POST combinations of photosystem II (PSII) and the 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors are effective for control of broadleaf weeds in corn. Field studies were conducted during 2007 and 2008 in Urbana and Dekalb, IL, to determine the nature of interactions between two PSII inhibitors, atrazine and bromoxynil, and the HPPD inhibitor mesotrione, based on control of common waterhemp, common lambsquarters, and giant ragweed. Two rates of each herbicide were evaluated, with the highest representing a typical recommended rate, and the lowest a fraction of that rate. Synergistic interactions occurred for common waterhemp control from 10 to 30 d after treatment (DAT) regardless of herbicide rates, rainfall accumulation, or plant height. Synergism between mesotrione and bromoxynil was not observed for common lambsquarters control at Urbana in 2008 at the lower herbicide rates, possibly due to taller weed heights at the time of herbicide application relative to 2007. Giant ragweed control indicated that a synergistic interaction occurred for all herbicides and rates in 2008. However, synergism between bromoxynil and mesotrione did not occur in 2007, likely due to limited rainfall before herbicide application. Reduced herbicide rates and adverse environmental conditions have the potential to regulate the expression of synergism between mesotrione and PSII inhibitors, and therefore may limit their effectiveness for weed management in corn.