That grand slam was the first of his career and the team's initial one this season. It also capped off a six-run eighth inning that gave the Giants their second consecutive victory in this four-game series.
"I was trying to be more patient at the plate," Bowker said. "That's the key to hitting, I think, swinging at good pitches and laying off the bad ones."
Giants manager Bruce Bochy said the rookie appears to be getting everything together at the plate since coming to Washington. Bowker now has five homers and 21 RBIs with a .248 average.
He's 3-for-8 in this series and has repeatedly hit the ball hard.
"He really has picked up his game," Bochy said. "He's quieted down at the plate. He's worked hard, he's out here early hitting today and it looks like he's seeing the ball much better now."
Bowker had shown power before, becoming the first Giant ever to homer in his first two Major League games, something he did on April 12 and 13 against the Cardinals.
The Giants needed the six-run outburst to help Jonathan Sanchez (5-3) get his third straight victory.
Sanchez blanked the Nationals in seven innings, giving up four hits. He struck out four and walked two. The left-hander never really got into trouble, but the Giants turned three double plays behind him to end innings.
Sanchez has given the Giants five consecutive quality starts. He's gone at least six innings and allowed two earned runs or less in those outings.
Bochy said Sanchez's improved focus on the mound has made a big difference in his work.
"He's so focused out there now, and he's maintaining it, no matter what's happening," Sanchez said. "Men on base, he's not losing that; he's not getting distracted. That was his biggest problem early, and as a starter, you've got to have that for the time you're out there."
Sanchez needed 98 pitches for his seven-inning effort. He worked hard at getting ahead in the count to control things against Washington hitters who are often impatient at the plate.
"When you get ahead, you make the hitters swing early, and you get ground balls," Sanchez said. "Today, I got three double plays that got me out of the jam."
The Giants couldn't do much to give Sanchez any offensive support for much of the game. They got four hits in the first two innings, but left two runners on, and Nationals starter Shawn Hill (0-3) then settled down.
Hill retired 15 in a row after Omar Vizquel's second-inning single. Rich Aurilia's one-out double in the seventh ended that streak, but the Giants finally broke through one inning later.
Jose Castillo started the rally with a single, and Randy Winn followed with a double that put runners on second and third with none out. Ayala then came on, and Bengie Molina lined an RBI single to center for a 1-0 lead.
Ray Durham walked before Aurilia outdueled Ayala and lined an RBI single to center. Aurilia fouled off three pitches before the hit, and Bochy said that at-bat was crucial to the inning.
The bases were then loaded for Bowker, who promptly broke the game open with his long grand slam, giving the Giants a 6-0 lead.
Tyler Walker and Pat Misch then came on to each throw an inning of scoreless relief and finish the shutout. The Giants have held the injury-plagued Nationals to just one run in the first 18 innings of this series.