Bechorot 57: When to Tithe

A new Mishna teaches us about when the tithes are offered.  There are three threshing floors where animals are tithed.  Tithing happens close to Peach, Shavuot and Rosh HaShana.  The Gemara connects this to when cattle usually give birth, in the beginning of the winter, in the spring and in the summer.  The Mishna states that all animals born in one year are considered to be a single group for tithing purposes.  It is permitted to sell or slaughter these animals outside of the tithing periods.

Rashi explains that harvesting produce is different because tithing animals fulfills a biblical mitzvah.  If one does not fulfil the mitzvah, s/he can still derive benefit from the animal.  The three times of year were established to help farmers with tithing.  The Rambam suggest that there is a biblical obligation to tithe animals and the rabbis had to find times to do that tithing.