet 5 1/4 inches in a competition that started in a light rain. Dan Taylor was second at 68-10 3/4 and former Dartmouth standout Adam Nelson third at 67-4 3/4.
"I was checking the weather every hour, waiting to see what would happen," Hoffa said. "I saw a front coming in. After that, I accepted it: 'It's going to be wet out here. There's nothing I can do about it.' "
James Carter, runner-up at the worlds two years ago, won the 400 in 47.72 seconds, with Kerron Clement second at 47.80. They were the event's two fastest times this year.
Treniere Clement won her third consecutive US women's 1,500 title at 4:07.04.
The top three finishers in each event make up the US team for the world championships Aug. 25-Sept. 2 in Osaka, Japan. Defending world champions also make the team, as long as they compete in some event at the US meet.
