I realize this probably won't weigh much with your decision as economics generally are the determining factor....however, the fact that hemlock is noted so strongly for splintering, would pose a concern for me with my horses.
I would prefer a material that did not splinter as badly. Horses will rub and chew and this could cause serious injury. We made a shelter out of cottonwood, Populus trichocarpa, specifically due to its properties of not splintering.
You may have already thought of this, but put the wood vertically as opposed to horizontally. That way when they do rub it will be crossgrain as opposed to with the grain; less potential for splintering. Mind the corners and possibly face them with a less splintering product for the vertical neck rubbing. And watch your edges so exposed surfaces do not pose an invitation to chew.
I have one horse who will leave everything alone and another that chews anything in sight...just have to figure out the individual.
Have fun this weekend.
Sylvia
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