For what period of time? This week? Last week? Last year? Last decade? etc.
Any time period you wish. It makes no difference in how it's measured.
I don't wish, you do. You are the one that brought it up. And, It does make a difference. The shorter the time period the harder it is to get a definite track on it. ex: still in transit, cannot be counted as shipped from N.O. Think about it. Go ahead. I'm sure you will eventually understand. I'm also sure you will not admit it if you do.
There are other details that need to be investigated.
Name them.
What form of corn are you measuring? At what stage of processing, etc. There are more. You are the one interested in corn. You research it. After this post, in this thread, I'm done doing your work for you.
In addition, the question is not expressed clearly. ex: not counting wastage?
Wastage is one component that can be measured. But the question is how or when it is measured.
Your posit is still not clear. If you could find a way to try your measurements you will see that you have fallen short in defining "the problem".
And I guess you are assuming all the corn loaded on the barges at Iowa ports is/was actually grown in Iowa?
No, but the origin of each load is easily documented if that's where you choose to measure it. After it's loaded on the ship it's not so easy.
No, the origin is not easily documented. Much of the corn coming from other states to Iowa ports is mixed in with Iowa corn before being loaded. So, for any given time period you do not have a precise measure.
I think you are being too simplistic.
You're being obtuse.
You are being simple minded. Be careful. You could be easily mistaken for a doofus.