Whenever the Secretary has reason to believe that any packer or swine contractor has violated or is violating any provision of this subchapter, he shall cause a complaint in writing to be served upon the packer or swine contractor, stating his charges in that respect, and requiring the packer or swine contractor to attend and testify at a hearing at a time and place designated therein, at least thirty days after the service of such complaint; and at such time and place there shall be afforded the packer or swine contractor a reasonable opportunity to be informed as to the evidence introduced against him (including the right of cross-examination), and to be heard in person or by counsel and through witnesses, under such regulations as the Secretary may prescribe. Any person for good cause shown may on application be allowed by the Secretary to intervene in such proceeding, and appear in person or by counsel. At any time prior to the close of the hearing the Secretary may amend the complaint; but in case of any amendment adding new charges the hearing shall, on the request of the packer or swine contractor, be adjourned for a period not exceeding fifteen days.