Mo. Rev. Stat. § 550.160

No officer, appointee or employee holding a state, county, township or municipal office, including police officers and policemen, either by election or appointment, shall claim, be allowed or receive any fee or compensation as a witness for testifying before a coroner's inquest, grand jury, or in any criminal cases. All officers, appointees and employees as aforesaid, shall be compelled to attend the trial of all criminal cases, coroner's inquest and grand juries, when legally subpoenaed; provided, that the provisions of this section shall not apply to any officer who is a witness in any case where the residence of such officer is five miles from the place where the trial or coroner's inquest is held, or where the grand jury is in session.