They will do a wonderful job of clearing if kept confined and forced to eat what is there in their confined area... As you found out, if there is stuff they prefer to eat that they have access to over the stuff that you want them the clear they will eat that preferred food first until it's all depleted...
I see it in my pastures, early spring and summer they make patch work out of the pasture, eating the stuff they like the most, checking off the list from good to bad... Some of the less desirable stuff is left to grow out of control to 4 or 6 feet tall by mid summer... But by the end of they year aka now, about 90% of my pasture is mowed down to about 6" the only thing left standing tall is a few thistle bushes that they won't touch and I need to be pull... The other stuff that was 4-6 feet that they ignored earlier this year is about half mowed down now and will be gone in the next month when I have to start feeding them hay for the winter... As long as the ground is snow free they will ignore the hay and continue to clear the pasture of anything left standing...
