Wood Chewing & Mineral Deficiency

[MileyMouse]

Both my mare and gelding and their field mate have started to chew our post and rail (it has an electric wire across the top but they still stick their heads under neath to get the lower rails!). Whilst I suspect it is just because they are bored (they do not do it ordinarily) what with the lack of grass and adundence of mud, does anybody know if this is a sign of a vit/mineral deficiency? if so which one?!

Many thanks if you can help.

 

[TGM]

Probably more likely to be due to lack of fibre if they are out with no grass! Are you able to put hay out in the field for them?