In the event the first such referendum or any subsequent referendum is carried by the votes of two thirds or more of the eligible farmers participating therein and assessments in pursuance thereof are levied annually for the period set forth in the call for such referendum, then the agency conducting such referendum shall in its discretion have full power and authority to call and conduct during the third year of such first period or the last year of any subsequent period another referendum in which the farmers and producers of such agricultural commodity shall vote upon the question  of whether or not such assessments shall be continued for the next ensuing three years or continued for the next ensuing six years. (1947, c. 1018, s. 17; 1965, c. 1046, s. 2.)