The broodmares are outside 24/7 with option to use their run in shed, which they generally do. Their run in shed is mostly enclosed so it can get pretty toasty in there if most of them choose to stay inside. Also, their roundbales are in there anyway so they normally just munch in their all day long...still hoping one day they will learn to come outside to poop as that would make things SO much easier
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Then, most of my stalls in the barn have a small paddock enclosed to them so they can be inside their stall or they may be outside if they like. Generally i put the "hotter" horses in those stalls as they tend to get to worked up if stalled for any long period of time. I can close off their paddock to keep them inside but generally leave them be...i like to think they are smart enough to come inside if its to harsh outside
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Then, the other horses...like today when we had atleast near 2ft of snow on the ground with it still coming down hard, i just keep them in ..mostly as i dont want to trudge threw the snow and chase them down to bring them inside in this.
As long as there is some way they can get out of it if it gets to bad, you'll be good
Really, the only thing that worries me is when it gets icy and slippery out. The broodmares i dont worry about with the ice as much just because they are outside 24/7 and just tend to stand around...however, the show horses who are stalled, as soon as i turn them out they take off running and running and running so i worry about them slipping and possibly hitting the fence or going threw the fence, so if its very icy they stay in.