select [year],[month],[rate]
from yourTable 
where datefromparts([year], [month], 01) between '2013-03-01' and '2014-05-01'

declare @year_from int=2013;
declare @month_from int=3;
declare @year_to int=2014;
declare @month_to int=5;

with cte (y,m)
  as (select @year_from
            ,@month_from
      union all
      select y+case when m>=12 then 1 else 0 end
            ,case when m>=12 then 1 else m+1 end
        from cte
       where y*100+m<@year_to*100+@month_to
     )
select a.[month]
      ,a.[year]
      ,a.rate
  from (select a.m as [month]
              ,a.y as [year]
              ,b.rate
              ,row_number() over (partition by a.y,a.m order by b.[year] desc,b.[month] desc) as rn
          from cte as a
               left outer join yourtable as b
                            on b.[year]*100+b.[month]<=a.y*100+a.m
       ) as a
 where a.rn=1
 order by a.[year]
         ,a.[month]