Following 15 yr of successful use, glyphosate failed to control a population of the widespread grass weed rigid ryegrass in Australia. This population proved to be resistant to glyphosate in pot dose-response experiments conducted outdoors, exhibiting 7- to 11-fold resistance when compared to a susceptible population. Some cross-resistance to diclofop-methyl (about 2.5-fold) was also observed. Similar levels of control of the resistant and susceptible populations were obtained following application of amitrole, chlorsulfuron, fluazifop-P-butyl, paraquat, sethoxydim, sirnazine, or tralkoxydim. The presence of glyphosate resistance in a major weed species indicates a need for changes in glyphosate use patterns.

The occurrence of glyphosate-resistant weeds has been reported after more than 20 yr of extensive use. Rigid ryegrass that evolved resistance to glyphosate was found in Australia and in California. Glyphosate-resistant rigid ryegrass plants were collected from northern California and selected through generations 8 and 5 to segregate the most resistant (R) and sensitive (S) biotypes. The eighth generation of R and the fifth generation of S biotypes survived 6.72 and died from 0.11 kg ae ha−1 glyphosate, respectively. The objectives of this study were to evaluate the role of metabolism in the observed resistance, to study the effect of glyphosate on the activity of 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS; EC 2.5.1.19), and to characterize the EPSPS gene in R and S rigid ryegrass. Neither quantitative nor qualitative difference was observed in the metabolism of 14C-glyphosate between the biotypes. Activity of constitutive EPSPS decreased more significantly in the S than R biotype in the presence of 5, 50, 500, and 5,000 µM glyphosate. Inhibition of 50% (I50) of the EPSPS activity by glyphosate was more than 90-fold in S compared to R biotype. Decreased EPSPS sensitivity in the R biotype appeared to be a major contributor to glyphosate resistance in rigid ryegrass from California. Fragments of the EPSPS gene containing 1,320 nucleotides were isolated from mRNA of S and R biotypes. A single nucleotide mutation from cytosine (C) to thymine (T) was identified at nucleotide 301 of the truncated EPSPS gene of the R biotype. This mutation changed the amino acid code from proline (Pro) to serine (Ser), which was similar to that reported for the glyphosate-resistant goosegrass from Malaysia and correlated with glyphosate insensitivity of EPSPS.

Glyphosate-resistant rigid ryegrass has been identified in California, but research has yet to elucidate the resistance mechanism. The objectives of this study were to examine the differences between sensitive and resistant rigid ryegrass in absorption and distribution of glyphosate, in vivo and in vitro absorption by chloroplasts, and shikimic acid accumulation after glyphosate treatment. Foliar absorption and distribution of 14C-glyphosate did not differ 1 to 3 d after treatment (DAT) between the susceptible (S) and resistant (R) biotypes. Absorption of 14C-glyphosate by isolated chloroplasts also did not differ between the S and R biotypes. After foliar application of 14C-glyphosate, chloroplasts were isolated from treated leaves from both biotypes. Accumulation of 14C-glyphosate in the chloroplasts did not differ between the two biotypes. Shikimic acid level increased significantly in the S biotype after treatment with glyphosate at 2.24 kg ai ha−1 to levels 10-fold greater than in the R biotype 11 DAT. Shikimic acid in the germination media at 2 to 5 mM did not affect seed germination of S and R biotypes but drastically decreased the length of coleoptiles of both at 5 DAT. Thus, biotype differences in sensitivity or metabolism of shikimic acid do not explain differences in sensitivity to glyphosate.

The efficacy of metolachlor on rigid ryegrass was determined under a no-till seeding system in an experiment conducted in 2004 and 2005 in South Australia. Metolachlor at a rate of 0.48 and 0.96 kg ai/ha was applied at 40 or 46 d (very early preplant, VEPP) and 20 or 23 d before crop sowing (early preplant, EPP), and at sowing (preplant, PP) in 2004 and 2005; the herbicide was incorporated by sowing. The control of rigid ryegrass was greater than 80% when metolachlor was applied PP; however, that application resulted in phytotoxic effects on emergence and yield of wheat; metolachlor was more phytotoxic when applied at 0.96 kg/ha than at 0.48 kg/ha. Metolachlor applied EPP provided 71 to 83% rigid ryegrass control, whereas VEPP application provided only 33 to 49% control. Reduction in wheat grain yield was not observed at these application times. This study indicates that metolachlor at 0.48 kg/ha could be safely applied around 20 d before crop sowing to selectively control rigid ryegrass in wheat.

