Tolpyralate is a new Group 27 pyrazolone herbicide that inhibits the 4-hydroxyphenyl-pyruvate dioxygenase enzyme. In a study of the biologically effective dose of tolpyralate from 2015 to 2017 in Ontario, Canada, tolpyralate exhibited efficacy on a broader range of species when co-applied with atrazine; however, there is limited published information on the efficacy of tolpyralate and tolpyralate + atrazine relative to mesotrione and topramezone, applied POST with atrazine at label rates, for control of annual grass and broadleaf weeds. In this study, tolpyralate applied alone at 30 g ai ha^-1 provided >90% control of common lambsquarters, velvetleaf, common ragweed, Powell amaranth/redroot pigweed, and green foxtail at 8 weeks after application (WAA). Addition of atrazine was required to achieve >90% control of wild mustard, ladysthumb, and barnyardgrass at 8 WAA. Tolpyralate + atrazine (30 + 1,000 g ai ha^-1) and topramezone + atrazine (12.5 + 500 g ai ha^-1) provided similar control at 8 WAA of the eight weed species in this study; however, tolpyralate + atrazine provided >90% control of green foxtail by 1WAA.Tolpyralate + atrazine provided 18, 68, and 67 percentage points better control of common ragweed, green foxtail, and barnyardgrass, respectively, than mesotrione + atrazine (100 + 280 g ai ha^-1) at 8 WAA. Overall, tolpyralate + atrazine applied POST provided equivalent or improved control of annual grass and broadleaf weeds compared with mesotrione + atrazine and topramezone + atrazine.

Nomenclature: Atrazine; mesotrione; tolpyralate; topramezone; barnyardgrass, Echinochloa crus-galli (L.) P. Beauv. ECHCG; common lambsquarters, Chenopdium album L. CHEAL; common ragweed, Ambrosia artemisiifolia L. AMBEL; green foxtail, Setaria viridis (L.) P. Beauv. SETVI; ladysthumb, Persicaria maculosa Gray. POLPE; Powell amaranth, Amaranthus powelli S. Watson AMAPO; redroot pigweed, Amaranthus retroflexus L. AMARE; pigweed spp., Amaranthus spp., AMASS; velvetleaf, Abutilon theophrasti Medik. ABUTH; wild mustard, Sinapis arvensis L. SINAR; corn, Zea mays L