@PotomacBob
In a scenario where there are no manufacturers producing commercial animal feed of any kind
...there's probably no manufacturer of sliced bread or ready made clothing either.
So you live in a homestead that only ever needs to import salt, metal tools and luxury goods. You have stacked hay, you have dedicated stores for various types of grain, beans, and peas, you have huge cellar full of beets, carrots, potatoes and apples, and what's not, and your clothing starts by sowing linen and breeding sheep. And your farm has a specialized room called animal's kitchen where you, yes, process various ingredients into precursors of the commercial feedstock you thought about as indispensable.
Livestock were domesticated for convenience. They do not, as species, depend on humans for survival.
That said, livestock can and is kept in places and intensities far outside natural ranges. And of course, there's huge space of gradual scale between fully independent homestead and industrialized feed producers with various methods and benefits of specialization and cooperation at every step. But even in most of those scenarios manufactured ready feed is convenience product on par with Macdonalds (only healthier, mostly).
Of course going back to basics needs some knowledge, but a lot of that can be picked up by intelligent, attentive and wiling (or forced) to learn human on the fly. If no other way then simply by monitoring foraging habits, mood and health of said animals.
And of course, there's the often overlooked fact overwhelming majority of animal source products are already luxury goods in terms of bare survival, although those do gain utility and importance as further into cold climates you try to penetrate. Vegetarianism and especially veganism are survivable, but only make good sense in tropical or otherwise well controlled and comfortable environments.