The gemara in Gittin 62A teaches that one should feed his animals before he, himself eats. I have seen others who write that the care of an animal including feeding it satisfies tza'ar ba'alei chayim.
As the feeding of the animal is elevated to the role of "mitzvah" (that is, the fulfilling of an halachic obligation) does the food used get elevated in its sanctity as well (having been used for a mitzvah)?
As a practical application, my 4 minim, having been used for a mitzvah on Sukkot, have to be treated with a certain level of respect, and according to some, should be used for a mitzvah. Can I feed my lulav to a cow, or my etrog to whatever animal eats etrogim because feeding is a mitzvah?
And does the food left in my dog's bowl, which I set aside to be used to feed him (thus it was established there for the sake of a mitzvah) need to be disposed of in a particular manner because it has been elevated through its use for a mitzvah?