@corco2000 - I have built with straw bale (a house, not a barn) and the exterior finish on my exterior walls has been complete for about 1.5 years. Lime plaster, or "render" if you will, in the traditional 3-coat manner, plus an underlying discovery coat, all applied by hand. Total thickness, 1"-1.5" in most places. Actually, I have not yet applied limewash, but otherwise it is complete. I
should mention that my walls are also non-loadbearing, as you propose for yours. Mine have 4x4 posts embedded flush with the exterior surface at 4'-6' spacing, supporting a 4x6 equivalent beam.
SherwoodOwl is correct, of course, that no lime plaster has the same strength as concrete stucco, but in my experience it is as hard as you could need. I'll admit I'm not personally familiar with how hard a horse will kick, but casual contact of horses bumping around in the stable would absolutely NOT make a mess of the wall, as you fear. You would need an angry and determined horse to take the wall down. Believe me, if you were to kick my wall with all of your strength, it is your foot that would come away broken, not the plaster. For maximum strength, make sure that you pay maximum attention to detail with the construction. I don't see the need to strap-compress the whole wall, if I am understanding you correctly, but be sure to use sound, well-baled straw. If you chose a good source, they should be tight enough right off of the farm for use; mine were. Use good quality, well-aged lime putty, and wait as long as possible between coats (months, if possible) to allow maximum calcification of all layers. I had really good luck with my own wall: there was hardly a crack to be seen in any of the layers. This will also increase overall strength if you can achieve it.
Having said all that, I still think that using 3/4" plywood or OSB on the bottom 4' of the interior of your wall is an excellent recommendation. It will provide extra protection from the moist environment of the stall (which over the long run might prove a greater threat) AND it will save you a lot of time and money. There is a lot of labor involved in mixing and applying plaster by hand, believe me, and since aesthetics are not a concern, why not?! It should also make it much easier and safer for you to clean the stall - you could even hose down the plywood walls. If you design the barn with the load-bearing posts flush to the inside of the wall, or even proud of it by an inch or less, you could nail the plywood directly to them. If you have windows in the same wall, you might think about that carefully, though, since good straw bale construction usually sets windows flush with the outside surface to minimize the accumulation of rain, and this is most easily achieved by setting the posts flush to the outside as well (assuming that you design your posts to double as rough opening for the window frames).
But I would DEFINITELY advise that you still apply a single coat of plaster on the straw beneath the plywood to better seal it against moisture and insects. That is what I am doing on my interiors behind wherever cabinetry will be attached. Or, you could save time and effort and achieve the same result by stapling a vapor-permeable house wrap across the wall before attaching the plywood. If you go that route, be advised that my own research revealed that cheap house-wrap available at any lumber
yard is not best for straw bale construction. You have to identify and call the manufacturer to know, but if you do you will learn that the perm ratings are often much much lower than for the expensive, name brand house wraps.
Also, setting your wall atop a raised curb, as SherwoodOwl mentioned, is always good detail for straw bale construction. But I would advise that you build your curb from treated
wood, though, not concrete. Generally speaking, part of a curb's function is to keep your bales removed from touching concrete, since a concrete foundation will wick
water up, slowly but surely. That is why residential building codes usually require all wood in contact with a concrete foundation to be treated. A simple and standard curb design that I used is just a double sill of two treated 2x4's set apart to match the outer edges of the bale width, with the space between filled by loose gravel or perlite to create a capillary break. You might even bump up to 4x4s for a little extra height and protection, in anticipation of hosing out the stalls. If you are building the barn from scratch, space your J-bolts or other anchors appropriately for the double sill. If retrofitting an older barn, I guess you will have to go in from above with Redheads or similar screw-in foundation anchors.
I would very much like to hear as well from someone who has actually used bales for this type of building. No doubt they would know about additional considerations that are not occurring to us.
~ Matthew N., Southern transplant