Another gift to be given to a kohein was the first shearing of a sheep's wool. This applies to each sheep, every time it is shorn. On each occasion, a portion of the wool must be given to a kohein. As with terumah gedolah, the Torah does not define a size for this gift; the Sages set it at one-sixtieth of the wool. If a farmer wants to divide his wool among several kohanim, he has to make sure that each one gets an amount that would be sufficient to make a small article of clothing.
Only someone who has at least five sheep is obligated in this mitzvah. Less than this and the sheep would not produce a great enough output of wool to oblige a person in this matter.
The reason behind this mitzvah is what we have said before: as designated functionaries in the Temple, the kohanim do not receive a portion of real estate in Israel or any booty plundered in conquest. They supported themselves through the special gifts that God designated for them. They could eat the terumah but they couldn't wear it. Therefore, God commanded that the kohanim should also receive wool.
This mitzvah applies in Israel at all times. It is discussed in the Talmud in tractate Chullin starting on page 135a and is codified in the Shulchan Aruch in Yoreh Deah 333. This mitzvah is #144 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos and #52 of the 77 positive mitzvos that can be fulfilled today in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim. As with the gifts to the kohein (in Mitzvah #506), our practice is generally to act leniently in this matter.