I say there's no way of reliably keeping water from getting between the concrete and the barrel (well.. if the concrete lapped over the top of the barrel then maybe
), so make a plan for dealing with the water that gets into that gap.
As far as water in the middle of the concrete surface, I liked Shark Grip
I added to sealer I used on some indoor concrete. Outdoors though you won't have to have much water standing on the surface before it'll freeze into ice thicker than the additive, and then there's no grip at all. I used a xylene-based sealer in case it seems relevant. But I'd expect if the hooves are grinding on the concrete much, they'll soon wear away the grip stuff and the sealer too in their favorite spots.