The Torah commands in Parashat Shoftim (Debarim 18:4), “Ve’reshit Gez Sonecha Titen Lo” – that we must give a Kohen the first wool sheared from a sheep.
This obligation applies when one shears at least five sheep – whether male or female, and whether young or old. Even if a sheep had been sheared many times previously, nevertheless, each time one shears new wool, he must give a portion to a Kohen.
On the level of Torah law, there is no minimum amount that one must give to fulfill this Misva. However, the Gemara in Masechet Hulin (137) teaches that the Sages enacted a requirement to give at least one-sixtieth of the wool.
The reason for this Misva is that the Torah wants us to support the Kohanim so they can devote their time and attention to serving G-d. Just as the Torah requires giving the Kohanim certain portions of food – such as percentages from agricultural produce, and portions of animals which are slaughtered – it likewise requires giving them wool so they can make garments for themselves. This helps ensure that the Kohanim are free to perform their service without having to worry about their material needs.
One who shears wool must give each Kohen at least the amount of wool weighing five Sela’im, which suffices to make a small sweater. If one does not have twice this amount of wool to give, then he must give to only one Kohen, and may not divide the wool he gives among multiple Kohanim.
This obligation applies only when one shears wool which is soft and thus suitable for making garments. If the wool is coarse and unsuitable for making garments, then there is no obligation to give a portion to a Kohen.
This Misva is binding upon both men and women, to Yisraelim and Leviyim, but it applies only in the Land of Israel.
There is considerable discussion as to whether or not this Misva applies nowadays, in the absence of the Bet Ha’mikdash. The Sefer Ha’hinuch writes that this obligation applies even when there is no Bet Ha’mikdash, though from the Rambam it appears that in the absence of the Bet Ha’mikdash, this Misva is binding only Mi’de’rabbanan (by force of Rabbinic enactment). Some Poskim, such as the Hatam Sofer (Rav Moshe Sofer, Pressburg, 1762-1839), suggested that this Misva might not be applicable nowadays, because it is linked to the fact that the Kohanim did not receive a portion in the Land of Israel. Nowadays, technically, no tribes have a portion in the land, and so there is no longer an obligation to support the Kohanim. Others claim that since Kohanim nowadays cannot verify their lineage, people are not obligated to give them a portion of their wool, as it is uncertain whether they are truly Kohanim and thus deserve these gifts. (See Peri Hadash, Yoreh De’a, 61; and Birkeh Yosef, Yoreh De’a 333:2.)
Regardless, the concept underlying this Misva is relevant nowadays in that we are obligated to support Torah scholars. They are, in a sense, today’s “Kohanim,” as they devote themselves to the service of G-d, and they must therefore be supported so they are relieved of the burden of supporting themselves and their families.