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Goosegrass [Eleusine indica (L.) Gaertn.] remains problematic for bermudagrass [Cynodon dactylon (L.) Pers.] turf managers due to the ineffective, selective control of mature plants with available postemergence herbicides and lack of sufficient residual activity from those herbicides to control seedling plants. Topramezone controls mature E. indica, but past efforts to suppress potential injury to bermudagrass turf have been inconsistent. We hypothesized that metribuzin at 210 g ai ha−1 in admixture with topramezone would improve bermudagrass tolerance while conserving mature E. indica control. In preliminary field studies, metribuzin mixed with topramezone at 1.2 or 2.5 g ae ha−1 applied twice at a 3-wk interval reduced bermudagrass injury and white discoloration compared with topramezone applied alone, but metribuzin did not safen bermudagrass to mesotrione. Topramezone at 3.7 g ha−1 plus 210 g ha−1 metribuzin applied twice at a 3-wk interval offered improved bermudagrass tolerance while it still controlled mature E. indica during 15 field and 2 greenhouse studies in Virginia. This program offered a 10-fold decrease in suprathreshold duration of white discoloration compared with topramezone alone at 6.1 g ha−1. Bermudagrass absorbed three times less radioactivity than E. indica at timings up to 48 h after treatment with [14C]topramezone. Bermudagrass also metabolized twice as much topramezone compared with E. indica at 48 h after treatment. Metribuzin reduced 14C absorption by approximately 25% in both species. These studies confirm the performance of a novel, low-dose topramezone plus metribuzin program for mature E. indica control in bermudagrass turf and suggest that selectivity between bermudagrass and E. indica to topramezone is due to differential absorption and metabolism. The fact that metribuzin reduces topramezone absorption in both species suggests that it may help reduce bermudagrass phytotoxic response to topramezone, but its role in altering selectivity between bermudagrass and E. indica may be due to other factors.

Palmer amaranth (Amaranthus palmeri S. Watson) resistant to atrazine [6‐chloro‐N‐ethyl‐N’‐(1‐methylethyl)‐1,3,5‐triazine2,4‐diamine] and 4‐hydroxyphenylpyruvate dioxygenase (HPPD)‐inhibiting herbicides was confirmed in a seed corn (Zea mays L.) production field in Nebraska, in 2014. Neither atrazine nor HPPD inhibitors (mesotrione [2‐(4‐mesyl2‐nitrobenzoyl)‐3‐hydroxycylohex‐2‐enone], tembotrione 2‐[2‐chloro‐4‐(methylsulfonyl)‐3‐[(2,2,2‐trifluoroethoxy) methyl]benzoyl]‐1,3‐cyclohexanedione, or topramezone [3‐(4,5‐dihydro‐3‐isoxazolyl)‐2‐methyl‐4‐(methylsulfonyl)phenyl](5‐hydroxy‐1‐methyl‐1H‐pyrazol‐4‐yl)methanone) applied post‐emergence were able to control resistant Palmer amaranth even at greater than label rates. However, their tank mixtures even at lower than the label rate provided more than 90% control under greenhouse and field conditions. The objectives of this study were to investigate the effect of atrazine on mesotrione absorption and translocation when tank mixed or vice versa in atrazine‐ and HPPD inhibitor‐resistant Palmer amaranth from Nebraska. Tank mixing commercial formulation of atrazine at 560 g ha−1 increased 14C‐mesotrione absorption to 51% compared to 39% with 14C‐mesotrione alone. However, 14C‐atrazine absorption or translocation was not affected by mesotrione at 26 g ha−1 in the tank mixture. Similarly, mesotrione did not affect the metabolism of 14C‐atrazine in resistant or susceptible plants when tank mixed compared to 14C‐atrazine applied alone. Increased absorption of mesotrione when tank mixed with atrazine could be one of the reasons of atrazine and mesotrione synergism besides their biochemical interaction in the atrazine‐ and HPPD inhibitor‐resistant Palmer amaranth biotype from Nebraska. Core Ideas Atrazine applied in tank‐mixture increased mesotrione absorption. Mesotrione applied in tank mixture did not affect atrazine absorption and translocation. Atrazine metabolism was not affected by mesotrione applied in tank mixture.

