In order to insure that applicators of chemicals have sufficient scientific and practical knowledge in the use of chemigation, the director shall conduct training sessions directed toward thorough comprehension and knowledge of the safe use of chemigation or contract with the Cooperative Extension Service of the University of Nebraska to conduct such training sessions through its county extension agents and specialists in the state. If the department contracts for the training sessions, the Cooperative Extension Service shall be reimbursed for conducting the training sessions. The director shall issue a certificate acknowledging the satisfactory demonstration of competency to be determined by the director through the use of a written examination prepared and administered by the department. Each applicator's certificate, including such certificates issued prior to July 9, 1988, shall expire on January 1 of the fourth year after the date of issuance and shall be renewed upon the satisfactory completion of training and testing.