You can't be a mouth breather as a competitive athlete.
THE BOTTOM LINE: Just breathe. It doesn’t matter whether you’re breathing out of your nose or your mouth while you exercise. When you’re working hard, you’ll have to breathe through your mouth to take in enough oxygen. But if concentrating on your breathing helps keep you calm or mentally focused while training and competing, go ahead. Just don’t force yourself to breathe out of your nose if it isn’t comfortable—it won’t improve your performance.
http://www.outsideonline.com/fitness/fitness-coach/Should-I-Breathe-Through-My-Nose-or-My-Mouth.html