Go and listen to them talk for a while. In your arsenal of calls make sure you have a calf. I have seen bulls come to that call also.
If you want a really simple call to use, get a hoochie mama (I might have spelled it incorrectly). They cost around $25 and work rather well. I use them to locate herds and then sneak up close enough to the herd to shoot. I do not actually call in cows with them, although cows and bulls have both come in. In the rut, bulls loose their minds.
I have hunted CO only in the second season and 3-4 times the rut has still been going really good. Bulls would come running in and cows were mostly indifferent. It seems the call really works well for cows that are alone or in distress.
I also agree that the hoochie mama is a great call. And also good to hear that some of you guys have had luck calling in cows.
Last year I got into a herd of elk in the dark timber, and there was a cow that sounded EXACTLY like a hunter using a hoochie mama. It was so similar that I thought we were waking into another group of hunters. Nope. it was a herd of about 40 elk.
I usually use a mouth reed and carry the hoochie mama in my pocket so I can do either call I want. But for guys that are new to calling or don't use mouth reeds, it's tough to beat the hoochie mama.