Gemara

Rav Yehudah: One may bring a box of dirt into his house (before Shabbos or Yom Tov) for all his needs;

Mar Zutra: This is only if he designated a corner (of the house for the dirt).

Question (Mishnah): One may not slaughter a Koy on Yom Tov. If one slaughtered, he does not do Kisuy ha'Dam (cover the blood).

According to Rav Yehudah he could use designated dirt!

Counter-question: The Mishnah allows Kisuy with oven ashes or dirt in which a shovel was inserted from before Yom Tov. Why can't he use these?

Answer: You must say that the case is, these are not available.

Answer #1: Likewise, the case is when he did not designate dirt!

Objection (Seifa): If one slaughtered, he does not do Kisuy.

Surely, this is when dirt is available. Even for a Vadai Chayah one may not dig new dirt for Kisuy!

Answer #2 (Rabah): Ashes are considered Muchan for a Vadai Chayah, but not for a Safek.

Objection: Surely, they are not considered Muchan for a Safek because he makes a hole (when he takes them). Also for a Vadai he makes a hole!

You must say that it is permitted for a Vadai like R. Aba taught (it is considered destructive). This also permits a Safek!

Answer #3: We decree to forbid a Safek lest he crush clumps of dirt.

Question: We should likewise decree to forbid a Vadai!

Answer: Regarding a Vadai, if he crushes clumps the Aseh of Kisuy ha'Dam overrides the Lav.

Objection: An Aseh overrides a Lav only if one fulfills the Aseh at the same time he transgresses the Lav, e.g. circumcision which cuts off Tzara'as or wearing a linen garment with woolen Tzitzis. Here, he crushes before Kisuy!

Answer: The case is, he does Kisuy at the same time (he crushes over the blood).

Objection: Melachah on Yom Tov is also forbidden by an Aseh (Shabboson); an Aseh does not override a Lav and an Aseh!

Answer #4 (Rava): A person intends to use ashes for a Vadai, but not for a Safek.

This is like Rava taught, that if one prepared dirt for Kisuy ha'Dam of a bird he may not use it to cover excrement (from his baby, for he was not sure that this would be necessary).

Amora'im argue about whether or not dirt Muchan for a Vadai is Muchan for a Safek.

Answer #5 (Rami brei d'Rav Yeiva): We decree to forbid Kisuy ha'Dam of a Koy on Yom Tov lest people (think that it is a Vadai Chayah and) permit its Chelev.

Question: If so, it should be forbidden also during the week!

Answer #1: If he covers during the week, people will think that this is for the cleanliness of his Chatzer.

Questions: This does not apply to one who slaughters near a wasteheap! It does not solve the problem regarding one who asks whether or not he must do Kisuy!

Answer #2: During the week people realize that even if it is a Safek, Chachamim obligate one to toil to do Kisuy. On Yom Tov, people assume that Chachamim would not require toiling for a Safek!

R. Zeira - Beraisa: Not only a Koy, but even if the blood of Behemos became mixed with blood of birds and Chayos, one may not cover it on Yom Tov (because some of his toil is for Dam Behemah, which is not a Mitzvah).

Rishonim

Rif and Rosh (1:11): One may not slaughter a Koy on Yom Tov. If one slaughtered, he does not do Kisuy ha'Dam, even if he has Muchan dirt or oven ashes. Rami brei d'Rav Yeiva explained, this is a decree lest people think that it is a Vadai Chayah and permit its Chelev. However, on Motza'ei Yom Tov one does Kisuy.

Rambam (Hilchos Yom Tov 3:1): On Yom Tov, if one has Muchan dirt or ashes that may be moved, he may slaughter a Chayah or bird. One may not slaughter a creature that is a Safek Chayah or Behemah. If he did, he does not do Kisuy until night, lest people think that it is a Vadai Chayah and permit its Chelev.

Rosh (ibid.): One may not slaughter a Koy and keep the blood to do Kisuy at night, because we do not slaughter into a Keli (Chulin 41a, lest it look like he is Mekabel the blood for Avodah Zarah). If he slaughters over dirt, perhaps the blood will be absorbed before night and no mark will remain (and the Mitzvah of Kisuy will be lost). We are not concerned lest one refrain from Simchas Yom Tov because he cannot do Kisuy immediately (there is no Isur to eat before Kisuy).

Poskim

Shulchan Aruch (OC 498:18): One may not slaughter a Koy (an animal born from a deer and goat) on Yom Tov.

Tzelach (Beitzah 8b DH Nimtza): There are three kinds of Koy. One is a species unto itself; it is a Safek Behemah Safek Chayah. Regarding this Chachamim forbade to cover its blood on Yom Tov, lest people (think that it is a Vadai Chayah and) permit its Chelev. One does not bless on Kisuy (for it is a Safek).

