Halakhah for Zevachim 112:1

The commandment of the tithe of pure beasts every year: To tithe all the pure beasts - which are cattle, sheep and goats (Bekhorot 53a) - that are born in our flocks each and every year, and to take that tithe and eat it in Jerusalem after the fat and blood have been offered on the altar (Zevchim 56b), as it is written (Leviticus 27:32), "And all the tithe of your cattle and sheep, all that passes beneath the rod, the tenth shall be holy to the Lord." And they, may their memory be blessed, said in Bekhorot 58b, "How do we tithe? He brings them to a pen and makes for them a small opening, so that two cannot go out at the same time. He places their mothers outside, and they would moan, so that the lambs would hear their voices and exit the pen to meet them on their own and not from the effort of another. And he counts them with a rod, 'One, two, three, four' and so on, until ten. And the one that comes out tenth he marks with red chalk and he says, 'Behold, this is tithe.'"