My vet recommended getting what Purina sells that comes in a tub. You sit it out in a sheltered area during the Winter months. She says they will lick that and get the nutrients they need and it will help with the tree chewing. Sometimes I think it's just the individual horse myself because last year I had my 2 yr old and my 12 yr old gelding out together and they chewed on the trees,,,,,,,,this year my 2 yr old isn't in the field with my gelding but another older (21) gelding is and they have barely chewed on the trees at all. I don't buy mineral blocks, I only buy white salt because I was told that for the small amount of minerals in the blocks it wasn't worth the extra cost. My horses I have noticed have been licking on their salt.
There are several things you could look at...size of the pasture (can they run and move around freely), how much hay do they have access to (do they have a round bale or fed flakes of hay at certain times) could it be boredom? My 2 horses nibble on trees when I feed them flake hay (which they eat up in a couple of hours) but if they have a round bale they don't. I gave them the amount my vet recommended they get (and more, so I knew they weren't hungry) but they would nibble...so I give them round bales and when they get bored or whatever, they nibble on the hay instead of the trees. I had thought about getting one of those big balls for them to play with, but my pasture is sloped and it would just roll down the hill and out the fence, which is electric or against the fence which would ground out the fence and possibly shock them. Mine would also nibble on the roots, which I couldn't put wire on without risking them getting a shoe hung up in it. So the round bales solved my problem. I might try the stuff in the tub, though.
I have the same problem with my horses as well. I may also try the Purina product that dusty414 mentioned. Can this be purchased somewhere like the Coop or TSC? Or is this a product that must be purchased from a vet? I might even try one of the big balls. 2 of my 3 horses would probably enjoy it!!!!
I am not 100% sure on the name of what Purina sells that the vet (Dr. Gillam in Shelbyville) told me about but she said it came in a tub and you can set it outside in your pasture. Any Purina dealer should be able to tell you what it is and I even asked at the Lewisburg Farm Center and they told me they sold it. If I had had a problem again this year I was going to buy some but so far this year my horses aren't chewing the trees.