For silage corn the entire plant is harvested, chopped up, and packed tightly in a pile and covered with plastic or very large plastic bag. This creates an anaerobic conditions and yeasts/bacteria convert the sugars to alcohol, then acids. This acid preserves the feed for cattle for up to 2 years.

Silage corn is the most common base feed for all dairies and feedlots.

This isn't even the tallest variety out there. There are others that are 3' (1m) taller.

The machine with the spout is a chopper. They are a million dollar gigantic woodchipper. These things are fucking terrifying and you should absolutely stay out the field when they are running. Every so often a deer jumps into them.....shudder.... nightmare fuel.