Yaahh - I have a few that will eat more than they need. So you're right there. But if Iowanic's horses are working during the day that will make a difference. He also might need a sacrifice area where he can put them to limit their access to pasture.
And then horses produce more manure depending on how much of the feed they eat is waste, which is probably why the city horses on more concentrated feed are producing somewhat less manure. Manure weight though also contains water weight (about 63% moisture) and isn't just sold feed waste.
http://www.cals.ncsu.edu/waste_mgt/smal ... Manure.pdf
Most of my horses are pretty good and only poop in one corner of their pasture so waste of pasture is minimal and limited to what they tromp on when they are running and playing
So now I guess Iowanic will have to decide what kind of horses he is going to get to suit his needs which will make a difference in how much land and maintenance he'll need to plan for. I'm gonna guess Iowanic that you're going to want a general utility type horse that can do some draft work like pulling and ploughing, but still be light and quick enough for transportation. You won't need a high maintenance 17-18 hh draft or jumper or a racehorse - but a stouter type small-medium sized horse like a Morgan, Canadian, Fjord - even some type of Quarter Horse
- or maybe even a bigger pony. Most of the average working horses in the pioneer days in America were only about 14 hh (56 inches) and 800-900 lbs.So right there you would need less land and less feed than for a larger horse and probably have to take your horses off the pasture part time if it is good grass and they aren't working. But you'll also hopefully have pasture to rotate and access to land to cut for winter forage unless you have a climate with year round fresh forage. Lush pasture, I'd say would be 1 to 1.5 acres per horse, more arid plains type sparser pasture could be up to 5 acres per horse. The better you can maintain and irrigate the land, the less land you will need too.