1. One is not allowed to use even the side of a tree on Shabbos.
2. One is allowed to use something that is nailed into a tree to support something else, since this is not a direct use of the true.
3. One is permitted to untie bundles of straw and hay in order to feed his animal on Shabbos.
4. One may not force feed a camel on Shabbos.
5. One is permitted to feed a dog on Shabbos, even if he is not the owner of the dog.
A BIT MORE
1. Therefore, one may not lean a ladder on a tree in order to climb the ladder on Shabbos.
2. Accordingly, one is allowed to support a ladder by leaning it on a peg that is nailed into the tree.
3. One is also permitted to spread out the fodder that is in the bundle, since the smell of the food will encourage the animal to eat.
4. However, one is allowed to place food in its mouth in such a way that the camel can spit it out (Rav Yehudah), or to place the food in the mouth in such a way that the camel cannot spit it out, provided that he places the food there by hand (Rav Chisda).
5. This is because dogs usually have no access to the type of food they like, and thus the obligation to feed them falls on every man.