Matthew Poole and continuators on Leviticus 27:32

32 And concerning the tithe of the herd, or of the flock, even of whatsoever passeth under the rod , the tenth shall be holy unto the LORD.

Either 1. the tithers rod, it being the manner of the Iews in tithing to cause all their cattel to pass through some gate or narrow passage, where the tenth was marked by a person appointed for that purpose and reserved for the Priest. Or 2. the shepherds rod, under which the herds and flocks passed, and by which they were governed and numbred. See Ier. 33. 13. Ezek. 20. 37.