Animals on Shabbos
13. It is permissible to have an animal graze on grass that is still attached to the earth. The fact that it must uproot the grass from the ground is not considered "work" for it; on the contrary, it is a pleasure for the animal (1). In contrast, grass that was cut by a Gentile on Shabbos is considered "muktzah" (2), and an animal may not be led in front of it to eat (3), unless it has nothing else to eat; in that instance, leniency is granted because of "tza'ar ba'alei chayim" (lit: "suffering of living creatures"). Similarly, if an animal has nothing to drink, it is permitted to tell a Gentile to bring water from a well situated in a "carmelis" (an area in which it is rabbinically prohibited to carry on Shabbos - see HY 81:4) (4).
(1) The verse prohibiting allowing one's animal to "work" on Shabbos is: "[...cease work on the seventh day] so that your donkey and your ox will REST" ("La'Nuach") [Exodus 23:12]. One may have thought that what is considered "melacha" (activity prohibited on Shabbos) for a person, is also "melacha" for an animal; that is, since a Jew is forbidden to uproot grass on Shabbos, so too, his animal would be subject to the same prohibition. However, the Midrash clarifies that forcing an animal to abstain from grazing, is not considered "REST" ("Noach") but rather "SUFFERING" ("Tza'ar"), and therefore, could not be derived from the aforementioned verse as a prohibited activity for the animal (Mishna Berura 324:12).
(2) Any piece of vegetation that is separated from its source of growth on Shabbos, whether it was cut by hand or fell off naturally, is considered "muktzeh" and cannot be moved on Shabbos.
(3) There is nothing intrinsically wrong with allowing one's animal to eat grass that is "muktza." The reason it is prohibited here is out of a concern that the owner might pick up the grass to feed the animal by hand. Therefore, it is permissible to lead the animal onto a narrow path such that the only direction it can go is towards the cut grass, because in that case, the owner is not standing close to the "muktza" item, and there is no concern that he will inadvertently feed the animal by hand (Shulchan Aruch 324:13).
(4) Even though it is rabbinically prohibited to instruct a Gentile on Shabbos to perform an activity that would be forbidden to a Jew, in this case, the suffering of an animal overrides that prohibition.