A method includes providing: a selective catalytic reduction (SCR) catalyst disposed in an exhaust gas stream of an internal combustion engine, a reagent injector operationally coupled to the exhaust gas stream at a position upstream of the SCR catalyst, and a NOx sensor coupled to the exhaust gas stream at a position downstream of at least a first portion of the SCR catalyst. The method includes operating an extremum seeking controller to determine a first reagent injection amount corresponding to a predetermined slope of δNOx/δANR, the δNOx/δANR determined according to the NOx sensor, providing a reagent injection command in response to the first reagent injection amount, and injecting an amount of the reagent in response to the reagent injection command.