Investigating the effects of soil properties on herbicide persistence can aid in evaluating the carryover potential of herbicides in soil and the consequent injury risk to rotational crops. Laboratory incubation experiments were conducted to quantify the persistence of atrazine, mesosulfuron‐methyl, and topramezone in five regional soils under aerobic conditions at 23 °C. Additionally, mesosulfuron‐methyl persistence was tested at 7 °C, which is representative of regional average winter soil temperature. Herbicide half‐life was calculated with the logarithmic form of first‐order rate of degradation using linear regression and was correlated with soil properties. Half‐lives of atrazine (37–73 d) and topramezone (15–19 d) varied among soil types at 23 °C. Mesosulfuron‐methyl half‐life varied among soils at 7 °C (8.8–9.8 d) and 23 °C (5.4–5.8 d) and between temperatures. Atrazine and topramezone half‐lives were shortest in Candor sand (4% clay, 1.8% organic matter [OM], pH 5.1) and longest in Portsmouth sandy loam (13% clay, 5.3% OM, pH 4.3). Mesosulfuron‐methyl half‐life was longer at lower soil temperature. Half‐lives of atrazine, mesosulfuron‐methyl, and topramezone were correlated with soil OM content (r = .83, −.53, and .63, respectively) and pH (r = −.86, .55, and −.57). Additionally, atrazine and topramezone half‐lives were positively correlated with soil clay content (r = .83 and .71), and mesosulfuron‐methyl half‐life was negatively correlated with temperature (r = −.97). Correlations between soil OM content, clay content, and pH among soil types may have influenced herbicide persistence. Core Ideas Half‐lives of atrazine and topramezone varied among soil types at 23 °C. Half‐life of mesosulfuron‐methyl varied among soils and between 7 and 23 °C. Herbicide half‐life was correlated with soil OM content, clay content, pH, and temperature. Correlations between soil OM content, clay content, and pH may influence herbicide persistence.

Few options are available for controlling bermudagrass invasion of seashore paspalum. Bermudagrass and seashore paspalum tolerance to topramezone, triclopyr, or the combination of these two herbicides were evaluated in both greenhouse and field conditions. Field treatments included two sequential applications of topramezone (15.6 g ai ha-1) alone and five rates of topramezone + triclopyr (15.6 + 43.2, 15.6 + 86.3, 15.6 + 172.6, 15.6 + 345.2, or 15.6 g ai ha-1 + 690.4 g ae ha-1). Secondary greenhouse treatments included a single application of topramezone (20.8 g ha-1) or triclopyr (258.9 g ha-1) alone, or in combination at 20.8 + 258.9 or 20.8 + 517.8 g ha-1, respectively. Greenhouse and field results showed that topramezone applications in combination with triclopyr present opposite responses between bermudagrass and seashore paspalum. Topramezone increased bermudagrass injury and decreased seashore paspalum bleaching injury compared to topramezone alone. In field evaluations, topramezone + triclopyr at 15.6 + 690.4 g ha-1 used in sequential applications resulted in >90% injury to bermudagrass, however, injury decreased over time. Furthermore, sequential applications of topramezone + triclopyr at 15.6 + 690.4 g ha-1 resulted in >50% injury to seashore paspalum. Application programs including topramezone plus triclopyr should increase bermudagrass suppression and reduce seashore paspalum injury compared to topramezone alone. However, additional studies are needed because such practices will likely require manipulation of topramezone rate, application timing, application interval, and number of applications in order to maximize bermudagrass control and minimize seashore paspalum injury. Nomenclature: Topramezone; triclopyr; bermudagrass; Cynodon spp.; seashore paspalum; Paspalum vaginatum Sw

