16. When setting out hay ("Mispoh") for one's animals, it is forbidden to pass it through a sieve first, in order to remove the chaff and to clean it (1). If, however, one does not have that intent in mind, one may carry it to the feeding trough in a sieve (2).
(1) One of the 39 "Avos melachos" (prohibited categories of activity on Shabbos) is "sifting" ("Meraked"). Since one usually wouldn't remove chaff from hay with a sieve, doing so would not be a violation of a biblical prohibition. However, since it is similar to the "melacha" of sifting, the sages prohibited it (Mishna Berura 324:1 and Sha'ar Hatziyun). The Oruch HaShulchan (324:5), however, rules that this would be a violation of the biblical prohibition against sifting.
(2) This is true even though some of the chaff will automatically fall away through the holes in the sieve. Since one's intent is not for the chaff to fall away, it is permitted. In general, even if the performance of a melacha may occur as a result of a particular action, as long as it is not inevitable ("Psik Reisha"), and one's purpose is not to perform the melacha, the action is permitted. In this case, the Aruch Hashulchan 324:5 explains that although some chaff might fall away, a large amount will remain in the sieve, and hence, it does not constitute a "Psik Reisha."