Whenever in any investigation under the provisions of this subchapter, or in any investigation instituted by petition of the stockyard owner, market agency, or dealer concerned, which petition is authorized to be filed, the Secretary after full hearing finds that any rate, charge, regulation, or practice of any stockyard owner, market agency, or dealer, for or in connection with the buying or selling on a commission basis or otherwise, receiving, marketing, feeding, holding, delivery, shipment, weighing, or handling, not in commerce, of livestock, causes any undue or unreasonable advantage, prejudice, or preference as between persons or localities in intrastate commerce in livestock on the one hand and interstate or foreign commerce in livestock on the other hand, or any undue, unjust, or unreasonable discrimination against interstate or foreign commerce in livestock, which is hereby forbidden and declared to be unlawful, the Secretary shall prescribe the rate, charge, regulation, or practice thereafter to be observed, in such manner as, in his judgment, will remove such advantage, preference, or discrimination. Such rates, charges, regulations, or practices shall be observed while in effect by the stockyard owners, market agencies, or dealers parties to such proceeding affected thereby, the law of any State or the decision or order of any State authority to the contrary notwithstanding.