S.D. Codified Laws § 47-21-57
Each member of a cooperative is entitled to one vote on each matter submitted to a vote at a meeting of the members. The spouse of a member may vote on behalf of the member unless the member has indicated otherwise. Members may vote by mail for any issue authorized by the bylaws of the cooperative, except the election of directors, purposes prohibited by § 47-21-20, or a vote required by § 47-21-45. If a cooperative's bylaws allow mail voting, they shall also prescribe any conditions or limitations of such voting. Member ballots voted by mail shall be returned before or at any membership meeting, and each ballot submitted counts as a member present and voting at a meeting for any quorum requirements of § 47-21-56 or a cooperative's bylaws. No cooperative member may vote by proxy on any issue.
SDCL 47-21-57