Milwaukee first baseman Prince Fielder's eighth home run this season, a solo shot in the bottom of the eighth, had created a 3-3 tie before Ludwick's game-winner.
Schumaker had two hits, while Brendan Ryan had a run and three hits for the Cardinals, who broke a season-high three-game losing streak.
The winner was reliever Russ Springer (1-0).
Ryan Braun had three of the seven hits for the Brewers, who have lost seven of eight.
The losing pitcher was Eric Gagne (1-2).
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