Ashley, I have been stewing over this very thing. I am wanting to get enough hay now while it's at its lowest price rather than run out in the spring and pay three times that. Your question made me actually sit down with the calculator and figure it all out. I'll run through all my math for you. The ranges are using 1.5% at the low end and 2% at the high end of feeding per body weight.
Clydesdale - 1800 pounds - needs 27-36 pounds per day
Pony - 400 pounds - needs 6-8 pounds per day
donkeys together - 900 pounds - need 13.5-18 pounds per day
minis together - 700 pounds - need 10.5-14 pounds a day
so in total, everyone weighs 3800 pounds so I rounded up to 4000 pounds
4000 pounds of horses need 60 to 80 pounds of hay a day (we do not have severe cold weather like you do, so you will need to factor in more for that, especially if you do not bring them in.)
They can feed totally on pasture (not the minis) April through September so I need hay October through March, so 6 months or 180 days. This would mean I need between 10,800 and 12,800 pounds of hay for the season.
Hay I currently have on hand:
10 bales at 80 pounds each left from last year: 800 pounds
300 bales at 40 pounds each: 12,000 pounds
So I have 12,800 pounds of hay and that meets the high end of my requirement. Whew!
Just to be safe I will ask my hay guy to set aside another 50 bales for me (a ton at 40 lbs each). I also feed beet pulp in addition to hay, but not much, maybe three pounds dry, then soaked and divided amongst everybody. It's probably a couple of gallons once soaked.
Lastly, I believe that all but one of my horses is probably carrying more weight than they should and I don't want to continue contributing to that. So I am going to weight tape everyone every couple of weeks and keep a chart. This way I can see if anyone is losing or gaining so I can make a quick adjustment.
Hope that helps! Your thread sure helped me sort it all out and now I'm not going to worry any more!