An exhaust after-treatment system is described. Generally speaking, the system includes separate primary and secondary NOx reducing systems for delivering reductant to an exhaust stream, a sensor system for determining relevant operating conditions and an electronic control module for activating the reducing systems. Methods for reducing NOx in an exhaust stream are also described. Generally speaking, the methods include the steps of determining a need for NOx reduction in an exhaust stream, determining exhaust temperature, determining ambient temperature, and injecting at least one of a primary reductant and a secondary reductant into the exhaust stream based on the determined temperature. The system and method are particularly useful during cold start conditions, as well as lightly loaded application in conditions where the ambient temperature is below 45 F where the SCR catalyst may not fully function.