I think always having Know (Nature) as a class skill is probably more useful than getting +2 to it. The race is martially focused, and few warriors would even put ranks in Know Nature to make use of the bonus (can't make Knowledge checks untrained so a +2 bonus might as well be a +0 bonus if you have no ranks in the skill).

I would suggest that Brenin barbarians get +1 round of rage per day as their alternate favored class ability. If you want them to be barbarians, you should help offset their racial penalty to Con (which determines their rounds of rage per day).