my horses are eating the bark off several of my trees in the pasture. does anyone have any suggestions to prevent this, and what can i do to protect the trees that have been gnawed so that they don't die?

#1 question, do the horses have plenty of grass in the pasture, put out a salt/mineral block, You will have to build a fence around your trees far enough out that the horses cant reach them. Dont know what to put on the trees. Horses usually chew things out of boredom or lack of certain minerals. One of mine cribs but the only thing he goes after is the fence post, which are t-post so he cant do much damage.

I know next to nothing about horses, however, goats will eat trees/wood/bark and sometimes even dirt (very unusual behaviour for a goat) if they have a phosphorous difficiency. Maybe it's the same for horses??

I solved my tree eating horse problems - I got rid of all the horses (but two pets stud and gelding), and started breeding miniature Herefords as grass fed beef. Horses just like to eat trees for fun, even if they are fed well with kelp and other trace minerals. Even if they have lots of pasture, they will eat trees where they loaf and sleep. I found no way to save trees, and we lost some really nice oaks. You will have to fence horses away from trees.
Bob Bard

I have seen people take goat fence (about 4' x 4' squares) and loosely wrap it around the tree trunk and secure it with zip ties. I have another idea you might consider. Have you see the orange plastic construction fencing? I found a source that has the same fencing in green for about $50.00/roll. That might work just as well and would flex with any movement or growth in the tree.