Many studies have documented the interaction between 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting and photosystem II (PSII)-inhibiting herbicides. Most studies have focused on the interaction between mesotrione and atrazine, with few studies characterizing the nature of the interaction between tolpyralate and atrazine. Therefore, five field experiments were conducted in Ontario, Canada over a three-year period (2019-2021) to characterize the interaction between three rates of tolpyralate (15, 30, and 45 g ai ha-1) and three rates of atrazine (140, 280, and 560 g ai ha-1) for the control of seven annual weed species in corn (Zea mays L.). Tolpyralate at 30 or 45 g ai ha-1 applied with atrazine at 280 or 560 g ai ha-1 controlled velvetleaf (Abutilon theophrasti Medik.), redroot pigweed (Amaranthus retroflexus L.), common ragweed (Ambrosia artemisiifolia L.), C. album (Chenopodium album L.), and wild mustard (Sinapis arvensis L.) >90% at 8 weeks after application (WAA). Tolpyralate and atrazine were synergistic at each rate combination for the control of A. theophrasti at 8 WAA. In contrast, A. retroflexus and S. arvensis control were additive with each rate combination at 8 WAA. At 8 WAA, C. album control was generally additive, but one rate combination was synergistic. Ambrosia artemisiifolia control at 8 WAA was synergistic with five rate combinations and additive with the other four. Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] control at 8 WAA was additive with seven of the rate combinations and synergistic with two of the rate combinations. Setaria spp. control at 8 WAA was synergistic with one more rate combination compared to E. crus-galli, but the two weed species shared the same synergistic rate combinations. This study concludes that extrapolation or broad classifications of the interaction between tolpyralate and atrazine would be inappropriate as the interaction can vary due to herbicide rate, weed species, and the response parameter analyzed.

The complementary modes of action of 4-hydroxyphenylpyruvate dioxygenase (HPPD)- and photosystem II (PSII)-inhibitors have been credited for the synergistic weed control improvement of several species. Recent research discovered that reactive oxygen species (ROS) generation and subsequent lipid peroxidation is the cause of cell death by the glutamine synthetase-inhibitor glufosinate. Therefore, a basis for synergy exists between glufosinate and HPPD-inhibitors, but the interaction has not been well reported. Four field experiments were conducted in Ontario, Canada in 2020 and 2021 to determine the interaction between HPPD-inhibiting (mesotrione and tolpyralate) and ROS-generating (atrazine, bromoxynil, bentazon, and glufosinate) herbicides on annual weed species control in corn (Zea mays L.). The ROS-generators were synergistic with the HPPD-inhibitors and provided ≥95% control of velvetleaf (Abutilon theophrasti Medik.), except for tolpyralate + glufosinate which was additive 8 weeks after application (WAA) and control was 87%. Tank-mixes of HPPD-inhibitors plus ROS-generators were synergistic for the control of common ragweed (Ambrosia artemisiifolia L.) except for tolpyralate + glufosinate which was antagonistic 8 WAA. Tolpyralate + glufosinate was antagonistic for the control of barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] and Setaria spp. 8 WAA. Common lambsquarters (Chenopodium album L.) control 8 WAA was synergistic and ≥95% with mesotrione plus atrazine, bromoxynil, or glufosinate and with tolpyralate plus bromoxynil or bentazon. Herbicide tank-mixes were generally additive for the control of wild mustard (Sinapis arvensis L.) 8 WAA except for the synergistic tank-mixes of tolpyralate plus atrazine or bromoxynil; however, each tank-mix provided 97-100% control of S. arvensis. Results from this study demonstrate that co-application of ROS-generators with mesotrione or tolpyralate controlled all broadleaf weed species >90% at 8 WAA with the exceptions of A. artemisiifolia and C. album control with tolpyralate + glufosinate. Mesotrione plus PSII-inhibitors controlled E. crus-galli and Setaria spp. 48-68 percentage points less than tolpyralate plus the respective PSII-inhibitor at 8 WAA; however, mesotrione + glufosinate and tolpyralate + glufosinate controlled the grass weed species similarly.

Tolpyralate is an herbicide that is usually mixed with atrazine for broad-spectrum weed control in corn. Previous research has provided information on the effective dose (ED) of tolpyralate applied alone and in a 1:33.3 mixture with atrazine; however, tolpyralate is commercially applied at a dose of 30 to 40 g ai ha−1 with a minimum of 560 g ai ha−1 of atrazine. Therefore, five field trials were conducted over 3 yr (2019 to 2021) to determine the ED of atrazine to complement 30 g ai ha−1 of tolpyralate to achieve 80%, 90%, and 95% control of seven weed species 2, 4, and 8 wk after application (WAA). Tolpyralate was applied alone and in a mixture with atrazine doses ranging from 50 to 2,000 g ai ha−1. At 8 WAA, the ED of atrazine for 95% control of velvetleaf, common ragweed, common lambsquarters, and wild mustard was below the minimum label dose of atrazine on the commercial tolpyralate label, ranging from 430 to 520 g ai ha−1, which supports the use of the minimum label dose of atrazine. In contrast, redroot pigweed required 1,231 g ai ha−1 of atrazine to complement tolpyralate for 95% control 8 WAA. At 8 WAA, barnyardgrass and a mixture of green foxtail and giant foxtail (Setaria spp.) were not controlled by 80%, 90%, or 95% with tolpyralate applied alone or co-applied with any dose of atrazine evaluated in this study. The results of this study conclude that tolpyralate + atrazine is highly efficacious on several weed species at atrazine doses of 40 to 130 g ai ha−1 below the label dose of 560 g ai ha−1, but the use of the higher dose of tolpyralate or another herbicide may be required to improve control of redroot pigweed and grass weed species.

