Also called beard burn. I hear it is caused by a fungus and usually ends up with a shortened beard with the individual "hairs" being a lighter brown in color right on the end.
About 5 years ago my then 14 or 15 year old son killed 3 mature gobblers in the first 5 days of the season. On the third one, on Wendnesday, he called me excited that he had killed #3 but a little disturbed too. I met him at home and he came in kind of whimpering, almost to tears. I said "son, what's wrong"? He sniffled and said "I didn't mean to kill a jake! I didn't want to kill a jake! I know I saw a full fan on this bird, but it's only got a 3" beard but it's got 1" spurs!" I said "son, it ain't a jake if it's got 1" spurs! It's got beard burn." Sure enough, nice mature gobbler, but with only a 3" beard! He felt better then! Guess that's what daddy's are for.