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
53. It is a positive commandment to sanctify the firstborn males that open the womb
As it states in Shemos (Exodus) 13:2, "Sanctify to Me all firstborn that open the womb of the Children of Israel and of animals." The commandment is to sanctify the firstborn animals of the kosher species by saying "This is holy", as it states in Devarim (Deuteronomy)15:19, "Sanctify to Hashem your G-d." If one does not sanctify it, it becomes sanctified automatically by itself.
Nowadays, the firstborn animal should graze in a field until it develops a defect, and then it belongs to the Cohain (priest).
This is obligated for men and for women.