A representative sample of vitamin D milk produced by each processor shall be obtained by the commissioner or by some person designated by him for such purpose at intervals of approximately six months, and may be procured more often if the commissioner deems necessary. The samples so obtained shall be delivered to the approved laboratory selected by the processor to be bio-assayed. The producer of the vitamin D milk shall in each instance bear the cost of such tests. In his discretion, however, the commissioner, if satisfied from accurate and continuous records showing the amount and type of concentrate added to milk and the quantity of vitamin D milk produced, may forego the procurement and the bio-assaying of one or more periodical samples.