The Commissioner, upon written application by the owner or operator of certain vehicles used exclusively for hauling farm animal feed, shall issue overweight permits for operation of such vehicle. Permits shall be issued under this section only for specially designed five-axle semi-trailer combinations with bulk feed compartments and at least 51 feet of axle spacing between the first and last axle. Such permits shall not be combined with any other overweight permits or extension of weight limits. No permits issued under this section shall authorize a tandem axle weight of more than 37,400 pounds or a total gross weight in excess of 84,000 pounds. Permits issued under this section shall be valid for one year from the date of issuance. No permit issued under this section shall designate the route to be traversed or contain restrictions or conditions not applicable to other vehicles in their general use of the highways. However, no such permit shall authorize violation of any weight limitation applicable to bridges or culverts, as promulgated and posted in accordance with § 46.2-1130. Nothing contained in this section shall authorize any extension of weight limits provided in § 46.2-1127 for operation on interstate highways. The fee for a permit issued under this section shall be as provided in § 46.2-1140.1.