Two field experiments were conducted during 2018 at Paskeville and Arthurton, South Australia, to identify effective herbicide options for the control of thiocarbamate-resistant rigid ryegrass in wheat. Dose–response experiments confirmed resistance in both field populations (T1 and A18) of rigid ryegrass to triallate, prosulfocarb, trifluralin, and pyroxasulfone. T1 and A18 were 17.9- and 20-fold more resistant to triallate than susceptible SLR4. The level of resistance detected in T1 to prosulfocarb (5.9-fold) and pyroxasulfone (4-fold) was lower compared to A18, which displayed 12.1- and 7.8-fold resistance to both herbicides, respectively. Despite resistance, the mixture of two different preplant-incorporated (PPI) site-of-action herbicides improved rigid ryegrass control and wheat yield compared to a single PPI herbicide only. Prosulfocarb + triallate and prosulfocarb + S-metolachlor + triallate did not reduce rigid ryegrass seed set when compared to prosulfocarb applied alone at the higher rate (2,400 g ai ha–1). Pyroxasulfone + triallate PPI followed by glyphosate (1,880 g ai ha-1) as a weed seed set control treatment reduced rigid ryegrass seed production by 93% and 95% at both sites, respectively. These herbicides also significantly improved grain yield of wheat at Paskeville (22%) and Arthurton (38%) compared to the untreated.

The effect of the organophosphate insecticide malathion on the response of resistant rigid ryegrass population SLR 31 to the herbicides trifluralin, pendimethalin, clomazone, and triallate was investigated. The insecticide and herbicides were soil-applied prior to emergence of plants grown under controlled conditions. In the absence of malathion, the resistant population exhibited significant resistance to the four herbicides compared with a susceptible population. Levels of resistance, as determined by comparison of herbicide rates required to inhibit growth by 50%, were 35, 11, 2.4, and 2.4 for pendimethalin, trifluralin, triallate, and clomazone, respectively. Malathion had a synergistic effect on pendimethalin in the resistant population but not in the susceptible population. Malathion had no synergistic effect on trifluralin, triallate and clomazone. Resistance to triallate and clomazone was found despite the fact that the resistant population had never before been selected with these herbicides. This resistance, selected by other herbicides, further indicates that the use of alternative herbicides to control multiple resistant weeds is unlikely to be a successful resistance management approach.

Harvest weed seed control (HWSC) systems have been developed to exploit the high proportions of seed retained at maturity by the annual weeds rigid ryegrass, wild radish, bromegrass, and wild oats. To evaluate the efficacy of HWSC systems on rigid ryegrass populations, three systems, the Harrington Seed Destructor (HSD), chaff carts, and narrow-windrow burning were compared at 24 sites across the western and southern wheat production regions of Australia. HWSC treatments were established at harvest (Nov. – Dec.) in wheat crops with low to moderate rigid ryegrass densities (1 to 26 plants m−2). Rigid ryegrass counts at the commencement of the next growing season (Apr. – May) determined that HWSC treatments were similarly effective in reducing emergence. Chaff carts, narrow-windrow burning, or HSD systems act similarly on rigid ryegrass seed collected during harvest to deliver substantial reductions in subsequent rigid ryegrass populations by restricting seedbank inputs. On average, population densities were reduced by 60%, but there was considerable variation between sites (37 to 90%) as influenced by seed production and the residual seedbank. Given the observed high rigid ryegrass seed production levels at crop maturity it is clear that HWSC has a vital role in preventing seedbank inputs in Australian conservation cropping systems.

Glyphosate is a broad-spectrum herbicide that has been used extensively for more than 20 yr. The first glyphosate-resistant weed biotype appeared in 1996; it involved a rigid ryegrass population from Australia that exhibited an LD50 value approximately 10-fold higher than that of sensitive biotypes. We have characterized gene expression levels and glyphosate sensitivity of 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS), the target enzyme for glyphosate inhibition, in sensitive and resistant lines derived from this population. Restriction fragment length polymorphism analyses were also performed to examine the distribution of EPSPS gene variants and the gene copy number. A two- to threefold increase in basal EPSPS messenger RNA (mRNA) and enzyme activity levels was observed in the most resistant lines analyzed; however, differences among lines in the sensitivity of EPSPS to glyphosate were not apparent. Induction of EPSPS was observed within 48 h after application of 1.5 kg ae ha−1 of glyphosate. This was reflected in elevated levels of both EPSPS mRNA and enzyme activity. Similarly, 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase mRNA levels increased after glyphosate treatment; however, basal and induced transcript levels were comparable for sensitive and resistant lines in this case. The restriction fragment length polymorphism analyses showed no evidence for gene amplification or cosegregation of a specific EPSPS gene variant with glyphosate resistance. EPSPS expression in lines exhibiting an intermediate level of resistance was indistinguishable from that in glyphosate-sensitive lines, suggesting that the mechanism could, at least in part, be non–target-based.

