Horses eating my fences!

[Wyrd]

My horses have never been ones to chew fences, but suddenly 3 of them are eating my fencing to pieces!
They are in a separate field to the others, the fence at the front is all post and rail, and there are trees and bushes around the rest of the field.
I got there today, my welsh cob was standing at the place I feed them, and the other 2 were by the gate, and by my welsh cob a rail was almost chewed all the way through, with the bits all over the floor.
How can I stop them? And why are they suddenly eating my fences!

[CityLights]

have they enough food and varriety of food to eat and also getting all the vitamins and minerals they need?

Often they say if they chew wood they arent getting all the nutrition they need,

Ollie chews wood like mad he has everything he needs but i think often its cause he doesnt have a varriety of things to eat if i give him some nice tasty wood to chew that arent my fences he likes that and tends to stop chewing my rails

[Wyrd]

They are getting the same food as the others are getting (who are not chewing wood). They have plenty of branches to chew. They usually have a salt lick but they have just finished it so I need to get them another.
Next time I go to get feed I will get a bag of pony nuts to put in their feed balls to keep them occupied.

[My Crazy Clan]

Spray tea tree on your fence, see if that stops them.