Animals on Shabbos
12. When a bundle of hay is not tied with a "permanent knot" ("Kesher Shel Kayama") (i.e., two knots, one on top of the other), one may untie it on Shabbos to feed one's animals (1). One may cut up hard squash to use as animal fodder, (2) provided it was picked on the previous day (3). If the squash is soft and edible whole, it is forbidden to cut it up for the animal [on Shabbos] (4).
(1) As we saw earlier, untying a permanent knot on Shabbos ("Matir"), is one of the 39 "Melachos" (prohibited categories of activity).
(2) One should be careful not to cut it into very small pieces, because one would then risk performing the Melacha called "grinding" ("Tochen"); the act of grinding is basically defined as taking a whole object, and breaking it down into tiny pieces (See Mishna Berura 324:14).
(3) Any fruit or vegetable which was not picked before Shabbos, is considered "Muktza" (forbidden to be moved), even if it was picked on Shabbos by a Gentile (it is forbidden for a Jew to pick anything from a tree on Shabbos).
(4) Cutting it up would be considered unnecessary bothersome activity ("Tircha"), which is prohibited on Shabbos.