183A.6  Assessment.
  The council shall make an assessment of not less than point zero zero two nor more than point zero zero three of the gross sale price of all porcine animals. The assessment shall be point zero zero two five of the gross sale price of porcine animals until consent to an assessment has been given through the initial referendum referred to in this chapter. After approval of the initial referendum, the rate of assessment shall be determined by the council. The assessment shall be made at the time of delivery of the animals for sale, and shall be deducted by the first purchaser from the price paid to the seller. The first purchaser, at the time of sale, shall make and deliver to the seller an invoice for each purchase showing the names and addresses of the seller and the first purchaser, the number and kind of animals sold, the date of sale, and the assessment made on the sale.
  Assessments shall be paid to the Iowa pork producers council or its designated agent by first purchasers at a time prescribed by the council, but not later than the last day of the month following the month in which the animals were purchased.
85 Acts, ch 199, §6; 86 Acts, ch 1100, §16