The sharp decline in the area of lupin grown in Australia is partly attributed to the failure to control herbicide-resistant weeds in narrow-leaf lupin crops grown with the conventional 25-cm-wide row spacing. Growing lupin with wider row spacing allows for interrow weed control by nonselective herbicides using a sprayshield or physical methods. During 2003 to 2006, two experiments conducted at five sites evaluated the efficacy of interrow weed control techniques in narrow-leaf lupin crops grown in 55- to 65-cm-wide rows within the Western Australia wheatbelt. Interrow herbicides were applied POST using sprayshields, intrarow herbicides were banded on lupin rows at seeding, and interrow weeds were mowed using a garden mower. The main weed species at each site was rigid ryegrass, blue lupin, or wild radish. Paraquat plus diquat applied on the interrow of the lupin crop with sprayshields controlled up to 100% of weeds between rows, leading to increases in lupin grain yield in most of the sites. Glyphosate alone, a mixture of glyphosate plus metribuzin, and glyphosate followed by paraquat plus diquat also controlled interrow weeds, but did not increase lupin grain yield at any site. Thus, paraquat plus diquat is a better choice for interrow weed control in wide row lupin than glyphosate. Mowing did not improve weed control, but mowing followed by paraquat plus diquat increased lupin grain yield at one site. Regression models predicted that there was a strong relationship between weed biomass and lupin grain yield.

Herbicide rate cutting is an example of poor use of agrochemicals that can have potential adverse implications due to rapid herbicide resistance evolution. Recent laboratory-level studies have revealed that herbicides at lower-than-recommended rates can result in rapid herbicide resistance evolution in rigid ryegrass populations. However, crop-field-level studies have until now been lacking. In this study, we examined the impact of low rates of diclofop on the evolution of herbicide resistance in a herbicide-susceptible rigid ryegrass population grown either in a field wheat crop or in potted plants maintained in the field. Subsequent dose–response profiles indicated rapid evolution of diclofop resistance in the selected rigid ryegrass lines from both the crop-field and field pot studies. In addition, there was moderate level of resistance in the selected lines against other tested herbicides to which the population has never been exposed. This resistance evolution was possible because low rates of diclofop allowed substantial rigid ryegrass survivors due to the potential in this cross-pollinated species to accumulate all minor herbicide resistance traits present in the population. The practical lesson from this research is that herbicides should be used at the recommended rates that ensure high weed mortality to minimize the likelihood of minor herbicide resistance traits leading to rapid herbicide resistance evolution.

Experiments were conducted at Minlaton on the Yorke Peninsula of South Australia in 2004 and 2005 to study the effect of four different seeding systems and dinitroaniline herbicides on the seedling emergence and growth of rigid ryegrass in wheat. The seeding systems were low soil-disturbance discs (DayBreak and K-Hart) and high soil-disturbance tines (narrow point and Ribbon seeder); and the herbicides were oryzalin, pendimethalin, and trifluralin at rate of 0.72 kg ai/ha. The total seedling emergence of rigid ryegrass was higher in nontreated plots planted with the tines compared with the discs. In general, oryzalin was the least-effective herbicide in controlling emergence of rigid ryegrass. All herbicides were more effective in reducing the number of plants, spikes, dry matter, and seed production of rigid ryegrass in combination with tines than with discs. However, in the absence of herbicides, plant and spike numbers, plant dry weight, and seed production of rigid ryegrass were significantly lower where discs were used to sow, rather than tines. In the absence of herbicide, rigid ryegrass was less competitive with wheat under the disc-sown systems. The response of grain yield to herbicides was greater under the tine-sown systems than the disc-sown systems.

