Those aren't going to work. One horse generates about 50# of waste per day. More if you bed stalls and they waste hay. That's about 15 cubic feet of waste per day per horse. You would also have to scoop everything out of the wheelbarrow into the tumbler which is going to be painfully labor intensive. You will also have yard and house waste that you will add to the mixture as well. You could easily fill one tumbler in a day. Just build yourself 2 4x8 bins somewhere close but out of sight. One that's curing and one you're working into. If your small pasture area is closed for a period of time, you can also just spread it out fresh. Spread it as thin as possible and give it a month to work into the soil.