Given that the lumps are generally consolidated since delivery(both from the producer and deliverer), the likely assistance with a suitable round pronged fork is usually sufficient to break down after weathering, especially as it dries out after rainfall. If all else fails , the "lumps" could be spread along a grassed path and run down with a rotary mower, but Do put the grass box on!!
If you know you have horsemanure and the horse "eggs" are the issue, its not rotted enough and/or too dry to break down.
Composting will deal with that.