Trotter,
I think a few of these guys are hitting on the point....it's a bush hog and not a rockhog /w3tcompact/icons/smile.gif. I have lots of rocks on my place and they seem to reproduce every few months and every few months I find myself climbing off the tractor and removing them. It's aggravating but that's part of it. I've accidentally hit some and had to go back to the barn and resharpen. I imagine if I hit them and they were really embedded, it's possible I could cause damage as severe as yours. If you have a FEL, someone mentioned you can use it as an "early warning device". That's a good idea as long as your speeds are reasonably slow...which they should be if your hitting many rocks.