Are there any sources of non-protein nitrogen in the cow feeding?

Yes and the amounts are highly controlled.  Non-protein nitrogen is simply nitrogen that is not incorporated into amino acids and protein.  The two biggest sources of non-protein nitrogen on our dairies are urea and haylage.  Particular quantities of non-protein nitrogen are necessary for rumen health but should not be overused.  It is converted to ammonia in the gut to be used by the rumen microbes to make microbial protein.