Menahot, Chapter Ten, Mishnah Eight
This mishnah brings in exceptions concerning the prohibition of harvesting grain before the omer.
1) [Before the omer] one may reap [grain] in irrigated fields in the valley, but one may not stack it.
2) The people of Jericho used to reap [before the omer] with the approval of the sages, and used to stack it without the approval of the sages, but the sages did not protest.
3) One may reap the unripe grain for cattle feed.
a) Rabbi Judah said: When is this so? If one had begun to reap it before it had reached a third of its growth.
b) Rabbi Shimon says: one may reap it and feed [his cattle with it] even after it has reached a third of its growth.
Section one: One doesnt bring omer grain from grain grown in irrigated fields in the valley because this is poor quality grain. However, the sages did not permit people to stack this grain. The only reason that they did permit harvesting this grain is that there was a fear it would be lost if it was not harvested. Therefore they were lenient when it came to grain that would not have been brought as the omer barley in any case. But there was no reason to be lenient with regard to stacking because it would not cause a loss to wait until the omer to stack the grain.
Section two: Jerichos fields are in the valley and therefore the rabbis allowed them to reap the grain before the omer was offered. However, the people of Jericho went further and stacked their grain as well. This was a no-no in the eyes of the sages, but despite this, they did not protest.
Section three: Unripe grain can be harvested before the omer, if it is to be used for cattle feed. Rabbi Judah says that the grain can be harvested as long as it has not yet reached one third of its growth. Rabbi Shimon is more lenient and allows the grain to be harvested even if it has not yet reached one third of its growth.