Castro made only one bogey, and that was from the fairway. After a weak drive, he put his second shot on the 11th into the water, and saved bogey with a chip to tap-in range. He bounced back with consecutive birdies, and twice made solid par saves before his run of birdies on his back nine.
But it was a quiet day for the most part.
"Two U.S. Opens in three weeks," said George McNeill, who had a 71 while playing with Jonas Blixt and Ben Curtis. "And before that, we got to play the U.S. Open at Muirfield (Village), too. It was fairly quiet out there. You have a few cheers here and there. But we had the 'hot dog' group. That's where the fans are looking at the pairing sheet and go, 'Curtis, Blixt, McNeill. Let's go get a hot dog.'"
It didn't help that Tiger Woods wasn't around, unable to play because of a sore left elbow that will keep him out of competition until the British Open next month.
Woods won last year at 8-under 276, one of the higher winning scores on tour in 2012.
"You don't usually see first-round scores on a PGA Tour event only be 3-under leading after the morning wave," Horschel said. "It shows you how tough this golf course is, shows you how long the rough is."
Horschel, though, said he likes it that way because it doesn't feel like a putting contest.
As for not having Woods around? Horschel doesn't look at the AT&T National any differently without him.
"Tiger is Tiger. He's just another guy," Horschel said. "He's just another player out there. For me, thinking about someone, how great he is, is just a distraction for me. But it is a disappointment that he's not playing out there because obviously it is his event. The crowds love him to death, and he does spice up the event a little bit."
It's the second time in the last seven weeks that Castro got off to a great start. He had a course record-tying 63 on the TPC Sawgrass for a three-shot lead at The Players Championship, and he wasn't sure which was tougher.
"They were totally different rounds," he said. "The one at Sawgrass, I hit it 3 feet eight or nine times. And the one today was more of a normal, lower round where I made some putts. It's hard to compare these two golf courses. That one was playing firm and fast. This one is just long and soft."
DIVOTS: Nick Watney holed out from 148 yards with a wedge on the fourth hole for an eagle on his way to a 70. ... K.J. Choi celebrated his birthday Thursday, at least based on the Korean calendar. The lunar calendar birthday for the 43-year-old was May 19. Either way, here's how Choi counts it: "Every year, I get shorter," he said. ... Every player made at least one bogey.