Brothers and partners are exempt from the tithe of cattle. Our Rabbis taught: "It shall be yours (Exodus 13:12)" - and not that which is held in partnership. But this is written concerning the firstborn! If it does not apply to the matter of the firstborn - since the firstborn does apply even in partnership, as it is written, "And the firstborn of your herds and your flocks (Deuteronomy 12:6)" - apply it to the matter of the tithe of cattle. Rabbi Yochanan said: Even if they divided nine against nine, or ten against ten, we do not say, "This is the portion that reached him from the first moment." What is the reason? It is by analogy to "your sons": just as your sons are clearly identified as yours, so too your flock must be clearly identified as yours.
Brothers and Partners Are Exempt From the Animal Tithe
Yalkut Shimoni on Torah 351:7
הָאַחִין וְהַשֻּׁתָּפִין פְּטוּרִין מִמַּעֲשַׂר בְּהֵמָה, תָּנוּ רַבָּנָן, (שמות יג, יב) "יִהְיֶה לְךָ", וְלֹא שֶׁל שֻׁתָּפוּת. וְהַאי בִּבְכוֹר כְּתִיב, אִם אֵינוֹ עִנְיָן לִבְכוֹר, דְּהָא אִיתֵיהּ בְּשֻׁתָּפוּת דִּכְתִיב (דברים יב, ו) "וּבְכֹרֹת בְּקַרְכֶם וְצֹאנְכֶם", תְּנֵהוּ עִנְיַן לְמַעֲשַׂר בְּהֵמָה. רַבִּי יוֹחָנָן אֲמַר, אֲפִלּוּ חָלְקוּ תִּשְׁעָה כְּנֶגֶד תִּשְׁעָה וַעֲשָׂרָה כְּנֶגֶד עֲשָׂרָה, אֵין אוֹמְרִים זֶהוּ חֶלְקוֹ הַמַּגִּיעוֹ מִשָּׁעָה רִאשׁוֹנָה. מַאי טַעְמָא, דּוּמְיָא (דְּבִנְךָ) [דְבָנֶיךָ], מַה (בִּנְךָ) [בָּנֶיךָ] בְּבָרוּר לְךָ אַף צֹאנְךָ בְּבָרוּר לְךָ.