Crossbreeds of a deer and goat are also called Koy. Chachamim argued about whether or not we are concerned for the father; surely we are concerned for the mother. Kisuy is required for even a partial Chayah; the Chelev of even a partial Behemah is forbidden. Therefore, we do Kisuy for a Koy born from a male deer and a female goat without a Berachah. No one will permit its Chelev, for surely it is at least a partial Behemah. Rava says that dirt is not prepared for such a Safek (perhaps it is a total Behemah), unless it was prepared specifically for it. We bless on Kisuy for a Koy born from a doe (female deer) and male goat, for it is at least a half Chayah. Even during the week, one who hears someone bless on Kisuy might think it is a full Chayah and permit its Chelev! Chachamim did not want to totally uproot the Berachah. Therefore, they did not make any safeguards lest people permit its Chelev. One may slaughter it and do Kisuy on Yom Tov with dirt prepared for a Vadai Chayah.

Mishnah Berurah (92,93): The Shulchan Aruch refers to a Koy from a male deer and a female goat; the same applies to the Koy that is a species unto itself, for it is also a Safek Behemah Safek Chayah. An animal born from a doe and a male goat may be slaughtered on Yom Tov.

Kaf ha'Chayim (156): If the mother was a doe, the Maharshal and Taz obligate Kisuy with a Berachah. We hold like the Bach and Drishah, who say that it is a Safek, so one may not slaughter it on Yom Tov.

Kaf ha'Chayim (161): If a Koy is missing a limb (but is not Tereifah), one may slaughter it on Yom Tov, collect the blood in a Keli and do Kisuy at night. There is no suspicion that he offers such an animal to idolatry.

Shulchan Aruch (ibid.): If one slaughtered, he does not do Kisuy, even if he has Muchan dirt or oven ashes.

Mishnah Berurah (95): This is even if the dirt was prepared for a Koy. Tzelach says that this is only for the separate creation, but one may slaughter an animal born from a male deer and female goat if he has Muchan dirt.

Kaf ha'Chayim (159): The Shulchan Aruch did not specify. This implies that one may not do Kisuy for any kind of Koy.

Shulchan Aruch (ibid.): We are concerned lest people think that it is a Vadai Chayah, and permit its Chelev. They would assume that Chachamim would not require toiling for a Safek. At night one does Kisuy if a mark of the blood remains.

Magen Avraham (34): We do not put dirt in a Keli and collect the blood because of the same concern.

Question: Why do we say that we do not do Kisuy lest someone permit Chelev? Perhaps it is forbidden to toil for a Safek!

Answer (Taz 19): One may toil for a Safek Mitzvah. The Gemara said that if ashes or dirt are not Muktzah, he should cover with them!

Question: Rashi (8b DH Lo Koy) says that the concern is toil!

Answer (Taz 19): Rami brei d'Rav Yeiva holds that the concern is lest people permit Chelev. The one who challenged him holds like R. Zeira's Beraisa, that the concern is toil.

Question: The Beraisa refutes Rami!

Answer (Taz 19): Rami says that the Mishnah that forbids slaughtering a Koy or (b'Di'eved, if it was slaughtered) doing Kisuy argues with R. Zeira's Beraisa. If the Mishnah forbids because of toil, it should have discussed instead (a case in which no Isur was done, i.e.) a mixture of Dam Behemah and Dam Chayah! (We assume that the text of the Taz should say '...v'Lamah Lo Nokat Ta'aroves'.)

Rema: This is only if he slaughtered in a corner of the Chatzer. One may cover blood even of a Vadai Behemah in the middle of the Chatzer with Muchan dirt. It is like a privy; one covers it so his clothes will not get dirty.

Question (Magen Avraham 35): We permit Shechitah and Kisuy ha'Dam of a Koy during the week, because people assume that he covers for cleanliness. This implies that on Yom Tov we do not say that he covers for cleanliness!

Answer #1: During the week it is permitted only in the corner.

Rejection (Magen Avraham 35): If so, even during the week people will assume that he covers because it is a (Vadai) Chayah!

Defense (of Answer #1 - Bi'ur Halachah DH Afilu citing Acharonim): During the week people clean even the corners, but on Yom Tov one may clean only where people walk.

Answer #2 (Magen Avraham 35): Perhaps the conclusion is that indeed, one may cover Dam Koy on Yom Tov if he intends for cleanliness. It is forbidden only if he intends for the Mitzvah.

Bi'ur Halachah (ibid.): The Gemara did not permit Shechitah of a Koy on Yom Tov, even in the middle of the Chatzer. Perhaps Chachamim did not want to make distinctions. The Rema permits even Dam Behemah. This suggests that all the more so Dam Koy, since there is also a Mitzvah! However, perhaps this is only if he would clean for cleanliness even if there was no Mitzvah. If he must intend primarily for cleanliness, since we hold that Mitzvos need intent, he Vadai is not Yotzei. It is better that he wait until night. If there is still a mark he can fulfill the Mitzvah then!