[thinkharder]

That's what is known as 'feed corn' and is grown specifically as livestock feed. The specific strains of corn are optimized for producing maximum plant material, which is mostly stalk and leaves. A field of feed corn plants will usually be very tall, 8-15 feet. In contrast a field of sweet corn (grown for human consumption and possibly for processing into other foods), will be much shorter, since it's optimized to produce the most, largest and highest sugar content corn kernels, and any energy the plant spends to grow a tall stalk is wasted.

Feed corn is harvested with big combines that just cut down the whole plant at ground level and chop everything up. The results are piled up and left to ferment. Once fermented, you have silage.

I'd be surprised to learn anyone was harvesting sweet corn and saving the waste material to make silage. The equipment isn't designed to keep that stuff it just gets dumped back onto the field. And those strains of corn don't produce very much plant material since they're optimized for small plants and big cobs. Additionally you'd have to load all the waste material into trucks, which would significantly raise the cost of harvesting. And dumping that stuff back onto the field is a good thing, it helps keep the dirt down for the winter and decomposes into usable nutrients and fibrous material which helps reduce compacting, etc. It'd be expensive and labor intensive to try to capture the waste material from feed corn. It's easier and more economical to just plant feed corn or buy silage.

You're right about straw, though. The harvesters are specifically designed to leave the straw in row pile behind them. Then you run a baler over that and it leaves a row of straw bales in the field. Then you run a stacker over that (or a flatbed trailer and buck 'em by hand) and you have a haystack.