Think about the great population centers in the past where the only energy where windmills and fire (for heating and cooking), and fertilisers where not common at all.
Rome had a population of about 1.000.000 people before the middle ages, and still around 100.000 while the middle ages.
I think one can learn a lot by just looking at the past (middle ages 'n such).
People back then didn't have the tech benefits they have now (tractors, fertilizers), but they do had knowhow of agriculture, passed down through generations by experiencing it first-hand. Seeing as to how the entire West will have to resort to agriculture all of a sudden, no-one has experienced farming like this firsthand, only a few read something in books. Keeping in mind that the Wasted West is a downright terrible spot for farming (with Fall-out, contaminated water/soil, and generally nasty to downright deadly weather). Keeping cattle was a luxury back then, and with all the monsters nowadays this will be even more so. Horses were normally only for the rich, who could afford maintenance of horses. Nowadays, most horses would've been eaten, 'xcept for those who were very well protected and cared for by their owners.
Back in the middle ages about every four years, depending on the weather, there is a terrible harvest. These years crime, rebellion, and plain old cannibalism were common as men lost all reason due to hunger.
I'd think a medieval condition is comparable, as any Tech assets are offset by lack of experience, terrible weather conditions and nasty critters.
For a more contemporary comparison, look at the developing countries: see for example how whole villages become dependent on the sole village tractor, and how they are in dire trouble 'cause there's no fuel left or a necessary component breaks down (with no spare parts, and no-one to repair it).
He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you.
— Friedrich Nietzsche, Beyond Good and Evil