For the source text click/tap here: Bechorot 53
To download, click/tap here: PDF
The Mishnah states that the Mitzvah of Ma'aser Behemah applies both when the Beis ha'Mikdash is standing and when there is no Beis ha'Mikdash. The Gemara asks that if the Mitzvah applies when there is no Beis ha'Mikdash, then why does it not apply nowadays?
The Gemara explains that the Mitzvah of Ma'aser Behemah does not apply today because, as Rav Huna says, the Rabanan enacted a decree that Ma'aser Behemah not be given because of the concern that one of the animals might be a "Yasom," an orphaned animal that was born after the death of its mother, which is not subject to the obligation of Ma'aser Behemah, as the Gemara later (57a) derives from the verse, "Tachas Imo" -- "with its mother" (Vayikra 22:27).
The institution of ma‘aser behemah occupies an unusual place within the sacrificial system. Whereas most offerings involve some degree of human selection, assessment, or intention, the animal tithe is consecrated through a ritual that deliberately restricts the owner's freedom to choose. The animal that emerges tenth becomes holy not because it is especially valuable, beautiful, healthy, or desirable, but simply because it occupies the tenth position in the sequence.