The cows come into heat, if they dont get successfully bred they will come back into heat in 25-35days(something like that) those are what cause the secondary and late ruts. The bulls are hard to get going w/ just a call after the primary rut is over. The estrus scent will usually do it if they catch wind of it. Even then they may not bugle, just come in silent and try to find the cow. Cows on the other hand can be EASILY brought in any time of year(bulls are harder except in the rut).