Section 61. Whoever sells, offers or exposes for sale any commercial feed or mixture thereof without complying with the requirements of sections fifty-two to sixty, inclusive, applicable thereto, or in violation of any rule or regulation promulgated thereunder, or whoever falsely marks, or labels any such feed or mixture, or impedes, or obstructs, or hinders the commissioner or any of his duly authorized agents in the discharge of the authority or duties conferred or imposed by any provisions of said section, shall be punished by a fine of not less than twenty-five dollars or more than one hundred dollars for the first violation, or not less than one hundred dollars or more than five hundred dollars for a subsequent violation. In all prosecutions involving the composition of a lot of commercial feed, a certified copy of the official analysis signed by the commissioner or his agent shall be accepted as prima facie evidence of the composition.