There is little information available on performance interactions for tank mixtures of topramezone and acetolactate synthase (ALS)-inhibiting herbicides. Controlled-environment and field experiments were conducted in 2006 and 2007 to determine the interactions of topramezone when tank-mixed with ALS-inhibiting herbicides. Controlled-environment experiments were conducted on four annual grass species treated at the five- to six-leaf stage. Dose–response curves for large crabgrass, barnyardgrass, yellow foxtail, and green foxtail were generated for nicosulfuron or foramsulfuron alone and in combination with label rates of topramezone or mesotrione (with or without atrazine). Eight field experiments were conducted using registered rates of two ρ-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting and three ALS-inhibiting herbicides alone and in combination. All herbicide treatments in the field were applied at the two- to three-leaf and five- to six-leaf stages of barnyardgrass, green foxtail, giant green foxtail, and witchgrass. In both the controlled environment and field experiments, antagonistic interactions were found to be species specific. In the controlled environment, nicosulfuron antagonized topramezone for the control of large crabgrass and barnyardgrass, but did not influence control of yellow or green foxtail. This antagonism was overcome with the addition of atrazine or an increased dose of nicosulfuron. Antagonism was not observed with tank mixtures of topramezone and foramsulfuron on the species tested under controlled-environment or field conditions. In the field, antagonism was not influenced by growth stage of the annual grasses. Antagonistic interactions were observed when topramezone was tank-mixed with nicosulfuron or nicosulfuron + rimsulfuron for the control of barnyardgrass and, to a lesser extent, giant green foxtail. Similar tank mixtures did not reduce control of green foxtail or witchgrass. HPPD-inhibiting herbicides are known to antagonize the activity of ALS-inhibiting herbicides for the control of annual grasses. This is the first report in the literature that an ALS-inhibiting herbicide can antagonize an HPPD-inhibiting herbicide. Thus, the chemistries of these herbicides exhibit a two-way antagonistic interaction.
Herbicide-resistant weeds are a growing concern globally; in response, new herbicide resistance traits are being inserted into crops. Isoxaflutole-resistant soybean [Glycine max (L.) Merr.] will provide a new mode of action for use in this crop. Ten experiments were conducted over a 2-yr period (2017, 2018) to determine herbicide interactions between isoxaflutole and metribuzin on soybean injury, weed control efficacy, and soybean yield on a range of soil types. Soybean leaf-bleaching injury caused by isoxaflutole was most severe at sites with higher levels of rainfall after application. Control of weed species with isoxaflutole (52.5, 79, and 105 g ai ha−1) and metribuzin (210, 315, and 420 g ai ha−1) differed by site based on amount of rainfall after application. At sites where there was sufficient rainfall for herbicide activation, isoxaflutole at all rates controlled common lambsquarters (Chenopodium album L.), Amaranthus spp., common ragweed (Ambrosia artemisiifolia L.), and velvetleaf (Abutilon theophrasti Medik.) >90%; metribuzin at all rates controlled Amaranthus spp. and witchgrass (Panicum capillare L.) >80%. Control of every weed species evaluated was reduced when there was limited rainfall after herbicide application. The co-application of isoxaflutole + metribuzin resulted in additive or synergistic interactions for the control of C. album, Amaranthus spp., A. artemisiifolia, A. theophrasti, Setaria spp., barnyardgrass [Echinochloa crus-galli (L.) P. Beauv], and P. capillare. Isoxaflutole and metribuzin can be an effective management strategy for common annual broadleaf and grass weeds in Ontario if timely rainfall events occur after herbicide application.
