If in such referendum a majority or more of the wheat, corn, grain sorghum, and oats producers who are eligible to participate and actually vote therein, shall vote in the affirmative and in favor of the levying and collection of the assessment proposed on such referendum, then such assessment shall be levied and collected in the manner hereinafter provided. All wheat, corn, grain sorghum, and oats producers who produce wheat, corn, grain sorghum, or oats in the crop year immediately preceding the referendum shall be eligible to participate in said referendum. Following the referendum and within 10 days thereafter the certified association shall canvass, tabulate and publicly declare and announce the results thereof. The amount of the assessment levied upon the sale of wheat, corn, grain sorghum, and oats shall not exceed $.02 per net bushel after deductions for foreign material on any wheat, corn, grain sorghum, and oats sold by the producers thereof.