Concise Bible Dictionary:

Various Hebrew words are translated “corn,” and usually signify any kind of grain. The “OLD CORN OF THE LAND” was what the Israelites began to eat after crossing the Jordan, when the manna ceased (Josh. 5:11-1211And they did eat of the old corn of the land on the morrow after the passover, unleavened cakes, and parched corn in the selfsame day. 12And the manna ceased on the morrow after they had eaten of the old corn of the land; neither had the children of Israel manna any more; but they did eat of the fruit of the land of Canaan that year. (Joshua 5:11‑12)). It typifies a heavenly Christ, on whom those feed who have spiritually passed through Jordan—who are experimentally dead and risen with Christ. The manna is rather heavenly grace for wilderness circumstances. In the New Testament Christ speaks of Himself as a “CORN OF WHEAT,” which had to die or it would abide alone: there could be no association in life with Christ except through death and resurrection (John 12:2424Verily, verily, I say unto you, Except a corn of wheat fall into the ground and die, it abideth alone: but if it die, it bringeth forth much fruit. (John 12:24)).
Wheat growing in Israel.