Corn and wheat futures have surged to their highest levels in more than three years. But the reasons for these recent rallies are notably different.

Corn rose again Tuesday as the Pro Farmer corn yield cut kept sinking in, soybeans rebounded on China buying, and wheat firmed on Black Sea disruption.

Chicago wheat prices hit a three-year high as escalating tensions between Russia and Ukraine threaten critical grain exports from one of the world's key agricultural regions.