(c) The Secretary shall terminate the provisions of this part at the end of any fiscal period whenever it is found that such termination is favored by a majority of producers who, during a representative period, have been engaged in the production of Vidalia onions:

Provided, That such majority has, during such representative period, produced for market more than fifty percent of the volume of such Vidalia onions produced for market, but such termination shall be effective only if announced on or before June 15 of the then current fiscal period.