It all depends on the type of grain. Some commodities are heavily exported, while others tend to stay in the U.S. I work in the rail transportation department for one of the largest grain companies in the U.S., so I see on a daily basis where a lot of this stuff goes. The big three commodities are corn, soybeans, and wheat (there are four types of wheat: Spring, Winter, Durum, and White)
- Corn can go one of many ways: Export through elevators in the PNW and Texas/Louisiana Gulf, export to Mexico, or to domestic customers such as ethanol plants and feed lots.
- Soybeans are mainly exported through ports in the PNW and Texas/Louisiana Gulf or across the border to Mexico. The rest is used domestically for biodiesel plants or soybean oil processors
- Wheat is roughly a 60/40 split. 60% tends to stay in the U.S. for flour mills, the rest is exported either to Mexico or through elevators in the PNW, Gulf, or Duluth/Superior to a lessor extent.
Others:
- Barley: There seems to be two grades of barley, lower grade Barley is used for animal feed while higher grade is for Malting. The vast majority is used domestically.
- Canola: A majority of the Canola I see is used domestically by oilseed processing companies. Some canola is exported through Duluth/Superior.
- Milo: Don't see a whole lot of this one, but the majority is exported through the Gulf. Some does stay domestically (mills of some sort use it, not sure for what though)
- Flax: Not very common, but generally used domestically.
- Sunflowers: Domestically for oilseed processing or confectionary uses.
- Peas/Lentils: I believe a fair amount is moved by rail to the Seattle/Tacoma area, where it is transloaded into containers and sent to Asia. The rest is used domestically.
This should give you a rough idea of what goes where. There are other types of grain out there, but I see very little of them so I left them out (Oats, Rye, and Millett for example). And, while my employer deals with all sort of grains, some are dealt with more so than others. For example, we don't deal much with Flax, so my example does not necessarily mean all Flax is used domestically.
Hope this helps!