Little information is published related to seed germination and seedling emergence of Japanese foxtail, a troublesome annual grass weed widely distributed in winter wheat fields in China. Three Japanese foxtail populations were studied under laboratory and greenhouse conditions, to determine the effects of different environmental factors on seed germination or seedling emergence. Chemical control is absolutely necessary in integrated management, and efficacy of POST herbicides against different growth stages of Japanese foxtail was evaluated. Germination rate was 90% or more when temperature ranged from 5 to 25 C, with germination onset shortened as temperature increased. Light was not required for germination to occur. For pH values ranging from 5 to10 there was no effect on seed germination. Japanese foxtail seed germination was sensitive to osmotic stress and completely inhibited at an osmotic potential of -1.1 MPa. The ‘1513’ population of Japanese foxtail demonstrated tolerance to soil salinity, with 98% germination at 80 mM NaCl compared with 25 and 40% germination for populations ‘1532’ and ‘1544’, respectively. High amounts of crop residue (10 t ha−1) suppressed Japanese foxtail emergence 38 to 55%. Germination of seeds placed at 160 C for 5 min was completely inhibited for dry seeds, with a similar effect at 130 C for pre-soaked seeds. Seed burial in the soil from 0 to 4 cm had no effect on seedling emergence, but burial at 7 cm completely inhibited seedling emergence. POST herbicides mesosulfuron-methyl (13.5 g ai ha−1), clodinafop-propargyl (67.5 g ai ha−1), pyroxsulam (13.5 g ai ha−1), pinoxaden (67.5 g ai ha−1) and isoproturon (1125 g ai ha−1) reduced plant dry weight 80% or more when applied at three- to seven-leaf stage, but control declined with application at later growth stages. The information from this study helps to develop an integrated approach to Japanese foxtail management.

Two adjuvants, Triton 11 XE (Isoctyl phenyl polyethoxy ethanol) and urea were tank mixed with four herbicides, methabenzthiazuron [N-(benzothiazol-2-yl)N,N′-dimethyl urea] at 1.0 and 1.5 kg ai/ha, terbutryn [N-(1,1-dimethylethyl)-N′-ethyl-6-(methylthio)-1,3,5-triazine-2,4-diamine] at 0.50 and 0.75 kg/ha, metoxuron [N-3-chloro-4-methoxyphenyl)N,N-dimethylurea] at 1.0 and 1.75 kg/ha, and isoproturon [(N-(4-isopropylphenyl)-N,N-dimethylurea] at 0.50 and 0.75 kg/ha and applied to weeds in wheat. Metoxuron reduced weed dry matter by 90% over a 2-yr period. Isoproturon, terbutryn, and methabenzthiazuron reduced weed dry matter by 82%. Tank-mixing of Triton 11 XE at 0.5% (v/v) with metoxuron at 1.0 kg ai/ha, terbutryn at 0.5 kg/ha, isoproturon at 0.5 kg/ha, and methabenzthiazuron at 1.0 kg/ha reduced weed density and biomass by 52 to 67% and enhanced grain yield by 33% over herbicide alone. None of the herbicides had any significant interaction with adjuvants in terms of weed control and grain yield.

The response of littleseed canarygrass biotypes to isoproturon, pendimethalin, and diclofop-methyl was evaluated in India, in pot studies and the field during the winters of 1991 to 1992 and 1992 to 1993. Some biotypes of littleseed canarygrass were resistant to isoproturon but cross-resistance to pendimethalin and diclofop-methyl was not confirmed. The resistant biotype required a higher dose of diclofop-methyl than the susceptible biotype. Variations in the response of littleseed canarygrass biotypes were not due to isoproturon formulation. Resistant biotypes required 2 to 8 times more isoproturon than a susceptible biotype for the same level of control. Diclofop-methyl at 1.0 kg ai/ha applied at the 2- to 3-leaf stage of littleseed canarygrass in pot experiments and PRE pendimethalin at 1.5 kg ai/ha in field trials controlled resistant biotypes. Field surveys of the affected areas revealed that resistance in littleseed canarygrass is more prevalent in rice-wheat rotations compared to other crop sequences. Control of littleseed canarygrass with isoproturon dropped from 78 to 21% from 1990 to 1993.

