It was a tight game until the first play of the fourth quarter, when Mettenberger found Beckham for a 33-yard touchdown pass to give the Tigers a 38-26 lead.
Mississippi State's Dak Prescott threw for 106 yards and rushed for 103 more. Jameon Lewis caught seven passes for 111 yards and a touchdown.
The Bulldogs' offense was good, especially early, but LSU's was even better.
The Tigers' receiver duo of Jarvis Landry and Beckham was unstoppable, even when Mississippi State appeared to have good coverage, and Mettenberger found them early and often.
Landry had eight catches for 96 yards as the Tigers finished with 563 total yards.
LSU needed all those big throws, because the defense was gashed for a second straight week. The Tigers gave up 44 points and nearly 500 yards last week in a loss to Georgia and Mississippi State had some early success on Saturday with a two-quarterback system.
Prescott started after two weeks of mystery. Coach Dan Mullen said senior Tyler Russell who returned after missing three games due to a concussion would be the starter after the team's win against Troy on Sept. 21, but waffled on that statement as the LSU game drew closer.
The 6-foot-2, 230-pound Prescott has been terrific in Russell's place, providing much more of a running threat. On the team's first offensive drive, he rewarded Mullen's confidence with a 28-yard touchdown run to pull the Bulldogs with 7-6.
Russell entered the game early in the second quarter and also had immediate success, throwing a 20-yard touchdown to Lewis to pull Mississippi State to 21-16 in the second quarter.
The Bulldogs took a 23-21 lead on Russell's 59-yard touchdown pass to De'Runnya Wilson, but LSU responded with a 14-yard touchdown pass from Mettenberger to Beckham to take a 28-23 lead into halftime.
Russell's second touchdown pass was the 40th of his career, which set a Mississippi State record. He finished 7 of 11 passing for 146 yards.
The teams combined for 608 total yards in the first half 334 for LSU and 274 for Mississippi State.
LSU's defense tightened in the second half, giving up just three points.
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