Hippolyta, it uses a bag, but it's not a bag that collects dirt you need to empty. The bag basically functions as a filter to keep dirt out of the motor. The dirt collects within the box.
I have an Electrogroom Dairy-Vac (same as the horse version, just a different decal) that is from the early 80s. It belonged to my grandparents who were dairy farmers, and it just happens that it wasn't sold at their auction when they retired. I found it dirty and dusty in a pile of junk in the barn. Mice had taken up residence, but I cleaned it out, got a new bag and hose for it, and it turned right on and works like a charm.
I love it, and I'm not quite sure how I functioned without it. With three furry pasture horses, it's the only way I can get them halfway clean in the winter. It sucks the dust right out of their long coats. None of my horses are bothered by it, and really adjusted to it right away. Two of mine hate to be brushed on their barrels, and they aren't pleased with being vacuumed there either, but that's not because of the vacuum. The third one will give the hairy eyeball to the hose occasionally, but the actual vacuuming doesn't bother him a bit.
What's nice about the Electrogroom is that you can affix the hose to the exhaust and use it as a blower. I do not body clip because my boys live outside, so this is handy when they get sweaty. I can blow them dry while I curry them, which greatly speeds my post-ride grooming time.
Edited to add: I had one of the dirt devil hand vacs posted above way back when. My horse (I only had one then) hated it. I think the rotating brush bothered him.
Last edited by OneonOne; Nov. 18, 2012 at 08:24 PM.
Reason: Add more info