Glufosinate efficacy was examined in two major grass weed species, rigid ryegrass and sterile oat. Dose–response pot experiments under controlled environmental conditions showed that sterile oat was more successfully controlled by glufosinate than was rigid ryegrass. Glutamine synthetase was extracted from both species and assayed in vitro. Glufosinate readily inhibited glutamine synthetase activity in both species, indicating no differential sensitivity to the target enzyme. Thin-layer chromatography analysis of glufosinate showed no significant metabolism of glufosinate in either species. Absorption and translocation studies with 14C-glufosinate showed that the radiolabel was rapidly absorbed into the leaves of both species. However, translocation of radiolabeled glufosinate from the treated leaf to the meristematic regions was significantly greater in sterile oat, whereas translocation to the tip of the leaf was significantly greater in rigid ryegrass. This indicates that there is a difference in glufosinate distribution between the two species. It is likely that this difference in the distribution of glufosinate results in sterile oat being more easily controlled by glufosinate than is rigid ryegrass.

Glyphosate resistance was found in a rigid ryegrass population in northern California. A sample of the resistant plants were collected and grown under greenhouse conditions. The objective of this study was to evaluate glyphosate resistance in the progeny of the collected plants by recurrent selection, obtain the homozygous resistant and sensitive lines to establish dose-response curves, and to determine the inheritance of glyphosate resistance in rigid ryegrass. Diverse levels of resistance were observed in the first generation with survival of 89, 59, 45, and 9% from glyphosate at 1x, 2x, 4x, and 8x respectively, where x = 1.12 kg ha−1 isopropylamine salt of glyphosate. Clones of plants that died from 1x were allowed to produce seed and were further subjected to recurrent selection to generate the most sensitive plants (S lines), which died from 0.125x glyphosate. The most resistant plants (R lines) were generated from the survivors receiving 8x glyphosate. The ratio between I50 rates for the glyphosate resistant and the glyphosate sensitive plants was > 100-fold. The R and S lines were crossed reciprocally and F1 progeny of both (R × S) and (S × R) showed intermediate resistance. These survived up to 2x glyphosate. The F2 progeny were generated by intercrossing of F1 plants. The ratio of sensitive, intermediate, and resistant plants in the F2 population before the treatment of glyphosate at 0.125x followed by 8x was 1 : 16, 14 : 16, and 1 : 16 respectively, which corresponded to the Mendelian segregation ratio of two genes. The results indicated that the inheritance of glyphosate resistance in rigid ryegrass from California appeared to be nuclear, incompletely dominant, multigenic, and pollen-transmitted with no indication of maternal inheritance.

Selection by herbicides has resulted in widespread evolution of herbicide resistance in agricultural weeds. In California, resistance to glyphosate was first confirmed in rigid ryegrass in 1998. Objectives of this study were to determine the current distribution and level of glyphosate resistance in Italian ryegrass, and to assess whether resistance could be due to an altered target site. Seeds were sampled from 118 populations and seedlings were treated with glyphosate at 866 g ae ha−1. Percentage of survivors ranged from 5 to 95% in 54 populations. All plants from 64 populations died. One susceptible (S) population, four putatively resistant (R) populations, and one S accession from Oregon were used for pot dose–response experiments, shikimic acid analyses, and DNA sequencing. Seedlings were treated with glyphosate at eight rates, ranging from 108 to 13,856 g ae ha−1. Shoot biomass was evaluated 3 wk after treatment and fit to a log-logistic regression equation. On the basis of GR50 (herbicide rate required to reduce growth by 50%) values, seedlings from putatively R populations were roughly two to 15 times more resistant to glyphosate than S plants. Shikimic acid accumulation was similar in all plants before glyphosate treatment, but at 4 and 7 DAT, S plants from California and Oregon accumulated approximately two and three times more shikimic acid, respectively, than R plants. Sequencing of a cDNA fragment of the EPSPS coding region revealed two different codons, both of which encode proline at amino acid position 106 in S individuals. In contrast, all R plants sequenced exhibited missense mutations at site 106. Plants from one population revealed a mutation resulting in a proline to serine substitution. Plants from three R populations exhibited a mutation corresponding to replacement of proline with alanine. Our results indicate that glyphosate resistance is widespread in Italian ryegrass populations of California, and that resistance is likely due to an altered target enzyme.

