An official certificate evidencing official inspection and designating the grade of potatoes inspected shall be issued by the appointed inspector or agent of the director to the person or persons for whom the inspection and grading service was completed. All inspections shall be made on the basis of the official grades established from time to time by the United States Department of Agriculture and such additional grades as may be duly adopted by the director; Provided, that when United States grades are used, they shall conform in all respects to the requirements and standards prescribed by the United States Department of Agriculture.