New Aggressive Behavior in Horse

Question
My gelding of 15 years has had a recent (2 yr) change in behavior.  To start, here's some background info - born & raised in a herd environment living with same herd for 11 years.  Then when I moved he moved with me and was stabled in a barn, no longer a part of the herd.  He has moved around to a couple of barns over the last 4 years, none of which elicit a herd environment.  Finally I have my own home with my own pasture, where he can now roam.  My husband got a horse, Red, and they share the pasture/barn.  My horse, Silk, has become incredibly aggressive towards Red, and either tries to bite/kick him or completely ignores him.  There's also a mare next door, and Silk walks up and down the fenceline keeping the mare in sight.  If he loses sight of her, he goes nuts with his whinnying.  

The problem is he never exhibited this aggressive behavior before.  Red is a very docile gelding, and does not fight back.  At first I thought it was just the horses determining their picking order, but Silk continues to harrass Red for no apparent reason.  And why is my gelding calling out to mares?  He was castrated at a young age.  He's been around mares his whole life, and not once has he grown attached to one.

The only thing that has changed is the environment in which he's kept (pasture turn out, run-in shed at night) and he now eats New Mexico vegetation, which I couldn't tell you what's included in that.  His grain portions have been cut back, and he gets fed grass hay.

Any insight is much appreciated, especially since now the horses have to get turned out separately.  We're dressing bite wounds on Red and don't want anymore of them.

Answer
Hello Erin,

Silk is demonstrating very normal behaviour for a gelding.

In the past herd situation, Silk may not have ranked high enough to do this or there may have been a good alpha horse keeping him and everyone else in line.  Now, many years later he's in a situation where he outranks his only pasturemate and is smitten with the neighbouring mare.  It may not even be the fact she's a mare.  To him it's his 'buddy'.

Essentially you have a three horses herd (even though they aren't all together, they are neighbours) and unfortunately that is the most difficult number of horses to deal with.  I've never seen it work out to every horse's advantage.  Somebody is always the outcast, getting chased and being ostracized.  Works a lot like it does with a group of three people.  :-)

Unfortunately, this situation can't be fixed easily.  There is no rule in the equine world that says horses have to get along.  In the wild Red would either have to start fighting back or he'd be chased out of the herd and he'd have to go join another one.  

You could add a fourth horse and hope Red and the new horse become buddies, but other than that, your handling the situation in the best way.....other than not having the mare around as a neighbour might also restore the peace.

Best wishes!

Sincerely,

Lana Reinhardt