Bechoros Daf 58 בכורות דַף 58

The Mishnah had listed three opinions on the three "גרנות" for מעשר בהמה, all of which were periods of time shortly before the three רגלים. These were chosen כדי שתהא בהמה מצויה לעולי רגלים – so that animals would be readily available for purchase for those who come up to Yerushalayim for the רגלים. Although animal merchants could sell or shecht their animals before the גורן without separating maaser, ניחא ליה דליתעביד מצוה בממוניה – [a person] prefers that a mitzvah should be done with his property (such as מעשר בהמה), and only afterwards sell or eat it. Thus, establishing the גורן shortly before the רגלים ensured a sufficient supply of animals for purchase. Rebbe Akiva said the three גרנות are the "פרוס" of each רגל. "פרוס" means “half,” and refers to the halachah that שואלין ודורשין בהלכות פסח קודם לפסח ל' יום – we ask about and expound the laws of Pesach for thirty days prior to Pesach, half of which is fifteen days. The Gemara proceeds to explain the various differences of opinion of the three גרנות in our Mishnah.

The next Mishnah describes the process for separating מעשר בהמה: he gathers them into a pen and makes a small opening for them so that only one can pass through at a time, and he counts them as they pass through (under a staff), saying, “One, two, etc.,” and the one which exits tenth he marks with red dye and says, "הרי זה מעשר" – This one is hereby maaser.” If he did not mark it, or did not count them with a staff, או שמנאן רבוצים או עומדים – or he counted them while they were crouching or standing (without passing through an exit), the maaser is effective. However, if he had a hundred animals and he simply took ten as maaser, or he had ten animals and took one as maaser, the Tanna Kamma says it is not maaser, but Rebbe Yose bar Rebbe Yehudah says it is. A Baraisa adds that when he makes the small opening in the pen, the mothers are placed outside the pen and cry out to their young, which then exit toward their mothers. He cannot remove them himself, because the passuk says "יעבור" – [any that] will pass, implying ולא שיעבירוהו – they shall not make it pass.

The Mishnah continued: קפץ אחד מן המנויין לתוכן – if one of [the animals] which had already been counted jumped back into the midst of [the uncounted animals in the pen], and we cannot identify it, all the animals are exempt from מעשר. This is because the previously counted animal may emerge as tenth, and it is unfit to be maaser, since it was already exempted. Since any animal which emerges as tenth would only be questionable maaser, they are all exempt, because of the principle (quoted above) that עשירי ודאי אמר רחמנא ולא ספק – Hashem said that only that a definite tenth one [shall be holy], but not a questionable tenth one. Wherever the maaser would be a matter of doubt, there is no maaser obligation. The Mishnah concludes: מן המעושרין לתוכן – if one of [the animals] which had been designated as maaser jumped back into the midst of [the animals in the pen], and it cannot be identified, כולם ירעו עד שיסתאבו – they all must graze until they develop blemishes, ויאכלו במומן לבעלים – and they may be eaten by their owners in their blemished state. Since one of the animals is a korban, none may be eaten before developing a מום.