I have some spring barley following spring oats that does look inexplicably poor in places. We baled the oat straw and drilled stubble turnips but many volunteer oats grew and left a straw mat by spring time, trodden down by the sheep. We paraplowed and drilled the barley with the Undrill. The same Spring barley following wheat and turnips looks much better so I would assume the oat straw does have an allopathic effect. It was all limed on the stubbles before the turnips were drilled and the pH is OK.