Iowa Code § 497.19: Additional dividends.

The remainder of said net profits shall be distributed by uniform dividends upon the amount of purchases of shareholders, and upon the wages and salaries of employees. In producing associations, such as creameries, canneries, elevators, factories, and the like, dividends shall be on raw material delivered instead of on goods purchased. In case the association is both a selling and a producing concern, the dividends may be on both raw material delivered and goods purchased by patrons.