Pretty sure that link is obsolete. " But on October 1, 2012, the way certain FMCSRs apply to agricultural operations changed with the implementation of federal reauthorization legislation called Moving Ahead for Progress in the 21st Century."  

 http://modot.mo.gov/MOAgCarrier/jsp/moAgGloss/MacBookletMap21.html

Large Farm Vehicles
Farm-vehicle-drivers of vehicles with a gross vehicle weight rating of 26,001 pounds or larger driven anywhere in Missouri intrastate commerce or within 150 air-miles of their farm in interstate commerce are exempt from:

• The Federal Motor Carrier Safety Regulation regarding drug and alcohol testing, commercial driver license requirements, general driver qualifications and driver qualification files, including the DOT medical certification requirement, driver hours of service, and vehicle maintenance, repair, and inspection requirements. Drivers must still obey applicable state laws regarding age, valid license, etc.