Prior to the promulgation of any rule or regulation contemplated by section 402 of the Act (7 U.S.C. 1592), notice shall be given by publication in the Federal Register of intention to promulgate such rule or regulation and of the time and place of a public hearing to be held with reference thereto. Such hearings shall be conducted by the Director or by such employee or employees of the Department of Agriculture as may be designated to preside thereat, except that hearings with respect to rules or regulations contemplated by section 402(b) of the Act relating to title III of the Act (Foreign Commerce), shall be conducted by the Secretary of the Treasury and the Secretary of Agriculture, acting jointly or separately, or by such employee or employees of the Department of Agriculture or the Department of the Treasury as may be designated to preside thereat. The presiding officer shall conduct the hearing in an orderly and informal manner, according to such procedure as he may announce at the commencement of the hearing. Any rule or regulation promulgated under section 402 of the Act shall become effective on the date fixed in the promulgation, which date shall be not less than 30 days after publication in the Federal Register. Any rule or regulation may be amended or revoked in the same manner as is provided for its promulgation.