Responsa על שבת 148:2

You have asked: Is it permitted to crumble pieces of bread to feed chickens on Shabbat? Should one be concerned about tehina (alt. “tohen”; grinding/milling/pulverizing, one of the types of labor prohibited on Shabbat), or not? It stands to reason that it is permitted, since we maintain that one may render something into food. We further learn in a mishna (Shabbat 156b): “Gourds may be chopped up for livestock….” This applies not only to livestock, for which large pieces suffice, but also to fowl, since any manner of turning it into food is permitted. The mishna simply spoke of the normal case. Yet, it have a difficulty with this based on what is stated in the chapter “Klal Gadol” (Shabbat 74b): “One who dices beets (paris silka) very fine is liable on account of tohen.” If we attempt to answer that in this case it was not for the needs of that day, but to prepare to cook it that evening—as Maimonides states: “One who dices a vegetable very finely in order to cook it, has violated a tolada of tohen.” This demands explanation: if this is truly tohen, how can we permit a type of labor that the Torah forbids for the sake of fowl and livestock? Similarly, regarding Maimonides’s ruling: “Therefore, one may not chop up carobs or unripe grain to place before livestock, whether large or small, since it resembles tohen. But one may chop up gourds for livestock and a carcass for dogs, because tohen does not apply to fruits.” I do not understand his opinion. Indeed, in Shabbat (155a), it is implied that the reason that one may not chop up carobs is because “we may not expend effort on food,” for carobs here are like unripe grain according to Rav, whose opinion we accept. Moreover, what does [Maimonides] mean by “tohen does not apply to fruits”? The quintessential tohen is of fruits like wheat and barley. And carobs are themselves fruits, so how could he say that this “resembles tohen”? Arukh explained “paris silka” as “like pressing, and it is not similar to the way it grows.” And regarding R. Alfasi (=Rif)’s explanation of one who dices beets—that the rationale is due to fine flour—the rabbi and kohen wrote that this requires explanation, for wood is no worse than beets.