7 U.S.C. § 1423

(a) In general The Secretary may make appropriate adjustments in the support price for any commodity (excluding cotton) for differences in grade, type, quality, location and other factors. The adjustments shall, so far as practicable, be made in such manner that the average support price for the commodity will, on the basis of the anticipated incidence of such factors be equal to the level of support determined as provided in this Act. Beginning with the 1991 crops of wheat, feed grains, and soybeans for which price support is provided under this Act, the Secretary shall establish premiums and discounts related to cleanliness factors in addition to any other premiums or discounts related to quality. (b) Adjustment in support prices for cotton The Secretary may make appropriate adjustments in the support price for cotton for differences in quality factors and location. Beginning with the 1991 crop, the quality differences (premiums and discounts for quality factors) for the upland cotton loan program shall be established by the Secretary by giving equal weight to (1) loan differences for the preceding crop, and (2) market differences for such crop in the designated United States spot markets. (c) Limitation on adjustments for wheat and feed grains Notwithstanding any other provision of this section, for each of the 1990 through 1995 crops of wheat and feed grains, no adjustment in the loan rate applicable to a particular region, State, or county for the purpose of reflecting transportation differentials may increase or decrease the regional, State, or county loan rate from the level established for the previous year by more than the percentage change in the national average loan rate plus or minus 3 percent.