To the best of my knowledge my agency (Forest Service) does not have an official dress code. Depending upon position some employees are required to wear uniforms (such as front desk personnel at visitor centers) and all employees are required to wear safety equipment for some tasks (such as nomex for firefighting, leather boots for flying in helicopters, etc.). Any dress code would need to be negotiated with the union for bargaining unit employees. Unofficially, field office generally have employees dressed very casually (jeans, etc.) but regional offices and our national headquaters tend towards more business attire.