You surely don't need a DOT to haul to your local elevator even if they rail it to the port and it's shipped out of the US.
You hauling it to the elevator consitutes the first leg of the interstate movement.  Since the shipment is interstate and your GVWR is 10,001 or greater you are required to have a DOT number.

Now deliver the grain to a feedmill that mixes it into feed and subsequently hauls the feed across state lines, the grain shipment would be an intrastate movement and does not require a DOT number because the product you deliver the to feedmill is processed into feed and is no longer grain and since it is changed substantially by the processor to produce a product, the raw material is not considered an interstate movement.  The same applies delivering grain to a soybean processor or ethonal plant.