Wis. Stat. § 604.05: Investments.

Assets of all funds under chs. 605 to 607 shall be invested by the state investment board under s. 25.17. Each January 1 the secretary of administration shall credit each fund with earnings on the invested assets in each fund for the preceding 12 months. If any fund is indebted to the general fund of the state, the fund shall be charged, at the end of each calendar year, with interest on the indebtedness at the average rate earned by the state upon its deposits in public depositories during the period of indebtedness and that sum shall be credited to the general fund.