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1. The sages knew that any grain that is harvested on Sukos was already one- third grown before Rosh Hashanah.
2. The Korban ha'Omer brought by Bnei Yisrael during their first year in Eretz Yisrael was from Jewish grown grain.
3. Produce in Eretz Yisrael grows faster than in other lands.
4. The Gemara states how the Ma'aser status of produce is determined by its categorization as a vegetable, fruit, or grain.
5. For example (see #4), the Ma'aser status of fruit depends on when it blossoms.
A BIT MORE
1. The Gemara lists similar amounts of Chazal that are very exact, such as the amount in which a person can immerse his entire body is exactly 40 Se'ah.
2. It could not have been from grain grown by Nochrim before Bnei Yisrael arrived, because the verse says that the Omer must be from "your harvest," implying not from the harvest of (or produce grown by) Nochrim.
3. The Gemara derives this from the verse describing Eretz Yisrael as, "Eretz ha'Tzvi." Rashi explains that just as a Tzvi runs quickly, Eretz Yisrael grows produce quickly.
4. In order to separate produce as Ma'aser for other produce, it must be determined that all of the produce belongs to the same year, which is determined by different factors for different types of produce. (See #5 below.)
5. The Ma'aser status of grain (and olives) depends on when it is one-third grown. The Ma'aser status of vegetables depends on when they are harvested.
Next Daf Index to Revach for Maseches Rosh Hashanah