By law Bourbon must be made from a mash bill of 51% corn, so what's the rest of it made of? The US requires that any spirit labeled as "Whiskey" must be made from a grain mash bill. which is the combination of grains that are milled and then mixed together with corn. The remainder of the mash bill is generally made up of what are called "flavor grains" or "small grains". The most often used flavor grains are rye, malted barley, and wheat. Most Bourbons are made from a mash bill of corn, rye, and malted barley. Wheated Bourbons generally replace rye, the largest portion of small grains, with wheat. The result is a softer, sweeter Bourbon.