יש או אין דחוי אצל מצוות – from dafyomi.co.il

(a) "Dichuy" means "pushed off". This Halachic concept applies to certain cases in which a specific object is necessary for the fulfillment of a Mitzvah. If at one point that item is unfit for use as required, then Dichuy says that it may no longer be used even if it should then become fit. If, for example, one sanctifies an animal that has a Mum (blemish), Dichuy would not allow that animal to be offered upon the Mizbe'ach even after it has healed. 

(b) There are two different scenarios in which Dichuy may apply. One, as in the previous example, is where the Dichuy was in existence from the beginning of the object's suitability for the Mitzvah. This is termed "Dichuy me'Ikara". If, however, the animal developed the Mum only after it was sanctified, this would be a question of "Nir'eh v'Nidcheh". This term refers to a situation in which the item was at first fit for use, end only afterward became unfit. Nir'eh v'Nidcheh is a more serious condition, as the object was actively deferred from the Mitzvah. 

(c) In our Gemara, Rebbi Yirmeyahu raises the question of whether or not the concept of Dichuy applies to all Mitzvos, or only to specific Mitzvos in which we know Dichuy to apply. The application of Dichuy to our Sugya is with regard to a Hadas that was unfit for use in the Arba Minim at the onset of the Mitzvah of Arba Minim due to that which its tip was cut off. If a Temarah then developed afterward, is the Hadas valid, or is invalid due to Dichuy? 

כיסוי הדם

(a) It is a Mitzvah to cover the blood of slaughtered birds and Chayos (non-domesticated Kosher animals) (Vayikra 17:13). The blood must be surrounded from above and below with earth or any other granular substance.