Goosegrass control options in bermudagrass are limited. Topramezone is one option that offers excellent control of mature goosegrass, but application to bermudagrass results in unacceptable symptoms of bleaching and necrosis typical of hydroxyphenylpyruvate dioxygenase inhibitors. Previous research has shown that adding chelated iron reduced the phytotoxicity of topramezone without reducing the efficacy of the herbicide, resulting in safening when applied to bermudagrass. Our objective was to examine additional iron sources to determine whether similar safening effects occur with other sources. Field trials were conducted in the summers of 2016 to 2018 (Auburn University). Mixtures of topramezone and methylated seed oil were combined with six different commercial iron sources, including sodium ferric ethylenediamine di-o-hydroxyphenyl-acetate (FeEDDHA), ferrous diethylenetriamine pentaacetic acid (FeDTPA), iron citrate, FeSO4, and a combination of iron oxide/sucrate/sulfate, some of which contained nitrogen. Bermudagrass necrosis and bleaching symptoms were visually rated on a 0% to 100% scale. Reflectance (normalized difference vegetation index) and clipping yield measurements were also collected. Application of FeDTPA and FeSO4 reduced symptoms of bleaching and necrosis when applied with topramezone. Other treatments that contained nitrogen did not reduce injury but did reduce bermudagrass recovery time following the appearance of necrosis. Inclusion of small amounts of nitrogen often negated the safening effects of FeSO4. The iron oxide/sucrate/sulfate product had no effect on bleaching or necrosis. Data suggest that the iron source had a differential effect on bleaching and necrosis reduction when applied in combination with topramezone to bermudagrass. Overall, FeSO4 and FeDTPA safened topramezone the most on bermudagrass. Nomenclature: Topramezone; goosegrass; Eleusine indica (L.) Gaertn.; bermudagrass; Cynodon dactylon (L.) Pers.

POST goosegrass and other grassy weed control in bermudagrass is problematic. Fewer herbicides that can control goosegrass are available due to regulatory pressure and herbicide resistance. Alternative herbicide options that offer effective control are needed. Previous research demonstrates that topramezone controls goosegrass, crabgrass, and other weed species; however, injury to bermudagrass may be unacceptable. The objective of this research was to evaluate the safening potential of topramezone combinations with different additives on bermudagrass. Field trials were conducted at Auburn University during summer and fall from 2015 to 2018 and 2017 to 2018, respectively. Treatments included topramezone mixtures and methylated seed oil applied in combination with five different additives: triclopyr, green turf pigment, green turf paint, ammonium sulfate, and chelated iron. Bermudagrass bleaching and necrosis symptoms were visually rated. Normalized-difference vegetative index measurements and clipping yield data were also collected. Topramezone plus chelated iron, as well as topramezone plus triclopyr, reduced bleaching potential the best; however, the combination of topramezone plus triclopyr resulted in necrosis that outweighed reductions in bleaching. Masking agents such as green turf paint and green turf pigment were ineffective in reducing injury when applied with topramezone. The combination of topramezone plus ammonium sulfate should be avoided because of the high level of necrosis. Topramezone-associated bleaching symptoms were transient and lasted 7 to 14 d on average. Findings from this research suggest that chelated iron added to topramezone and methylated seed oil mixtures acted as a safener on bermudagrass. Nomenclature: Topramezone; triclopyr; crabgrass; Digitaria spp.; goosegrass; Eleusine indica (L.) Gaertn.; bermudagrass; Cynodon dactylon (L.) Pers.

The control of multiple-resistant wild radish (Raphanus raphanistrum L.) populations in no-till Australian wheat (Triticum aestivum L.) crops has relied upon 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides over the last decade. Two R. raphanistrum populations identified as putatively resistant to pyrasulfotole + bromoxynil in an initial large-scale screening trial were characterized and confirmed to be 5- to 8-fold (comparison of LD50 values) less sensitive than the susceptible control population to the HPPD inhibitor pyrasulfotole when plants were treated at the 4-leaf stage. The two pyrasulfotole-resistant populations exhibited up to 4-fold resistance to the coformulated herbicide mixture pyrasulfotole + bromoxynil and up to 9- and 11-fold cross-resistance to mesotrione and topramezone postemergence, respectively. A small-plot trial was conducted in the field from which of one of the populations suspected of resistance was originally collected. Pyrasulfotole + bromoxynil or topramezone + bromoxynil applied postemergence delivered reduced R. raphanistrum control (79% to 87%), whereas mesotrione applied preemergence was >99% effective. We report here the first case of field resistance to HPPD-inhibiting herbicides in R. raphanistrum, caused by 12 yr of continuous reliance on that mode of action. The mitigation of herbicide resistance in continuous no-till cropping requires a constant optimization of the herbicide technology via alternation and mixtures of multiple sites of action, use of preemergence herbicides, and ensuring postemergence herbicides are applied at the most sensitive plant growth stages.

