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The objectives of this study were to estimate the residual and half-life of [imazapic + imazapyr] and to infer on the impact of these residuals over time. The first experiment comprised the application of [imazapic + imazapyr] to Clearfield ® rice. On the following summer cropping season (365 days later), undeformed soil samples 0-5 cm depth were collected and seeds of six species or varieties were sown as bioindicators of residuals (experiment 2), being assessed plant height and dry mass 20 days after emergence start. The third experiment comprised the cultivation of the same species submitted to ten increasing herbicide doses (0-280 g ha -1 ) to establish standard response curves, also assessing plant height and dry mass 20 days after emergence start. About 2.1-5.8% of the applied imazapic remains in soil after one year, for the label doses. Imazapyr was considered to be at negligible doses as its half-life is short, and less than 0.0000001% of the applied dose is expected to be in soil 365 days later. The expected imazapic half-life in lowland areas of Southern Brazil is longer than for dryland, being estimated as between 63 and 77 days (95% confidence interval), contrasting to the 60 days half-life previously estimated for dryland soils.

Clearfield® technology is an important tool for chemical weed management in rice fields, however, carryover of these herbicides is still a concern for crops in succession. In this study, we aimed to evaluate the impact of the herbicide Kifix® (imazapic + imazapyr) associated with Clearfield® technology of rice crops on ryegrass in succession, as well as on non-Clearfield® rice and soybean grown in rotation, in addition to the control potential of jointvetch (Aeschynomene rudis and A. denticulata) from 2016 to 2018. The experiments comprised the application of Kifix® to Clearfield® rice grown in the first experimental year, evaluating its impact (1st summer) on ryegrass grown the following winter, as well as on soybean, rice and non-Clearfield® rice grown the following summer (2nd summer). The results reported reductions in jointvetch density and infestation as Kifix® doses increased; as well as mild damage from Kifix® residues on ryegrass sown in succession to Clearfield® rice, as well as on non-Clearfield® rice and soybean in rotation to Clearfield® rice. In addition, the possibility of residue accumulation of Kifix® in the soil after repeated applications should be verified in further studies. [ABSTRACT FROM AUTHOR]

Selection pressure caused by continuous use of imazapyr and imazapic herbicides in rice fields without crop rotation, has selected for resistant biotypes of Echinochloa crusgalli var. mitis. To know the mechanism of resistance involved becomes important in order to define effective management strategies. Therefore, the objective of this work was to verify if enhanced metabolism of herbicide is responsible for imazapyr and imazapic resistance in Echinochloa crusgalli var. mitis biotypes from Rio Grande (ECH38 resistant and ECH44 susceptible), Pelotas (ECH1 resistant and ECH14 susceptible) and Arroio Grande (ECH27 resistant) in the state of Rio Grande do Sul. For this purpose, forty-nine biotypes were collected from the southern region of the state in areas with barnyard grass control difficulties with ALS inhibitor herbicides. The treatments of study consisted of application of malathion (1000 g a.i. ha-1) and piperonyl butoxide (525 g a.i. ha-1), inhibitors of the cytochrome P450 mono-oxygenase enzymes and after 60 minutes, preceding herbicide application (73.5 + 24.5 g a.e. ha-1. Visual control evaluations were taken at 14 and 28 days after treatments (DAT) and shoot dry mass (SDM) determination at 28 DAT. The results indicate that the application of inhibitors does not alter the control response of the ECH38, but shows a significant increase in phytotoxicity in 28 DAT, with values of 78% for the ECH1. The use of Malathion associated with the herbicide provided a reduction in the SDM of 1.34 g.plant-1 in relation to the isolated application of herbicide and for piperonyl butoxide (PBO) of 1.28 g.plant-1 for the resistant biotype from Pelotas. For ECH27, the SDM reduction was lower than the values of 0.87 g.plant-1 for Malathion and 1.04 g.plant-1 for PBO. The resistance mechanism to imazapyr + imazapic herbicide in Pelotas and Arroio Grande biotypes is related to increased metabolism of the herbicide, and it cannot be stated that this mechanism is involved in the resistance of the ECH38 biotype. [ABSTRACT FROM AUTHOR]

