Menachos 10:8-9

Menachos 10:8

One may reap crops from irrigated fields in the plains before the omer is brought but one may not stack their grain. The people of Jericho (who had such fields) used to reap with the Sages’ consent but they would stack the grain without the Sages’ consent. Nevertheless, the Sages didn’t chastise them. One may harvest unripened grain and feed cattle with it; Rabbi Yehuda says that this is true so long as he started before the grain reached one-third of its growth. Rabbi Shimon says that one may harvest unripened grain and feed cattle with it even if he starts after it has reached one-third of its growth.

Menachos 10:9

Before the omer is brought, one may harvest produce: that grows between saplings for the benefit of those saplings; for mourners to sit on; to prevent the study halls from being impeded. One may not tie the harvested grain in bundles; it must be left in small heaps. The mitzvah of the omer requires that it be harvested from standing grain. If standing grain cannot be found, it may be brought from sheaves. The mitzvah requires that it be brought from fresh grain. If fresh grain cannot be found, it may be brought from dried grain. The mitzvah requires that it be reaped at night but if it was reaped by day, it is valid. Harvesting/offering the omer supersedes Shabbos.