Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
First Section – the Positive Commandments
56. It is a positive commandment to break the neck of a male donkey that was not redeemed
As it states in Shemos (Exodus) 34:20, "If you do not redeem it, then break its neck." The breaking of the neck is by hitting it with a butcher's knife on the back of its neck until it dies. It must be with a butcher's knife and not with any other instrument nor shall it die by any other way. It is forbidden to derive pleasure from it even after its neck is broken. It requires burial and its ashes are forbidden to derive any pleasure from.
This applies in all places and at all times for men and for women. Cohanim and Levi'im are exempt.