There are two estrus cycles, around 4 weeks apart. Cows who aren't bred in the first cycle will initiate a second rut around the 3rd week of October. That is why you will see some calves still spotted out during the first of September, and why October rifle hunters come off the mountain with bulls in the back of their pickup, commenting that the bulls were "bugling up a storm". My grand daughters rifle hunt, and my son who bow hunts with me, will comment that they heard more bugling in one morning than we heard all of the bow season.