Mature bucks
 

Just curious as to what time of day most big (140"+) big whitetail bucks are killed....

The thing about your bigger bucks is the unpredictability of them. I've taken them at all times from dawn to dusk. One of the biggest whitetail I've ever taken I spotted while inhaling a ham sandwich at around 12:30. During close pre-rut (about a week before the Does really start hitting estrus) is when you will see some crazed looking bucks running around with their tongues hanging at any time day or night. That would be the times for ALL DAY sits.