Not later than 24 months after the effective date of the plan first issued under section 6003 of this title, the Secretary shall conduct a referendum among growers, grower-shellers, and importers, who during a representative period determined by the Secretary have been engaged in the production or importation of pecans, for the purpose of ascertaining whether growers, grower-shellers, and importers favor continuation, termination, or suspension of the plan.