Mishnayos Bechoros Perek 9 Mishnah 6

According to the opinion of Rabbi Elazar and Rabbi Shimon, with regard to all animals that are born from the first of Tishrei until the twenty-ninth of Elul, those animals join to be tithed together. If five were born before Rosh HaShana and five after Rosh HaShana, those animals do not join to be tithed together. If five were born before a time designated for gathering and five after that time designated for gathering, those animals join to be tithed together. If so, why were three times stated for gathering the animals for animal tithe? The reason is that until the time designated for gathering arrives it is permitted to sell and slaughter the animals. Once the time designated for gathering arrives one may not slaughter those animals before tithing them; but if he slaughtered an animal without tithing it he is exempt.

כָּל הַנּוֹלָדִים מֵאֶחָד בְּתִשְׁרֵי עַד עֶשְׂרִים וְתִשְׁעָה בֶּאֱלוּל, הֲרֵי אֵלּוּ מִצְטָרְפִין. חֲמִשָּׁה לִפְנֵי רֹאשׁ הַשָּׁנָה וַחֲמִשָּׁה לְאַחַר רֹאשׁ הַשָּׁנָה, אֵינָן מִצְטָרְפִין. חֲמִשָּׁה לִפְנֵי הַגֹּרֶן וַחֲמִשָּׁה לְאַחַר הַגֹּרֶן, הֲרֵי אֵלּוּ מִצְטָרְפִין. אִם כֵּן לָמָּה נֶאֱמַר שָׁלשׁ גְּרָנוֹת לְמַעְשַׂר בְּהֵמָה, שֶׁעַד שֶׁלֹּא הִגִּיעַ הַגֹּרֶן, מֻתָּר לִמְכּוֹר וְלִשְׁחוֹט. הִגִּיעַ הַגֹּרֶן, לֹא יִשְׁחוֹט. וְאִם שָׁחַט, פָּטוּר: