I have found (as mean as it sounds) that pepper spray works for my horses. I have several cans from where I used to work and decided to try it after some of my guys started chewing the lip of my new water tanks. I just sprayed some around the outside edge and they don't chew anymore. The only problem with using that though, would be that it will stain your wood orange.
Oh, and if you use that, make sure that you don't get any on your fingers and stand beside the wind (neither upwind nor downwind) when you are spraying it
This is the brand that I have. Just to give you an idea of how powerful it is, the hotness of peppers is rated on the Scoville heat scale. Your typical jalapeņo rates between 2,500 and 8,000. This stuff is rated at over 2 million LOL. Top Cop Pepper spray: Primary Users: Law Enforcement, Military, Correctional Personnel.Top Cop Pepper spray is distribbuted by security pro USA