I divide mine up first by age, (because of what they are fed), weanlings in one field, and yearlings in another until 20 months, then the twos are usually in the barn to start prepping for breaking. at 3 and up, I divide by sex keeping mares and geldings seperate. I do this because the geldings establish a pecking order, but add 1 mare in heat and the gelding try to gain control of "their mare", and thats when they tend to beat the crap out of each other (like Divine said). Plus, there doesn't seem to be as much herd bound behavior when you pull one horse out of the field to ride. When intermingled the geldings seem to worry if someone will "take their woman" while they are gone, so it keeps the guys focused more. Just what I have experienced.