Beet pulp would be better than straw, but some straw might stretch the hay a bit and some horses positively love the stuff. Straw is a tough fiber- they won't get as much out of it nutrition-wise as hay, so you may see weight loss even if you let them eat their fill, especially if you have tough keepers or older horses with iffy teeth. Those ones would require individual supplementation to get them through the winter. You wouldn't be able to substitute it for hay, but it might help you stretch out the hay or cubes you can get until spring. Pretty sure many of the bagged 'hay stretcher' feeds have chopped straw as one of their ingredients.