Menachos 10:6

Offering the omer caused the new crop to be permitted throughout the land, while the two loaves permitted it in the Temple (i.e., for offerings). Flour offerings, first fruits and the libations of animal offerings may not be brought before the omer. If one brought them from the new crop before this, it was invalid. They may not be brought before the two loaves but if one did so, it was valid.

Menachos 10:7

Wheat, barley, spelt, oats and rye are liable to challah (the portion of dough due to the kohanim) and these grains combine with one another to form the minimum volume. These grains are prohibited before Passover because of the prohibition against using the new crop before the omer was offered, and they may not be reaped before the omer. If they took root before the omer, then the omer renders them permitted; if not, they remain prohibited until the following omer.