The Secretary shall conduct a referendum among persons who, during a representative period determined by the Secretary, have been engaged in the production of cotton for the purpose of ascertaining whether the issuance of an order is approved or favored by producers. No order issued pursuant to this chapter shall be effective unless the Secretary determines that the issuance of such order is approved or favored by not less than two-thirds of the producers voting in such referendum, or by the producers of not less than two-thirds of the cotton produced during the representative period by producers voting in such referendum and by not less than a majority of the producers voting in such referendum.