The Secretary of Agriculture, prior to prescribing any term in any marketing agreement or order, or amendment thereto, relating to milk or its products, if such term is to fix minimum prices to be paid to producers or associations of producers, or prior to modifying the price fixed in any such term, shall ascertain the parity prices of such commodities. The prices which it is declared to be the policy of Congress to establish in section 602 title shall, for the purposes of such agreement, order or amendment, be adjusted to reflect the price of feeds, the available supplies of feeds, and other economic conditions which affect market supply and demand for milk or its products in the marketing area to which the contemplated marketing agreement, order, or amendment relates. Whenever the Secretary finds, upon the basis of the evidence adduced at the hearing required by section 608b of this title or this section, as the case may be, that the parity prices of such commodities are not reasonable in view of the price of feeds, the available supplies of feeds and other economic conditions which affect market supply and demand for milk and its products in the marketing area to which affect market supply and demand for milk and its products in the marketing area to which the contemplated agreement, order, or amendment relates, he shall fix such prices as he finds will reflect such factors, insure a sufficient quantity of pure and wholesome milk to meet current needs and further to assure a level of farm income adequate to maintain productive capacity sufficient to meet anticipated future needs, and be in the public interest. Thereafter, as the Secretary finds necessary on account of changed circumstances, he shall after due notice and opportunity for hearing, make adjustments in such prices.