Jerusalem Talmud Shabbat 20:4

משנה: גּוֹרְפִין מִלִּפְנֵי הַפְּטָם וּמְסַלְּקִין לַצְּדָדִין מִפְּנֵי הָרֵיעִי דִּבְרֵי רִבִּי יוֹסֵה. וַחֲכָמִים אוֹסְרִין. נוֹטְלִין מִלִּפְנֵי בְהֵמָה זוֹ וְנוֹתְנִין לִפְנֵי בְהֵמָה זוֹ בַּשַּׁבָּת׃ MISHNAH: One cleans up after an animal to be fattened²⁰Food outside the barn., and one removes it²¹The remaining food, but one may not clean the barn. to the sides for one grazing, the words of Rebbi Dosa, but the Sages forbid. One may take from before one animal and put it before another one on the Sabbath²²Inside one trough or inside the barn..

הלכה: גּוֹרְפִין מִלִּפְנֵי הַפְּטָם וכו׳. מִפְּנֵי שֶׁהַפְּטָם מוֹתִיר הָרֹעֵי אוֹכֶל. בְּעָיָא דָא מִילְּתָא. מָהוּ לִיטּוֹל מִלִּפְנֵי בְהֵמָה זוֹ וְלִתֵּן לִפְנֵי בְהֵמָה זוֹ. וְלָאו מַתְנִיתָה הִיא. נוֹטְלִין מִלִּפְנֵי בְהֵמָה זוֹ וְנוֹתְנִין לִפְנֵי בְהֵמָה זוֹ בַּשַּׁבָּת: מַתְנִיתָא מִין בְּמִינוֹ. צְרִיכָה לֵיהּ מִין בְּשֶׁאֵינוֹ מִינוֹ. תַּנֵּי. נוֹטֵל אָדָם מִלִּפְנֵי הַחֲמוֹר וְנוֹתֵן לִפְנֵי פָרָה. מִלִּפְנֵי בְהֵמָה שֶׁפִּיהָ רַע וְנוֹתֵן לִפְנֵי בְהֵמָה שֶׁפִּיהָ יָפֶה. HALAKHAH: “One cleans up after an animal to be fattened,” etc.⁵⁷Addition of the corrector in the ms.; missing in E. Because what the animal to be fattened leaves over, the grazing one eats⁵⁸Explanation of Ritba (ed. M. Goldstein, Jerusalem 1990, col. 915). Since it remains animal feed it cannot become muqṣeh.. The following is in question: May one take from before one animal and give it before another animal? Is that not the Mishnah, “one may take from before one animal and put it before another on the Sabbath”? The Mishnah is about the same kind; what he asks is about another kind. It was stated, a person may take from before a donkey and put before a cow; from before an animal whose mouth is repelling to an animal whose mouth is nice⁵⁹In this form the second statement contradicts the Babli 140b. Since also it contradicts the first statement, probably one should switch the terms “repelling” and “nice” (it is not impossible that this was E’s reading.) As the Babli explains, a donkey eats in a orderly manner, therefore food taken from before the donkey and put before a cow will be eaten. But cattle eat in a very disorderly way from a trough; the cow’s spittle in the grain will make it repellent to a donkey. Since remainders of cow feed are not donkey feed, one may not take food from before a cow and put it before a donkey..