Why is it forbidden to buy the firstborn calf, sheep, and goat? Why are they holy?
ב"ה Shalom, The Torah commanded us to sanctify the firstborn son of his mother and the first born calf born of kosher domesticated animals such as sheep and goats and also the firstborn calf of the "chamor" which is the only non kosher animal that has to be redeemed. According to Sefer Hachinuch Mitzva18, there is a twofold purpose for this mitzva. One is to understand that all that we have is from G-d especially after our great expectations waiting for the birth of the fitstborn infant or calf. The other reason is to always remember that G-d saved our firstborn while in bondage in Egypt while the firstborn of the Egyptians was slain. In the time of the Bet Hamikdash the Bechor was given to the Cohen and then he would bring it as a sacrifice and then the Cohen and his family were allowed to eat its meat. Today since we have no Bet Mikdash, there is no way for the Cohen to bring it as a sacrifice. On the one hand the firstborn still retains its holiness and must be given to a Cohen, but on the other hand the Cohen cannot benefit from its meat until the animal becomes blemished making it unfit for a sacrifice. Taking care of the firstborn therefore can become a great burden. Therefore, there is a custom today to sell the animals bearing the firstborn to a non-Jew or to make him a partner in the ownership of such an animal, thereby divesting the firstborn of any holiness. (See Peninei Halach – Kashrut 1:19) All the best