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.

The inheritance and linkage of enhanced metabolism-based herbicide cross-resistance was examined in a multiple resistant population of rigid ryegrass. F1 hybrids between resistant and susceptible populations showed an intermediate response to all the four herbicides tested, with no indication of maternal inheritance. Segregation of F2 families fitted a single-gene model for resistance to simazine, chlorotoluron, and chlorsulfuron. But there was more than the expected mortality from the low dose of tralkoxydim. Segregating F2 populations were selected by high rates of each of the four herbicides to create selected F2 families. Analysis of the response of these families demonstrated that simazine resistance is linked to chlorotoluron resistance. No other herbicide resistances were linked. Therefore, in this population of rigid ryegrass, at least three separate genes are responsible for metabolism-based cross-resistance. This study shows that multiple herbicide resistance in rigid ryegrass is the result of accumulation of a number of different resistance genes.

In Australia, glyphosate resistance has been observed in rigid ryegrass in several states including New South Wales (NSW) and South Australia (SA). Several populations of glyphosate-resistant rigid ryegrass were analyzed for the inheritance of glyphosate resistance. Eight glyphosate-resistant populations were crossed to the same susceptible population to create first filial generation (F1) families. Individuals from the F1 families were subsequently treated with glyphosate. The response to glyphosate of F1 families from all eight crosses was more similar to the resistant parent than the susceptible parent. Within crosses, dose responses of reciprocal F1 families were not significantly different from each other, indicating glyphosate resistance is encoded on the nuclear genome in all eight populations. The level of dominance observed in dose–response experiments ranged from partial to total within the herbicide doses tested. F1 individuals from five of the populations were crossed with susceptible (S) individuals to create backcross (BC) populations. Most of the families from these BC populations segregated in a manner consistent with a single gene controlling glyphosate resistance. These results indicate that resistance is inherited as a single dominant allele in four out of the five glyphosate-resistant rigid ryegrass populations. Such information is vital in the development of management strategies for glyphosate resistance in Australia.

Continuous use of herbicides has triggered a phenomenon called herbicide resistance. Nowadays, herbicide resistance is a worldwide problem that threatens sustainable agriculture. A study of over a decade on herbicides in Iran has revealed that herbicide resistance has been occurring since 2004 in some weed species. Almost all the results of these studies have been published in national scientific journals and in conference proceedings on the subject. In the current review, studies on herbicide resistance in Iran were included to provide a perspective of developing weed resistance to herbicides for international scientists. More than 70% of arable land in Iran is given over to cultivation of wheat, barley, and rice; wheat alone covers nearly 52%. Within the past 40 years, 108 herbicides from different groups of modes of action have been registered in Iran, of which 28 are for the selective control of weeds in wheat and barley. Major resistance to ACCase-inhibiting herbicides has been shown in some weed species, such as winter wild oat, wild oat, littleseed canarygrass, hood canarygrass, and rigid ryegrass. With respect to the broad area of wheat crop production and continuous use of herbicides with the sole mechanism of action of ACCase inhibition, the provinces of West Azerbaijan, Tehran, Khorasan, Isfahan, Markazi, and Semnan are at risk of resistance development. In addition, because of continuous long-term use of tribenuron-methyl, resistance in broadleaf species is also being developed. Evidence has recently shown resistance of turnipweed and wild mustard populations to this herbicide. Stable monitoring of fields in doubtful areas and providing good education and training for technicians and farmers to practice integrated methods would help to prevent or delay the development of resistance to herbicides.

