Chapter 6. Animal Food on Pesach

In the straightforward understanding, since it has been ruled as practical halacha [1] that food which has become unfit for human consumption but is fit for a dog to eat is subject to the obligation of disposal, it follows that animal food containing chametz must be disposed of, since it is fit for a dog to eat [and it seems that when they mentioned a dog, this was not meant specifically, but the same applies to all animals]. 

However, the reason why chametz that has become unfit for human consumption but is fit for a dog to eat must be disposed of is that it is fit to leaven other doughs; and this follows the opinion of the Rambam [2] that chametz which has become unfit for human consumption, if it is fit to leaven other doughs, is included in the category of se’or. But if it is not fit to leaven other doughs, since it is not fit for human consumption, the prohibition of chametz does not apply to it at all. Therefore, if the animal food contains an admixture of chametz in such a way that the form of the chametz has changed, since it is crushed and mixed with other substances and is not fit to leaven other doughs [3] there is no obligation to dispose of it, and it is permitted to keep it and use it.

However, if the chametz mixed into the animal food is not crushed, such as wheat or barley, even if it is not fit for human consumption — since it is fit for animal consumption [a dog] and for leavening other doughs, it must be disposed of.

One may give animals alternative food on Pesach that is not chametz, such as millet for parrots, worms for fish, poultry meat for cats and dogs, and the like.

[1] In the Gemara, Pesachim (45b), and in the Tur and Shulchan Aruch (section 442, paragraphs 2 and 9).
[2]  Laws of Chametz and Matzah (chapter 1, halacha 2).
[3]  See Chazon Ish (section 116, subsection 8).