Similarly naive follow-up: how much hay is needed per animal per month, assuming no other sources of food? (or does one always provide hay plus something else?)
Of course it varies by quite a lot. But 2% of body weight per day is in the area.
And what you feed is based on nutrition, cost, availability, and the specifics of your operation. It can be all one thing for a while, but there’s usually more of a strategy to it.
The answer to this also depends a lot on the ambient outdoors temperature. Horses can stay warm through the winter outside in almost any weather (down to single digits F easily). But in order to do so, they use their GI system like a furnace. So they basically are CONSTANTLY eating and pooping. So the colder it gets the more they eat.