Shortawn foxtail is a competitive annual grass weed widely spread in east, south-central, and southwest China and parts of the Yellow River basin. One shortawn foxtail population (JSQT-1) resistant to fenoxaprop was identified in Jiangyan, Jiangsu province. Whole-plant experiments determined that the resistant population conferred high-level resistance to fenoxaprop (93-fold), clodinafop (21-fold), sethoxydim (107-fold), mesosulfuron (41-fold), and pyroxsulam (12-fold); moderate-level resistance to haloxyfop (8-fold), clethodim (9-fold), and pinoxaden (8-fold), and no resistance to isoproturon. Molecular analyses confirmed that the Ile-1781-Leu mutation was present in the resistant population. A dCAPS marker was used to detect the Ile-1781-Leu mutation. All 97 plants of the resistant population analyzed were homozygous mutants at the 1781 position. Our study established the first case of fenoxaprop resistance in shortawn foxtail, determined cross resistance to other herbicides, and elucidated that the molecular basis of resistance resulted from, at least partly, an Ile to Leu mutation at amino acid position 1781 in the plastid ACCase.

American sloughgrass is a troublesome grass weed in winter wheat fields after rice in China. Mesosulfuron-methyl failed to control American sloughgrass in Danyang County in 2012. The purpose of this research was to determine the resistance level to mesosulfuron and other herbicides in American sloughgrass and to identify the molecular basis of resistance. Dose–response experiments indicated that this population was moderately resistant to mesosulfuron-methyl (7.6-fold) and pyroxsulam (6.0-fold), highly resistant to flucarbazone-sodium (20.3-fold), fenoxaprop-p-ethyl (565.0-fold), clodinafop-proargyl (19.5-fold), and pinoxaden (45.9-fold), and susceptible to isoproturon. Part of the acetolactate sythase (ALS) gene was cloned and sequenced to confirm the molecular mechanism of resistance to ALS-inhibiting herbicides. A Pro197Ser substitution was identified. This substitution is likely the molecular mechanism of resistance to mesosulfuron-methyl in the Danyang population in which it is cross-resistant to flucarbazone-sodium and pyroxsulam. This study established the first report of mesosulfuron-methyl resistance likely caused by a Pro197 substitution in American sloughgrass and a potential herbicide to control this resistant weed.

Overreliance on tribenuron has resulted in resistance evolution in water starwort. This study investigates the resistance mechanisms to tribenuron in water starwort populations from China. The cytochrome P450 monooxygenase (P450) inhibitor malathion increased tribenuron sensitivity in all populations. The decrease in the amount of herbicide dose that causes 50% growth reduction (GR50) for the sensitive (S) population JS24 and the resistant (R) populations JS16 and JS17 were 2.3-, 2.5-, and 4.1-fold, respectively. However, the GR50 values for the R populations were still much higher than those of the S population. This observation indicates that P450-mediated enhanced metabolism is one mechanism for resistance in water starwort. The glutathione-S-transferase (GST) activity could be induced by tribenuron for all tested populations. In particular, the GST activity of JS16 is inherently greater and is more rapidly induced than that of JS17 or JS24. Resistance attributed to mutant acetolactate synthase (ALS) alleles was identified by sequence analysis for each population. Pro197Ser substitution was detected in JS16 and JS17. Molecular markers were also developed to rapidly identify resistance as well as individuals carrying the specific Pro197Ser mutation in water starwort populations. The resistance patterns experiment revealed that the R populations exhibited different levels of resistance to pyrithiobac sodium salt, florasulam, pyroxsulam, and flucarbazone-Na; however, R populations were sensitive to imazethapyr, fluroxypyr-meptyl, 2,4-D butylate, isoproturon, and diflufenican. This study establishes that either one or at least two resistance mechanisms are involved in herbicide resistance in water starwort. Moreover, these mechanisms might contribute to the different levels of resistance to tribenuron among water starwort populations.

