Abstract
The most effective adjuvant is often herbicide-specific and dependent on the target weed species. Tolpyralate is a 4-hydroxyphenylpyruvate dioxygenase-inhibiting herbicide that is usually tank-mixed with atrazine for improved weed control in corn (Zea mays L.). Previous research has reported that a methylated seed oil (MSO) adjuvant (MSO Concentrate®) enhances the efficacy of tolpyralate + atrazine on several annual broadleaf and grass weed species. The efficacy of tolpyralate can also be improved with the addition of bromoxynil instead of atrazine; however, there is no information in the peer-reviewed literature that tests the efficacy of tolpyralate + bromoxynil with different adjuvants. Therefore, four field trials were conducted in 2020 and 2021 in Ontario, Canada to evaluate the efficacy of tolpyralate + bromoxynil without an added adjuvant and with MSO Concentrate®, Agral® 90, Assist® Oil Concentrate, Carrier®, LI 700®, or Merge® on six annual weed species in corn. The adjuvants did not enhance control of wild mustard (Sinapis arvensis L.) or velvetleaf (Abutilon theophrasti Medik.) with tolpyralate + bromoxynil. At 8 weeks after application (WAA), common ragweed (Ambrosia artemisiifolia L.) control was improved with the addition of MSO Concentrate® or Merge® to tolpyralate + bromoxynil. At 8 WAA, all adjuvants enhanced the control of common lambsquarters (Chenopodium album L.) with tolpyralate + bromoxynil similarly except for Assist® Oil Concentrate; Merge® was a better adjuvant than Assist® Oil Concentrate for common lambsquarters control. At 8 WAA, control of barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] was enhanced with the addition of Assist® Oil Concentrate, Carrier®, MSO Concentrate®, or Merge® to tolpyralate + bromoxynil. Carrier®, MSO Concentrate®, and Merge® enhanced the efficacy of tolpyralate + bromoxynil on Setaria spp. at 8 WAA. Control of velvetleaf, wild mustard, common ragweed, Setaria spp., and barnyardgrass with tolpyralate + bromoxynil was similar with the six adjuvants evaluated at 8 WAA.
Highlights
Activator adjuvants such as oil adjuvants and surfactants can improve the efficacy of foliar-applied herbicides (Harbour et al, 2003; Langdon et al, 2020)
Nelson et al (1998) reported that control of common ragweed (Ambrosia artemisiifolia L.), common lambsquarters (Chenopodium album L.), and velvetleaf (Abutilon theophrasti Medik.) with imazethapyr was greater when methylated seed oil (MSO) was used instead of nonionic surfactant; with imazamox, only common ragweed control was greater with MSO than with nonionic surfactant
Langdon et al (2020) reported that velvetleaf control with tolpyralate + atrazine was enhanced with the addition of MSO Concentrate®; when glyphosate was co-applied with tolpyralate + atrazine, MSO Concentrate® did not improve velvetleaf control
Summary
Activator adjuvants such as oil adjuvants and surfactants can improve the efficacy of foliar-applied herbicides (Harbour et al, 2003; Langdon et al, 2020). Bunting et al (2005) documented that foramsulfuron + atrazine efficacy was similar on several weed species when applied with either MSO or crop oil concentrate; control of giant foxtail (Setaria faberi Herrm.), fall panicum (Panicum dichotomiflorum Michx.), and waterhemp [Amaranthus tuberculatus (Moq.) J. To improve the efficacy of tolpyralate + atrazine, it is recommended to add MSO Concentrate® to the spray solution (Anonymous, 2021). Langdon et al (2020) reported that the addition of MSO Concentrate® did not improve weed control when tolpyralate + atrazine was co-applied with glyphosate. Weed control efficacy of HPPD-inhibitors can be improved with the addition of bromoxynil to a similar or greater level as the addition of atrazine. Determining the most efficacious adjuvant to add to tolpyralate + bromoxynil could improve weed control with this tank-mix. The objective was to identify the most effective adjuvant to add to tolpyralate + bromoxynil for the control of four annual broadleaf and two annual grass weed species
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