For the source text click/tap here: Rosh Hashanah 13
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Our Daf states that the Jewish people brought the Korban ha'Omer as soon as they entered Eretz Yisrael. The Gemara questions how they were able to grow grain for the Korban in such a short amount of time. From the time they entered the land until the time of the Korban ha'Omer was a period of just five days.
They could not have bought the barley from the inhabitants of the land, because the Korban ha'Omer must be brought from barley that ripened in the possession of Jews.
The Gemara proves from here that barley is able to ripen within a very short amount of time, the amount of time that passed from when the Jews entered Eretz Yisrael and became field-owners until the time they harvested the barley for the Omer five days later.
We explore the Mitzvah of Omer offering and its relationship to Sefiras Ha'Omer.