Corn is a commodity - which, by definition, means that buyers do not differentiate different parcels of corn by quality. There are certain minimum regulations that designate safety, etc., but once a shipment passes inspection, corn is corn.
This came up as I listened to corn breeders discuss efforts to increase the nutritional qualities of corn. "Identity preservation," has been hotly contested for decades as some entities hoped to release corn varieties with various improved qualities while others balked at the enormous difficulty and cost associated with trying to keep track of different batches of corn. Right now, virtually all corn that passes inspection is dumped into a massive, undifferentiated national stream of grain. Changing this would require everything from duplicating equipment at local elevators to developing a national tracking system.
Although there is some traction to be made for improving quality for human consumption, apparently there is none for animal feed. I was shocked to hear that the job of a feed purchaser for a poultry operation is to simply find the cheapest possible corn available with complete indifference to quality. Whatever standards the operation holds with regard to feed are addressed by running chemical assays on the cheap, purchased feed and adding in any nutrients, flavor or color compounds that are lacking!
It wasn't clear to what extent this occurs with corn meant for human consumption. It is, of course, a familiar story to us plant pathologists that a farmer whose grain doesn't pass inspection (e.g. due to pest contamination/damage) doesn't just throw out that year's harvest - he tries to mix it in with completely unblemished grain so that the average then falls below the rejection threshold. This has been going on as long as the marketplace and it makes me thankful I live in a country with stringent regulatory thresholds!*
*Back in the Middle Ages, gingerbread was supposedly invented to disguise a batch of flour that was rank with smut fungus.