A limited number of herbicides and sites of action are registered for use on sugarcane in Louisiana. Repeated use of the same sites of action can lead to the evolution of herbicide resistance by weeds. Therefore, it is critically necessary to evaluate additional sites of action to provide growers with options for rotating herbicides to reduce the risk of resistance. Topramezone, indaziflam, and a formulation that includes mesotrione, bicyclopyrone, atrazine, and S-metolachlor, along with more common herbicides (pendimethalin, and metribuzin, clomazone, and diuron), were evaluated in the spring for injury to sugarcane, weed control, sugarcane yield, and sugar yield. Of these treatments, clomazone applied with diuron was the only herbicide combination to consistently injure the crop, with injury estimates ranging from 11% to 36%, which frequently resulted in reduced sugar yield with losses between 2.3% to 24.1% of the nontreated control. In most treatments, an increase in itchgrass counts was observed between harvests, indicating that additional control strategies will be needed in fields infested with this weed. However, topramezone alone and with triclopyr was well tolerated by sugarcane, with injuries ranging from 0% to 11% 2 wk after treatment. Indaziflam and combined application of mesotrione, bicyclopyrone, atrazine, and S-metolachlor injury was at or under 10% 2 wk after treatment. The tolerance of sugarcane for these herbicides suggests that they can be incorporated into weed management strategies in sugarcane production. These herbicides would increase the sites of action available to be applied to sugarcane and help mitigate the risk of herbicide-resistant weeds. Nomenclature: Atrazine; bicyclopyrone; clomazone; diuron; indaziflam; mesotrione; metribuzin; pendimethalin; S-metolachlor; topramezone; triclopyr; itchgrass; Rottboellia cochinchinensis (Lour.) W.D. Clayton; sugarcane; Saccharum spp. hybrids

The continued development of herbicide-resistant weeds, such as Palmer amaranth, represents a growing concern across the United States Cotton Belt. To mitigate this issue, BASF Corp. developed Axant™ Flex cotton, the first quadruple-stacked herbicide resistance germplasm to improve the control of troublesome weed species in cotton. Field studies were conducted in 2022 and 2023 at the Texas Tech University Research Farm near New Deal, TX, to evaluate the response of Axant Flex cotton to topramezone applied alone or in combinations when applied to three-leaf cotton (early-postemergence or EPOST) or to seven-leaf cotton (mid-postemergence or MPOST). No difference in cotton stand was observed between isoxaflutole or prometryn preemergence treatments compared to the nontreated control. In 2022, no EPOST treatment caused greater than 6% crop response at 7 and 14 d after application (DAA). When treatments were made to seven-leaf cotton, crop response did not exceed 18% at 7 and 14 DAA. In 2023, crop response was ≤2% at 28 DAA regardless of application timing. No differences in lint yield were observed following any herbicide treatment when compared to the nontreated control in either year. Additionally, fiber length and strength were not adversely affected by treatments containing topramezone EPOST or MPOST in 2022 and 2023. These results support the potential use of topramezone in Axant Flex cotton to help manage troublesome weeds without detrimental effects on yield and fiber quality. Nomenclature: Isoxaflutole; prometryn; topramezone; Palmer amaranth; Amaranthus palmeri S. Watson; cotton; Gossypium hirsutum L.

Glyphosate-resistant (GR) horseweed [Conyza canadensis (L.) Cronquist; syn.: 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 postemergence control of GR C. canadensis in corn. Four field trials were studied over 2 yr (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 wk 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.