in a pinch,  i've welded  them.    typically breaks at a  rivet hole,   making the  repair a  little more  complicated.      stress relieving with a ball peen hammer  is  good.

    if the bar is badly worn on the backside,   I'll probably pass and just  get a new bar..  depending how much crop is  in front of the  combine for that season. 
But,  I live near a  shop that makes  custom bars and  sickle assemblies.    I can get complete  assemblies or just components in less than  a  couple hours. 
I'd suspect  mud or dirt built up limiting the  arc-  or the wrong  end-guard(s).    The first  guard(s) also  has to be able to allow the  bar to arc  (move  fore and aft)  as the sickle  moves  back and  forth.      If  either is the case,  the binding effect will  snap a bar  fairly quick.