I currently have 14 horses.
I keep the mares and foals by themselves until the foals are older.
In pasture two I have a gelding, mare and two yearling geldings. They all get grain so they are kept seperate from those who don't.
In pasture three I have old cranky Mustang gelding, two year old gelding and three year old stud (for now, could change at a moments notice

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As soon as the grass grows up a bit more the two yearling geldings will be going out into the big pasture with the other geldings.
Normally the three mares (currently with foals at side) would be out in the big pasture also but since there is a stud out there I DON'T want them bred to the will not be until they are checked back in foals.