With the Reds leading 7-3, Andrew McCutchen had drawn a leadoff walk off reliever Manny Parra and Pedro Alvarez followed with a grounder to first baseman Joey Votto.
Votto flipped to shortstop Zack Cozart for the force on McCutchen. Cozart's throw back to first, with Parra covering, was ruled in time by Walcott.
While Hurdle was engaging Walcott, he got the thumbs up from his bench coach, Jeff Banister, and formally challenged the play.
Neil Walker homered to left field later in the inning, scoring Alvarez for a two-run jack.
Tom Singer is a reporter for MLB.com and writes an MLBlog Change for a Nickel. He can also be found on Twitter @Tom_Singer. This story was not subject to the approval of Major League Baseball or its clubs.