It was not against FDA rules to use corn over 300 ppb for feed.   It could be used in a ration with other ingredients.  Even if the corn was all 300ppb, that is a small percentage of a feedlot ration.  The total ration is what had to be below 300ppb to be considered 'safe' for beef feed.     There is no requirement that I am aware of for a feeder to test his ration.  His concern will be animal health..and possible liver damage is not a huge concern in a finishing lot.    
High testing corn could also be blended with other farm stored corn to produce a saleable level.  
Also, the corn could be sold to a salvage buyer.

I am positive that last year's rules allowed over 300ppb to be harvested, stored on farm, and the claim worked at 50% QA.  That is what the rules stated, and I am certain that it was done that way.  

If you want to learn about aflatoxin, talk to people in Texas, Arkansas, and Mississippi.....and listen to what they say, they have experience...there is no need to tell them how they are "wrong"