I never realized that there may be an issue mixing mares & geldings. I have always mixed the two genders in large paddock areas (I live in So Cal so large pasture areas have never been an option). I have seen personality conflicts between two individuals - sometimes it is the mare who is agressive and sometimes the gelding. Never noticed a distinct pattern. I just moved horses around again yesterday and paid no attention to gender and just avoided the individual personality issues that I know occur. It seems I have to be more careful in the mare pasture (2 acres) then anywhere else to make sure everyone can live together. Though, I will say that putting my stallion in with the mares solved all the "mare to mare" issues I was having.
