I've had really good luck soaking them in corn or vegetable oil for like 10 minutes, then currying them, then more oil, then a good strong bath. For some less intense ones, WD40 or Pink Oil works pretty well too.
Any type of oil works well to soft and release from hair shaft. We have used Ez-All the past few years and let it set and soak. It does a great job of loosening the manure and mud. Some folks have used fabric softener on difficult caked spot.
oil or just rinse...we leased a bull one time that had dingle-berries clear up on his back...i couldnt stand havin' somethin' that "dirty" in my field, lol...so we rinsed him...took about 4 hours of rinsing over two days, but we finally got him cleaned up...he went home lookin' way better than when we got him...