Metalleat is a crop added by Thaumic Bases. It is a renewable source of Iron Nuggets. It grows similarly to Wheat, except instead of dropping Wheat when harvested it drops Iron Nuggets and Metalleat Seeeds. It has 4 growth stages before maturing.
Sometimes a thaumaturge needs some metallum for his ore processing. And there goes an iron ingot. Sometimes a thaumaturge needs metallum for his crafts. And there goes more iron. You have discovered a way to put an end to this misery - by infusing a regular wheat seeds in a crucible with some metallum and victus to keep the life in the seeds you have managed to create a plant, that grows exactly like wheat, but gets tough and metallic in the latest stages of growth. It then can be harvested to obtain some Iron Nuggets. This discovery asks more questions than it answeres[sic] though, and you might need to research it further... As far as you can tell you can't replicate the same with thaumium.