The future release of 'Balance GT' soybean, which is resistant to isoxaflutole and glyphosate, opens up the possibility for control of glyphosate-resistant (GR) Canada fleabane using HPPD-inhibiting herbicides (Group 27) in soybean. Field trials were conducted over two years to evaluate the dose response of an isoxaflutole plus metribuzin tank mix, as well as each chemical applied alone, to assess their response using Flint's adaptation of Colby's equation. Factorial experiments were performed in growth room and greenhouse environments to assess isoxaflutole versus glyphosate, isoxaflutole versus metribuzin, and isoxaflutole plus metribuzin versus glyphosate. Tank mixes of isoxaflutole plus metribuzin in a 1:4 ratio provided 80% control of GR Canada fleabane at a dose range between 420 (84 + 336) and 611 (122 + 489) g a.i. ha 1 at 8 WAA (weeks after application). Tank mixes achieved an 80% reduction in biomass at a dose range between 498 and 738 g a.i. ha 1 , while 80% reduction in density was obtained with doses from 96 to 423 g a.i. ha 1 , 8 WAA. With glyphosate as a constant tank partner, field treatments of isoxaflutole plus metribuzin were mostly synergistic with some analyses showing an additive response. When tested in the growth room, isoxaflutole plus glyphosate tank mixes indicate additivity in the majority of treatments on glyphosate-susceptible (GS) fleabane.Key words: additive, antagonistic, Canada fleabane, glyphosate, herbicide resistance, soybean, synergistic, yield.Résumé : L'homologation prochaine du soja 'Balance GT' résistant à l'isoxaflutole et au glyphosate ouvre la porte à la lutte contre la vergerette du Canada résistante au glyphosate (RG) par les herbicides qui inhibent la HPPD (groupe 27) chez le soja. Les auteurs ont effectué des essais au champ pendant deux ans afin d'évaluer la doseréponse d'un mélange d'isoxaflutole et de métribuzine ainsi que celle de chacun de ces herbicides en recourant à l'équation de Colby adaptée par Flint. À cette fin, ils ont réalisé des expériences factorielles dans une chambre de croissance et en serre, et comparé les effets de l'isoxaflutole à ceux du glyphosate, ceux de l'isoxaflutole à ceux de la métribuzine, et ceux du mélange isoxaflutole-métribuzine à ceux du glyphosate. Le mélange d'isoxaflutole et de métribuzine dans un rapport de 1:4 détruit 80 % de la vergerette du Canada RG à un taux d'application situé entre 420 (84 + 336) et 611 (122 + 489) grammes de matière active par hectare, huit semaines après l'application. Le mélange entraîne une diminution de 80 % de la biomasse à un taux d'application de 498 à 738 g de matière active par hectare, alors qu'on observe une baisse de 80 % de la densité avec des doses de 96 à 423 g de matière active par hectare, huit semaines après application. Lorsque le glyphosate fait partie du mélange, l'isoxaflutole et la métribuzine appliquées au champ fonctionnent en synergie, certaines analyses indiquant même que leurs effets s'additionnent. Lors des essais en chambre de croissance, les mélanges d'isoxaf...
Five field trials were conducted over a two-year period (2013, 2014) to determine the control of glyphosate-resistant (GR) giant ragweed with isoxaflutole (IFT) and metribuzin (MTZ) applied alone and in combination. Treatments were designed to assess the dose response of an IFT plus MTZ tank-mix as well as each chemical applied alone to classify the response using Flint's adaptation of Colby's equation. Two factor factorial experiments were performed in the growth room to ascertain the response of IFT versus glyphosate, IFT versus MTZ, and IFT plus MTZ versus glyphosate on single plants. Field experiments evaluated the control of GR giant ragweed with IFT plus MTZ in tank-mix in a 1:4 ratio. The rate of IFT plus MTZ for 80% control of GR giant ragweed at 4 and 8 weeks after application (WAA) was 518 (104 g a.i. ha −1 IFT + 414 g a.i. ha −1 MTZ) and 631 g a.i. ha −1 (126 g a.i. ha −1 IFT + 505 g a.i. ha −1 MTZ), respectively. A rate of 668 and 467 g a.i. ha −1 was required to reduce GR giant ragweed density and biomass by 80%, respectively. Field experiments evaluating the control of GR giant ragweed with tank-mixes of IFT plus MTZ, where glyphosate was a constant tank-mix partner, were mostly synergistic. However, the low tank-mix rate (52.5 + 210 g a.i. ha −1) had an additive response for GR giant ragweed biomass reduction. When tested in the greenhouse and growth room, glyphosate susceptible (GS) giant ragweed showed some antagonism with glyphosate and isoxaflutole tank-mixes at rates less than commercial field rates. GR giant ragweed showed an additive response across all treatments in the growth room. Greenhouse experiments evaluating IFT versus MTZ and IFT plus MTZ versus glyphosate revealed all tank-mix treatments to be synergistic at 2 WAA.
Isoxaflutole-resistant soybean is currently in development for commercialization in North America. Proposals to use isoxaflutole + metribuzin as the main herbicide tank-mixture raise concerns as there is limited grass control with these herbicides. Strategies are needed to improve grass control with isoxaflutole + metribuzin. Nine experiments were conducted over a two-year period (2017, 2018) to determine the efficacy of isoxaflutole + metribuzin (52.5 + 210 g a•i• ha −1) applied alone and co-applied with pendimethalin, dimethenamid-P, pethoxamid, pyroxasulfone or S-metolachlor applied preemergence (PRE). Comparisons were made with isoxaflutole + metribuzin at a low rate (52.5 + 210 g a•i• ha −1), medium rate (79 + 315 g a•i• ha −1) and a high rate (105 + 420 g a•i• ha −1). Eight weed species were evaluated including common lambsquarters, green and redroot pigweed, common ragweed, velvetleaf, green and giant foxtail, yellow foxtail, barnyardgrass and witchgrass. All herbicides were affected by amount of rainfall following application; less rainfall resulted in reduced weed control. The addition of pendimethalin, dimethenamid-P, pethoxamid, pyroxasulfone or S-metolachlor to the low rate ofisoxaflutole + metribuzin provided equivalent control of all weed species evaluated compared toisoxaflutole + metribuzin at the low, medium, or high rate.
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