Control of multiple-resistant Palmer amaranth populations in corn will rely heavily on the use of POST 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides. Therefore, field and greenhouse experiments were conducted to: (1) evaluate Palmer amaranth control with four HPPD inhibitors alone and in combination with atrazine at two application timings and (2) investigate the joint activity of HPPD-inhibiting herbicides and atrazine in atrazine-resistant (AR) and atrazine-susceptible (AS) Palmer amaranth populations. Control of the AR Palmer amaranth population varied among the HPPD-inhibiting herbicides with tolpyralate > tembotrione = topramezone > mesotrione based on GR₅₀ values in the greenhouse. In the field, Palmer amaranth control was lower when the HPPD-inhibiting herbicides, with the exception of tolpyralate, were applied to 15- vs. 8-cm-tall Palmer amaranth. Tolpyralate controlled Palmer amaranth ≥95% at both application timings. The addition of atrazine (560 g ai ha^-1) improved Palmer amaranth control with mesotrione and topramezone at the 8-cm application timing and with mesotrione and tembotrione at the 15-cm application timing. In the greenhouse, joint activity of mesotrione and atrazine and tembotrione and atrazine was synergistic with both the AR and AS Palmer amaranth populations. In the AR population, an additional 980 g ai ha^-1 of atrazine (8X) was needed to cause a synergistic response compared with the AS population. Synergistic responses with mesotrione were detected with all atrazine rates for the AS population and for atrazine rates ranging from 280 to 2,240 g ai ha^-1 for the AR population. Only additive responses were observed when atrazine was applied with tolpyralate and topramezone, indicating that joint activity in the form of synergism occurs more readily with the triketones compared with the benzopyrazoles. When faced with an AR Palmer amaranth population, the addition of atrazine to HPPD inhibitors may increase the overall success of weed management due to joint activity.

Nomenclature: Atrazine; mesotrione; tembotrione; tolpyralate; topramezone; Palmer amaranth, Amaranthus palmeri S. Wats.; corn, Zea mays L.