Littleseed canarygrass is a major weed of winter-season crops, although it is most dominant in wheat-growing regions in the Indo-Gangetic Plains of India, Pakistan, Nepal, and Bangladesh. Resistance in this species to photosystem II–inhibiting herbicide isoproturon was first recorded in 1992, and has since spread to several Indian states covering more than a million ha. Genetic studies and resistance characterization from multiple locations indicate independent evolution of resistance due to continuous use of isoproturon and monoculture rice–wheat-cropping system. Isoproturon-resistant biotypes were found cross-resistant to diclofop, but not to chlortoluron, which has the same mode of action as isoproturon. The isoproturon-resistance mechanism is metabolic degradation, mediated by P-450 monooxygenase enzymes. This type of resistance could become serious and lead to the evolution of multiple resistances to herbicides of different modes of action. Adoption of fenoxaprop-P, clodinafop, and sulfosulfuron in isoproturon-resistant areas since 1997 initially led to high yields, but resulted in a weed flora shift which eventually reduced yields and increased the cost of weed management. Although isoproturon recommendation has been withdrawn from rice–wheat cropping zones, resistance in littleseed canarygrass is spreading in other areas where isoproturon has been used for several years because it is inexpensive and has broad-spectum weed control. Management factors, such zero or minimum tillage, early planting after rice harvest, and alternative herbicides provide effective control of resistant biotypes. However, lower efficacy of these herbicides has been observed in the field, although multiple resistances have yet to be confirmed. Herbicide rotations, mixtures, and sequences are beneficial, but only in the short term. Improved cultivation practices are also helpful; however, no current single system is sustainable. An integration of tillage method, planting time, varietal selection, crop rotation, timing and method of herbicide application, optimum dose, and sanitation practices is crucial in managing herbicide-resistant littleseed canarygrass.

A diagnostic test (Syngenta Quick-Test, QT) used for testing grass weed survivors to herbicides in the field for resistance was evaluated. Cuttings from grass weeds were transplanted into pots to regenerate new leaves, then treated with herbicide. In greenhouse experiments, resistance of known herbicide-resistant blackgrass biotypes to the aryloxyphenoxypropanoate herbicides CGA 184927 and fenoxaprop-ethyl and to the phenylurea herbicide isoproturon was verified by the QT. The findings were similar to those for seedlings grown from seed. Rigid ryegrass from suspect resistant fields in South Australia was sampled and sent by post to Switzerland for QT analysis. Resistance was confirmed in less than 4 wk, which verified resistance as responsible for the field failures. The added features of the QT over current resistance tests suggest a likely fit for in-season testing of surviving weeds and possible follow-up action.

Herbicide-resistant weeds are a constraint to weed management in many cropping regions around the world. Of the numerous populations of weeds with resistance to herbicides, it appears that most have resistance due to an alteration to the target enzyme. Use of herbicides with alternative modes of action has relatively easily controlled these populations. In stark contrast are a much smaller number of populations with resistance due to increased rates of herbicide detoxification. These populations may be cross-resistant to herbicides with other modes of action. Such cross-resistance can severely compromise weed control because alternative herbicides may fail on their first use. It has proved extremely difficult to predict cross-resistance due to increased herbicide detoxification in weed populations and hence, difficult to provide adequate advice to growers on how to avoid or manage the problem. Most commonly, such cross-resistance has been selected by certain aryloxyphenoxypropanoate herbicides such as diclofop-methyl and phenylurea herbicides such as chlorotoluron and isoproturon; however, other herbicides can also act as selecting agents for this type of resistance. Illustrative examples from rigid ryegrass in Australia and blackgrass in Europe demonstrate the breadth of the problem and the magnitude of the effort required to understand increased herbicide detoxification as a resistance mechanism. Recent work has elucidated the genetic basis of cross-resistance in some populations, but this has so far not provided new predictive tools useful to growers. Despite more than a decade of research aimed at unraveling the complexities of cross-resistance due to increased herbicide detoxification, management of these cross-resistant populations remains a significant challenge.

In 1998, Indian regulatory agencies approved the registration of CGA 184927, MON 37500, and fenoxaprop for postemergence control of isoproturon-resistant littleseed canarygrass. Herbicides used in rice and wheat before 1998 were generally mixed with sand or urea and were applied by hand. Foliar pesticide spray applications consisted primarily of insecticides and fungicides that were applied to high-value crops. These pesticides were often sprayed to runoff with backpack sprayers that were equipped with single hollow-cone or flood nozzles. Applicators walked through the fields, swinging the wands in sweeping motions resulting in uneven pesticide distribution and overapplication. The newly registered postemergence herbicides were applied with the same equipment and in the same fashion. After these applications, control of littleseed canarygrass was strikingly inconsistent, and the growers blamed the lack of control on the manufacturers. It was later clear that basic understanding of the application techniques was lacking. In response to this, an application training workshop was organized and conducted in India and Nepal in 2000. The workshops focused on teaching the participants how to use and fabricate multiple-nozzle booms, the importance of flat-fan nozzles, calibration, drift avoidance, and applicator safety. To date, approximately 3,000 farmers, extension agents, scientists, and industry representatives have attended more than 30 workshops. The participants were unanimously enthusiastic about the value of the workshops. Although simplistic, the adoption of this technology will significantly decrease the amounts of herbicides applied and will increase efficacy and efficiency.