Positive Commandment 78 (Digest)
The Tithe of the Herd
"And all the tithe of the cattle and livestock, the tenth shall be holy to G‑d"—Leviticus 27:32.
We are commanded to separate a tenth of the kosher cattle and livestock born to us each year. Their blood and fats are sprinkled and offered on the altar, and the rest of the flesh is consumed in Jerusalem by the animals' owners.
According to biblical law, this mitzvah applies both in the land of Israel and outside of it, whether the Temple is standing or not [i.e., we are commanded to separate and sanctify the animals even though we cannot sacrifice them]. The Rabbis, however, instituted that we not perform this mitzvah when the Temple is not standing in Jerusalem—for fear lest someone consume the sanctified animal (before it becomes blemished and technically unfit for sacrifice).
When the Temple will be rebuilt, however, this mitzvah will be practiced in all locations.
Translation of (the unabridged text of) Sefer Hamitzvot by Rabbi Berel Bell, member of the Rabbinical Court of Montreal and director of Teacher Training for the Jewish Learning Institute.
From "Sefer Hamitzvot in English," published by Sichos in English