Mich. Comp. Laws § 141.31
That from and after the passage of this act, before any county in the state of Michigan shall issue any bonds except serial bonds and bonds having a special security, the legislative body of such county shall pass a resolution providing for the levying and assessment in each year during the life of the bonds to be issued of a tax which shall raise annually a sufficient sum to pay the interest on said bonds as the same shall accrue and to pay the principal of said bonds at maturity. Said resolution may contain a proviso that the amount levied and assessed in any 1 year under the provisions of the resolution may be reduced by the amount of money earned by the sinking fund during the immediately preceding year. The county treasurer shall keep a separate and distinct account for each item of the sinking fund set aside and pledged to the retirement of each issue of county bonds.
MCL 141.31