This might be a fantastic idea. The starch content of oat straw is lower and the lignin content higher. Therefore the horses won’t like it as much. In addition, the manure becomes more yellow than brown. When I exercised polo ponies back in the 70’s in Calif that is all they fed the horses.

Here are 2 images. The 1st is for oat straw (4% protein) and the 2nd is for grass hay at 10% protein.