Glyphosate-resistant (GR) horseweed [Conyza canadensis (L.) Cronquist); synonym Erigeron canadensis L. interference can substantially reduce corn (Zea mays L.) yield. The complementary activity of 4-hydroxyphenylpyruvate dioxygenase (HPPD)- and photosystem II (PSII)-inhibitors has been investigated for the control of several weed species, and in many cases has been synergistic; however, there is little information on the interaction of HPPD- and PSII-inhibiting herbicides for POST control of GR C. canadensis in corn. Four field trials were studied over a two-years (2019, 2020) in Ontario, Canada in commercial corn fields with natural infestations of GR C. canadensis to evaluate the level of GR C. canadensis control with three HPPD-inhibiting herbicides (mesotrione, tolpyralate, and topramezone) and three PSII-inhibiting herbicides (atrazine, bromoxynil, and bentazon) applied individually and in tank-mix combinations, and to document the interaction of the three HPPD-inhibitors tank-mixed with the three PSII-inhibitors. Mesotrione, tolpyralate, and topramezone controlled GR C. canadensis 83, 84, and 72%, respectively, at 8 weeks after application (WAA). Bromoxynil and bentazon controlled GR C. canadensis 71 and 79%, respectively, while atrazine provided only 31% control at 8 WAA. The joint application of atrazine, bromoxynil, or bentazon with mesotrione increased GR C. canadensis control from 83% to 100% at 8 WAA. Tolpyralate tank-mixed with atrazine, bromoxynil, or bentazon controlled GR C. canadensis 96, 98, and 98%, respectively, which was comparable to the mesotrione tank-mixes at 8 WAA. Topramezone plus atrazine, bromoxynil, or bentazon controlled GR C. canadensis 91, 93 and 95%, respectively at 8 WAA. Interactions between HPPD- and PSII-inhibitors were synergistic for all combinations of mesotrione or tolpyralate with atrazine, bromoxynil, or bentazon. The interaction between topramezone and PSII-inhibitors was additive. All nine tank-mixes controlled GR C. canadensis >90%. This study concludes that bromoxynil or bentazon, instead of atrazine, can be co-applied with mesotrione, tolpyralate, or topramezone without compromising GR C. canadensis control in corn.

Tolpyralate is a 4-hydroxyphenylpyruvate dioxygenase-inhibiting herbicide that is applied postemergence for control of annual broadleaf and grass weeds in corn. Current Canadian label recommendations for tolpyralate specify the addition of a methylated seed oil (MSO) adjuvant (MSO Concentrate®) for improved weed control. The efficacy of tolpyralate applied with other proprietary adjuvants has not been widely reported in the peer-reviewed literature. Therefore, four field trials were conducted in corn over 2020 and 2021 in Ontario, Canada, to evaluate MSO Concentrate®, Agral® 90 (nonionic surfactant), Assist® Oil Concentrate (blended surfactant), Carrier® (blended surfactant), LI 700® (nonionic surfactant), and Merge® (blended surfactant) as adjuvants with tolpyralate for the control of annual broadleaf and grass weeds. At 8 weeks after application (WAA), tolpyralate applied with MSO Concentrate®, Agral® 90, Assist® Oil Concentrate, Carrier®, or Merge® controlled velvetleaf, wild mustard, barnyardgrass, and foxtail species similarly. These adjuvants also enhanced the efficacy of tolpyralate similarly for the control of common ragweed at 8 WAA with the exception that Agral® 90 was inferior to Merge®. At 8 WAA, tolpyralate controlled common lambsquarters the greatest when applied with MSO Concentrate®, Agral® 90, Carrier®, or Merge®; these adjuvants with the exception of Agral® 90 were superior to Assist® Oil Concentrate. At 8 WAA, tolpyralate applied with LI 700® controlled common ragweed, barnyardgrass, and foxtail species lower than when tolpyralate was applied with the other adjuvants tested; control of these weed species with tolpyralate was not improved with LI 700® when compared to tolpyralate applied without an adjuvant. Overall, tolpyralate applied with either MSO Concentrate®, Carrier®, or Merge® controlled all annual broadleaf and grass weed species similarly or greater than tolpyralate applied without an adjuvant or tolpyralate with Agral® 90, Assist® Oil Concentrate, or LI 700® at 8 WAA.