(a) Amendments to order
Subject to subsection (b), the Secretary may issue such amendments to an order as may be necessary to carry out this chapter.

(b) Amendment to assessment rates

(1) In general
The Secretary may conduct at any time a referendum of persons who, during a representative period as determined by the Secretary, have been fluid milk processors on adjusting the assessment rate under the order issued under this chapter then in effect, and shall hold such a referendum on request of the Board or any group of such processors that among them marketed during a representative period, as determined by the Secretary, 10 percent or more of the volume of fluid milk products marketed by all processors.

(2) Adjustment to assessment rate
The Secretary shall adjust the assessment rate under the order whenever the Secretary determines that the adjustment is favored—

(A) by at least 50 percent of fluid milk processors voting in the referendum; and

(B) by fluid milk processors that marketed during a representative period, as determined by the Secretary, 60 percent or more of the volume of fluid milk products marketed by all processors;

In no event shall the rate of assessment prescribed by the order exceed 20 cents per hundredweight.

(3) Effective date
The adjusted assessment rate shall be effective on a date, as determined by the Secretary, after the results of the referendum are known, but not later than 30 days after the referendum.

(4) Costs; manner
Subsections (c) and (d) of section 6413 of this title shall apply to a referendum conducted under this subsection.