Echinochloa species are one of the most troublesome weeds in rice crops, and their control is hampered due to herbicide resistance. The aim of this study was to identify the cross-resistance pattern and the differential resistance level for pre- and post-emergence applications of imazethapyr and penoxsulam in populations of barnyardgrass [ Echinochloa crus-galli (L.) Beauv.] with different ALS gene mutations. Of 26 biotypes, 23 were imazethapyr-resistant, and 10 were cross-resistant to penoxsulam. The resistance index (RI) to imazethapyr was 5.7–19.5 for Ser653Asn, 26.7–68.3 for Ala122Thr and Ala205Asn, and 70.9–252.9 for Trp574Leu. Only Trp574Leu also resulted in resistance to penoxsulam. The double ALS mutation Ala122Asn + Trp574Leu resulted in a RI for imazethapyr and penoxsulam higher than 2800. The ED 50 for penoxsulam applied at pre-emergence was three and six times lower than at post-emergence for the susceptible and resistant biotypes, respectively. The application of imazethapyr at pre-emergence was more effective than at post-emergence only for the biotypes with low resistance level mutation Ser653Asn. The efficacy of the herbicide quinclorac was similar for the application at pre- and post-emergence for susceptible and resistant biotypes. The ALS mutations Ala205Asn and Ala122Asn + Trp574Leu are reported for the first time in barnyardgrass. The use of ALS inhibitors at pre-emergence should be prioritized over post-emergence in fields with resistant barnyardgrass and in need of using these products to control other non-resistant weeds. [Display omitted] • Of 26 biotypes, 23 were imazethapyr-resistant, and 10 were cross-resistant to penoxsulam. • Ala122Thr, Ala122Asn, Ala205Asn, Trp574Leu, Ser653Thr, and Ser653Asn ALS mutations were found. • Penoxsulam applied at pre-emergence is up to six times more efficient than at post-emergence. • Imazethapyr is more effective at pre-emergence for only the low-level resistance mutations. • Timing of application should be considered for managing herbicide-resistant weeds. [ABSTRACT FROM AUTHOR]

The continuous use of herbicides with the same mechanism of action has caused the selection of resistant weed biotype that may present differences in fitness when compared with susceptible biotype. The objective of the present study was to identify and distinguish the fitness of barnyardgrass biotypes resistant and susceptible to imazapyr + imazapic herbicide under controlled and noncompetitive conditions. For that, there were used barnyardgrass biotype susceptible (ECH1 and ECH38) and resistant (ECH14 and ECH44) to ALS inhibitors, collected at Pelotas/RS and Rio Grande/RS cities, respectively, in experiment installed in a greenhouse in a completely randomized experimental design during the months from October to December 2015. The biotype were evaluated during the period of 15 to 120 days after the emergence related to plant height, foliar area, shoot dry matter mass and root dry matter mass. The results showed insignificant differences between the biotypes during the development of the plants. The susceptible biotype ECH44 showed a higher number of panicles per plant and seeds along with the resistant ECH1. There is variability among Echinochloa crusgalli var. mitis biotypes resistant and susceptible to ALS inhibitors, indicating an absence in fitness penalty caused by resistance when cultivated without the selection of pressure (herbicide). [ABSTRACT FROM AUTHOR]

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Abstract: During the 2006/07 and 2008/09 growing seasons in Southern Brazil, we evaluated crop selectivity and weed control efficacy of clomazone on rice when applied alone or in a mixture with other pre- and post-emergence herbicides (BRS Querência variety). All herbicide treatments caused some degree of rice injury during both years; however, in no case was the injury still visible 21 days after application. Rice injuries were observed when application rates exceeded the recommended dose, and particularly when the herbicide was applied pre-emergence. Density reduction and panicle sterility served as measures of rice injury. Echinochloa crus-galli was the primary weed; it reduced rice yields by approximately 50% compared treated plots. In both growing seasons, clomazone herbicide (400 g ai ha−1) controlled the weed 87.0%–99.6%, and it provided 8.06 t ha−1 to 9.44 t ha−1 of rice yield. [Copyright &y& Elsevier]

Florpyrauxifen-benzyl (FPB) is a new class of auxinic herbicide developed for selective weed control in rice. This study aimed to evaluate the effect of environmental conditions, P450 inhibitors, rice cultivar response, and gene expression on FPB selectivity in rice. Field experiments established in a randomized block design showed that rice plant injury due to two FPB rates (30 and 60 g ai ha−1) was affected by planting time and rice stage at herbicide application. The injury was higher at the earliest planting season and more in younger plants (V2) than larger (V6 and R0). However, no yield reduction was detected. Under greenhouse conditions, two dose-response experiments in a randomized block design showed that spraying malathion (1 kg ha−1) before FPB application did not reduce herbicide selectivity. The addition of two P450 inhibitors (dietholate and piperonyl butoxide, 10 g a.i. seed-kg−1 and 4.2 kg ai ha−1, respectively) decreased the doses to cause 50% of plant injury (ED50) and growth reduction (GR50). However, it seems not to compromise crop selectivity. BRS Pampeira cultivar showed lower ED50 and GR50 than IRGA 424 RI. A growth chamber experiment was conducted in a completely randomized design to evaluate the gene expression of rice plants sprayed with FPB (30 and 60 g ai ha−1). Results showed downregulation of OsWAKL21.2, an esterase probably related to bio-activation of FPB-ester. However, no effect was detected on CYP71A21 monooxygenase and OsGSTL transferase, enzymes probably related to FPB degradation. Further research should focus on understanding FBP bio-activation as the selective mechanism. [ABSTRACT FROM AUTHOR]