System and method of treating exhaust gas from an internal combustion engine using selective catalytic reduction (SCR) and adaptive control of diesel exhaust fluid (DEF) injection. Adaptive control of DEF injection includes intentionally underdosing the injected DEF based on an amount of DEF determined by an electronic control unit as an amount needed to reduce nitrogen oxides (NOx) to a compliance threshold. Since underdosing prevents ammonia (NH3) slip from occurring due to low levels of DEF, a sensor accurately senses NOx present in the exhaust gas at an output of an SCR chamber. An electronic control unit increases the amount of injected DEF based on the sensor.