The U.S. Agriculture Department’s headquarters off the National Mall is closed today because of a fire in an adjoining structure, a spokesman said this morning.  Some 6,300 employees are on administrative leave. A small electric in the complex’s South Building occurred during routine maintenance early this morning, Justin DeJong said in an email. Firefighters responded, he said, and Potomac Electric Power Co. (Pepco) shut off electricity. Besides the South Building, the Jamie L. Whitten Building, which houses USDA Secretary Tom Vilsack’s office, and the Cotton Annex are closed, DeJong said, adding later that employees at the Reporters Building have also…