It's next to impossible to accomplish. You obviously don't know how a grain elevator works. Most grain elevators are 'this elevator takes wheat shapes' or 'this elevator takes corn shapes'. Or the like. Everyone throws their grain in and takes an equal sized chunk out. Whose grain is GM? Do we then restrict who puts what into which elevator? That's a logistics hurdle, especially since grain elevators tend to be few and far between.
Then we have the supplier problem. Now that nobody knows which grain belongs to who, or what's even in it, we go to market. We say 'I have corn shapes here'. Someone you sold the futures to takes the corn away and puts it with all the other corn, and then bags it. The fiddling over who sold what corn to who is a nightmare. You night as well be asking if there was any kidney in the pink goop that your chicken nuggets came from. You can only make guarantees when you keep the supply chains apart from the very beginning. If you don't isolate supply chains, you are guaranteed to be serving up something that's been genetically modified.
The only people who DO that are the non-GMO producers who already label.
You are mighty unparticular in what you eat aren't you? You also seem to have an affinity for GMO's.
I agree there will be costs. It is up to the producers to keep track of what they produce. Have you ever seen a label on a candy bar or a cookie package..."Processed in facilities that also are used for peanuts and peanut products?" The candy bar is not a peanut containing product, but still it bears that label. The public's interest should supercede the interests of agribusiness.
Your real argument is that government cannot be allowed in any case to interfere with the profits of industrial agriculture.
Peanuts are a demonstrated risk - people are known to have suffered ill-effects, up to and including death, from traces of peanut. GMOs, on the other hand, have not been implicated in a single case of human illness.
Labels warning that a product 'May contain peanuts' are therefore not as pointless as labels warning of GMO content; However given that many food manufacturers label foods as 'May contain peanuts' as a matter of course, without any tracking or testing to determine whether peanuts are present, the labels are still fairly useless -
A 2007 study showed that 75% of cookies sold in Europe that carry the warning 'May contain peanuts' do not, in fact, contain any traces of peanut at all.
The only way to be sure that a product doesn't contain peanuts (or that it doesn't contain GMOs) is for there to be strict segregation of the products, and labelling of those items certified NOT to contain the material in question.
If you force food companies to label GMOs, then the CYA effect will result in the same situation we have with peanuts - everything not explicitly manufactured for the specialist 'no-nuts' or 'no-GMO' market will be labelled 'may contain...', and the information provided to the consumer will be no better than it is with only those specialist products bearing a label.
Government have a duty to interfere with the profits of industrial agriculture, where public health is served by doing so; but they should not be brow-beaten by fanatics into interfering when there is no public benefit whatsoever.
Peanuts are dangerous to some; so they are labelled. Show that GMOs are similarly dangerous, and I will join you in calling for labelling of GMOs too. But first, you must show cause. Otherwise we will have to label everything: 'May contain negative thought energy from disgruntled employees'; 'May have been packaged on equipment while Scorpio was in the ascendant'; 'May contain NSA mind-control nanobots'; etc.; etc.
It is not sufficient for some cranks to think that something might be harmful. First show that harm exists - as with peanuts. THEN you can demand a label.