SAN FRANCISCO Madison Bumgarner and Brandon Belt hit their first home runs of the season and the Giants opened a three-game series against the Houston Astros with a 6-3 win.Starting pitching report: Madison Bumgarner struck out the side in the first inning and cruised through 7.2 dominant innings, as he allowed two runs, just one earned, en route to his sixth win of the season.The Astros unearned run came in the third inning, when Jose Altuve reached on a two-out throwing error by Brandon Crawford on a routine ground ball. Crawfords throw sailed wide of Brandon Belt, allowing Altuve to advance to second, where he didnt stay for long. After stealing third base, Altuve trotted home on Brian Bixlers bloop single.Bumgarner was in cruise control from that point on, as he put together 1-2-3 innings in the fourth and fifth. The Giants young southpaw didnt allow an extra-base hit until the eighth inning, issued no walks and struck out 12, his seventh career double-digit strikeout game. All but one of Bumgarners strikeouts were swinging.His final line: 7.2 IP, 6 H, 2 R, 1 ER, 0 BB, 12 K (98 pitches, 70 for strikes).Bumgarner is now 13-2 with a 1.99 ERA in his last fifteen home starts after going 0-5 with a 5.82 ERA in his first seven starts at 3rd & King to start the 2011 campaign.Bullpen report: Sergio Romo entered the game with two outs in the eighth, a 4-2 lead and a runner on second. Five pitches later, all Frisbee sliders, he returned to the dugout with his standard flash and gave way to Clay Hensley in the ninth.Hensley walked pinch-hitter Jordan Schafer to lead off the final frame, but rebounded with two quick outs, including a fielders choice that could have been a game-ending double play if fielded cleanly by Brandon Belt. With two outs and a runner on second, Bruce Bochy turned to Javier Lopez to face lefty Jason Castro, who doubled down the right field line on the second pitch he saw from Lopez to cut the Giants lead to 6-3.Bochy turned to closer Santiago Casilla for the final out. Pinch-hitter Brett Wallace flew out to Melky Cabrera in left to end the game. Casilla earned his 17th save in 18 chances filling in for Brian Wilson.At the plate: Novato native Bud Norris overmatched the Giants in the first inning with a fastball that touched 95 miles per hour at times. Buster Posey got San Francisco into the hit column with an opposite field single in the second inning, but the Giants didnt get into the run column until the third, thanks to an unlikely hero.Entering Tuesdays game, the Giants had gone 16 straight contests at AT&T Park without hitting a home run. Madison Bumgarner stepped to the plate to leadoff the third and sent a 2-1 Bud Norris fastball deep to left field for the Giants first home dinger, ending a homerless streak of 517 at-bats and 138 innings.Bumgarners bomb tied the game 1-1 and the Giants came back in the fourth to take the lead, thanks to a wild Norris. Posey worked a walk to lead off the inning, and after a Pablo Sandoval pop out, Norris issued back-to-back walks to Nate Schierholtz and Brandon Belt to load the bases. With Bumgarners big bat lurking in the on-deck circle, Brandon Crawford redeemed himself for an earlier error with a seeing eye single past Norris and a diving Jose Altuve at second base. Crawford came into the game 9-for-52 with runners in scoring position, but his clutch hit drove in two for a 3-1 lead.Norris had to leave the game midway though the third inning with a left knee sprain sustained when chasing down a foul pop up by Bumgarner, and the Giants didnt fare well against his replacement. David Carpenter worked 2.2 scoreless innings, despite giving up two hits and issuing a walk.The Giants added an insurance run in the seventh, when leadoff man Gregor Blanco worked a walk, stole second, and advanced to third when catcher Jason Castros throw trickled into centerfield. Blanco scored the Giants fourth run on Melky Cabreras second single of the night in his first game after missing three with a balky hamstring.Even with Bumgarner ending the Giants home homerless streak, Brandon Belt entered Tuesdays game stuck in his own season-long dinger drought. It came to a merciful end in the eighth inning, as the first baseman took Wesley Wright deep with a towering shot to right that hit the arcade and bounced into McCovey Cove.Nate Schierholtz and Crawford joined Cabrera with two hits each. The Giants totaled nine hits, drew five walks and went 2-for-8 with runners in scoring position.In the field: Brandon Crawford committed his Major League-leading 11th error by a shortstop when he threw wide of Brandon Belt in the third inning, leading to an Astros unearned run. It was the Giants 57th error of the season, tied with the Baltimore Orioles for the most in the Majors.In the third inning, Madison Bumgarner threw to first as Brian Bixler took off for second, but Brandon Belts throw to Crawford at second was high and wide, allowing Bixler to slide under the tag.Pablo Sandoval has taken some heat for his defensive efforts since returning from the disabled list, but he made an impressive catch of a screaming line drive off the bat of Bixler. Diving to his right, Sandoval caught the ball just off the ground, robbing Bixler of a likely double.Sandoval came close to another amazing play when Matt Downs laid down a bunt in the seventh, but a charging Sandoval bobbled his attempt to catch the ball with his bare hand and Downs was credited with a single.Belt had the opportunity to turn a game-ending double play, but bobbled Downs grounder and settled for the out at first. Two batters later, Brett Wallace fouled a ball in play, but Joaquin Arias backed off and Brandon Crawford slipped, extending the game.On the bases: Nate Schierholtz took off for second base after leading off the bottom of the sixth inning with a single, but was hung out to dry when Astros first baseman and former Giant Matt Downs snagged Brandon Belts liner for an easy double play.Gregor Blanco led off the seventh inning with a walk, and was on third base in no time when he stole his seventh base of the season and advanced to third on a bad throw from Jason Castro that made it into centerfield.Attendance: The Giants announced a crowd of 41,200 on Irish Heritage Night for their 117th consecutive sellout.Up next: Matt Cain will take the mound Wednesday in search of his eighth win of the year. At 7-2 with a 2.41 ERA, Cain is off to a stellar start to the season, but is 1-3 with a 4.69 ERA in his career against the Astros. Houston will start J.A. Happ (4-6, 4.54 ERA) opposite Cain. Happ is coming off a rough outing against the St. Louis Cardinals in Houston and is 0-2 with a 3.75 ERA in 12 career innings pitched against the Giants.