Surviving rigid ryegrass plants were collected from a cropping field at Pindar, Western Australia (population WALR 50), after inadequate control by glyphosate applied at the normal field rate. Plants were grown to maturity in pots and seeds were collected. Glyphosate dose–response experiments with known susceptible and resistant control populations confirmed the resistant status of the WALR 50 population. The glyphosate rate resulting in 5% mortality (LD50) and GR50 (the glyphosate rate required to reduce mean growth of individuals to 50% of the untreated control) values for this population were 1,069 and 217 g ae ha−1, respectively, corresponding to R:S ratios of 3.4 and 1.9 for mortality and growth. In addition, a novel root growth–based assay of glyphosate resistance was developed and validated, giving a root growth GR50 R:S ratio of 3.4. A resistance profile was established by assessing population-level survival of WALR 50 after applications at recommended rates of a range of herbicides commonly used for rigid ryegrass control in Australia. High levels of resistance to the acetolactate synthase (ALS)–inhibiting sulfonylurea herbicides chlorsulfuron and sulfometuron, moderate resistance to the acetyl coenzyme A carboxylase (ACCase)–inhibiting herbicide diclofop, and low levels of resistance to the imidazilinone herbicide imazethapyr were found. More detailed dose–response experiments confirmed resistance to chlorsulfuron, sulfometuron, and diclofop. In vitro enzyme-inhibition studies demonstrated that ALS resistance in WALR 50 is due to an insensitive target enzyme and that ACCase resistance is due to a nontarget site–based mechanism. WALR 50 is the first glyphosate-resistant weed population with confirmed resistance to ACCase- and ALS-inhibiting herbicides.

Glyphosate is widely used for weed control in the grape growing industry in southern Australia. The intensive use of glyphosate in this industry has resulted in the evolution of glyphosate resistance in rigid ryegrass. Two populations of rigid ryegrass from vineyards, SLR80 and SLR88, had 6- to 11-fold resistance to glyphosate in dose-response studies. These resistance levels were higher than two previously well-characterized glyphosate-resistant populations of rigid ryegrass (SLR77 and NLR70), containing a modified target site or reduced translocation, respectively. Populations SLR80 and SLR88 accumulated less glyphosate, 12 and 17% of absorbed glyphosate, in the shoot in the resistant populations compared with 26% in the susceptible population. In addition, a mutation within the target enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) where Pro106 had been substituted by either serine or threonine was identified. These two populations are more highly resistant to glyphosate as a consequence of expressing two different resistance mechanisms concurrently.

Glyphosate resistance was first discovered in populations of rigid ryegrass in Australia in 1996. Since then, glyphosate resistance has been detected in additional populations of rigid ryegrass and Italian ryegrass in several other countries. Glyphosate-resistant rigid ryegrass and Italian ryegrass have been selected in situations where there is an overreliance on glyphosate to the exclusion of other weed control tactics. Two major mechanisms of glyphosate resistance have been discovered in these two species: a change in the pattern of glyphosate translocation such that glyphosate accumulates in the leaf tips of resistant plants instead of in the shoot meristem; and amino acid substitutions at Pro 106 within the target site, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). There are also populations with both mechanisms. In the case of glyphosate resistance, the target site mutations tend to provide a lower level of resistance than does the altered translocation mechanism. Each of these resistance mechanisms is inherited as a single gene trait that is largely dominant. As these ryegrass species are obligate outcrossers, this ensures resistance alleles can move in both pollen and seed. Some glyphosate-resistant rigid ryegrass populations appear to have a significant fitness penalty associated with the resistance allele. Field surveys show that strategies vary in their ability to reduce the frequency of glyphosate resistance in populations and weed population size, with integrated strategies—including alternative weed management and controlling seed set of surviving plants—the most effective.

Rigid ryegrass and wild radish dominate and coexist throughout southern Australian dryland cropping regions. Widespread herbicide resistance in these species has led to adoption of diverse and complex integrated weed management practices, which require evaluation of their impact on farming systems. Therefore, a multispecies version of the bioeconomic model resistance and integrated management (RIM) has been developed to compare long-term economic and weed population outcomes of various integrated management scenarios. We have extended the original single-species ryegrass RIM model to include wild radish biology and additional weed management practices used to control this species. The multispecies model can be used to evaluate weed management scenarios for coexisting herbicide-resistant species by investigating the implications of different crop–pasture rotational sequences and of varying herbicide availability. Multispecies RIM shows that economic differences between the scenarios are not due to differences in weed densities but to differences in total weed control costs.