No marketing order or amendment thereto directly affecting producers or producer marketing shall be issued unless the director determines (in accordance with any of the procedures described at RCW 15.65.160) that the issuance of such order or amendment is assented to or favored by producers within the affected area who during a representative period determined by the director constituted either (1) at least sixty-five percent by numbers and at least fifty-one percent by volume of production of the producers who have been engaged within the area of production specified in such marketing order in the production for market of the commodity specified therein, or who during such representative period have been engaged in the production of such commodity for marketing in the marketing area specified in such marketing order, or (2) at least fifty-one percent by numbers and at least sixty-five percent by volume of production of such producers: PROVIDED, That producers shall be deemed to have assented to or approved a proposed amendment order if sixty percent or more by number and sixty percent or more by volume of those replying assent or approve the proposed order in a referendum.