Systems and methods are provided for controlling a selective catalyst reduction (SCR) system having a SCR reactor. A reductant slip set point indicative of a target slip is received as a control variable is to be utilized for maintaining an amount of a NOx composition included in a combustion gas below a selected limit. A signal transmitted by a sensor is received, and is indicative of the reductant slip downstream of the SCR reactor. An amount of a NOx reductant to be introduced to the SCR reactor is determined based, at least in part, on a relationship between the target reductant slip and the reductant slip downstream of the SCR reactor indicated by the signal. A control signal is transmitted to cause introduction of the amount of the NOx reductant to the SCR reactor to maintain the amount of the NOx composition in the combustion gas below the selected limit.