would agree to use topper, keep going over it until the old dead grass is cut up small pieces, it then needs a damn good chain harrowing probably 2 or 3 times to get the old dead grass out and let new growth come through. we had exactly the same with a field i took on previously grazed by horses that were taken away due to cruelty. Its now very good grazing, once you get the circle of grazing ,animal manure, it will soon come back.