Mich. Comp. Laws § 38.1621

Sec. 21. The department shall at least annually allocate all or a portion of undistributed investment income to the individual balances in the reserve for employee contributions, the reserve for employer contributions, and the reserve for retired benefit payments and on the aggregate balance in the reserve for excess casualty experience. The amounts allocated shall be charged to the reserve for undistributed investment income. The allocation rates shall be determined by the department. Allocation rates may vary by reserve account but shall be uniformly applied to each subaccount within a reserve account.