In Europe, 18 weedy grass species had been confirmed to have biotypes with resistance to herbicides. The most frequent is that of atrazine resistance, with nine resistant biotypes found. These biotypes are mainly resistant because of changes in the D1 protein of photosystem II. All atrazine-resistant biotypes, except that of bristly foxtail, show cross-resistance to s-triazine and as-triazines. From an agriculture point of view, the most important cases of resistance are those found in blackgrass, wild oat, Italian ryegrass, rigid ryegrass, and barnyardgrass. In these species, cross- and multiple resistances were observed due to metabolism or changes in the target protein by genetic mutations or both. These biotypes are extremely difficult to control with alternative herbicides.

Acetolactate Synthase- (ALS) inhibiting herbicides are important components for the control of ryegrass species infesting cereal-cropping systems worldwide. Although resistance to ALS herbicides in ryegrasses has evolved more than 25 yr ago, few studies have been dedicated to elucidate the molecular mechanisms involved. To this end, we have investigated the molecular basis of chlorsulfuron, sulfometuron-methyl, and imazapyr resistance in AUS5 and AUS23, two ryegrass populations from Australia. Comparison between whole-plant herbicide assays and DNA sequencing results showed that resistance to the nonmetabolizable herbicide sulfometuron-methyl was associated with four different proline mutations at ALS codon position 197 (P197) in AUS23. In addition to three P197 amino acid changes impacting on the efficacies of the two sulfonylurea herbicides, the tryptophan to leucine target-site mutation at ALS codon position 574 (W574L) was present in AUS5, conferring resistance to both sulfometuron-methyl and imazapyr. The samples were also characterized by non target-site-based resistance impacting on the metabolizable herbicide chlorsulfuron only. Interestingly, compound mutant heterozygotes threonine/serine at ALS position 197, and plants with double mutations at positions 197 and 574 were detected, thus reflecting the ability of this outcrossing species to accumulate mutant alleles. Whole-plant dose-response assays conducted on predetermined wild-type and mutant genotypes originating from the same populations allowed for a more precise estimation of the dominant and very high levels of resistance associated with the proline to serine target-site mutation at ALS codon position 197 (P197S) and W574L mutations. The two highly efficient polymerase chain reaction- (PCR) based derived cleaved amplified polymorphic sequence (dCAPS) markers developed here will allow for quick confirmation of 197 and 574 ALS target-site resistance in ryegrass species field samples and also contribute to identify populations characterized by other likely resistance mechanisms in this important weed species.

The aim of this study was to test whether herbicide resistance in rigid ryegrass has led to increased densities of this weed in Western Australian (WA) cropping fields. A total of 503 wheat fields with previously unknown management history and weed status were visited prior to harvest across 15 agronomic areas of the central WA cropping belt in 1998 and 2003. Rigid ryegrass density was visually assessed and, where possible, seed was collected from the population. Ryegrass was found in 91% of the wheat crops sampled. Ryegrass populations were tested in the following year for resistance to chlorsulfuron, sulfometuron, diclofop, and clethodim. With the use of nonparametric and regression statistical methods, resistance status, including multiple-resistance status, was not found to be associated with higher weed density. The results show that growers are generally maintaining low densities in fields with herbicide-resistant rigid ryegrass. The most common rigid ryegrass density at harvest time was less than 1 plant m−2 in both resistant and susceptible populations. Field and model-based studies of weed and herbicide resistance management that allow populations to continue at very high densities are unlikely to reflect common grower practice.

A field study was undertaken to investigate the influence of different management strategies on rigid ryegrass plant density and seedbank dynamics over 4 yr. Even though weed seedbank declined by 86% after oaten hay in year 1, the residual seedbank enabled rigid ryegrass to reinfest field peas the next year, and the population rebounded sharply when weed control relied solely on PPI trifluralin. However, use of POST clethodim followed by crop-topping for seed-set prevention of rigid ryegrass in field pea was highly effective and caused a further decline in the weed seedbank. Integration of effective management tactics over 3 yr significantly reduced rigid ryegrass weed and spike density (90 and 81%) in the final year of the 4-yr cropping sequence. Use of oaten hay in year 1, followed by effective weed control in field pea and wheat crops, depleted the high initial seedbank (4,820 seeds m−2) to moderate levels (< 200 seeds m−2) within 3 yr. Effective weed-management treatments depleted the rigid ryegrass seedbank, reduced in-crop weed infestation, and returned higher grain yields and profitability. The results of this study clearly show that large rigid ryegrass populations can be managed effectively without reducing crop productivity and profitability provided multiyear weed-management programs are implemented effectively.

Rigid ryegrass is a major weed of southern Australian cropping systems. Increased knowledge about the germination biology of rigid ryegrass would facilitate development of effective weed control programs. The influence of different environmental factors on seed germination of rigid ryegrass was studied. The level of seed dormancy in five different populations, collected from different fields on the same farm, was found to be similar, suggesting consistency in seed dormancy in populations at a single location. Dormancy release was more rapid for seeds after-ripened in the field compared with those after-ripened dry in a greenhouse. Seed decay was found to be an important contributor to the loss of seeds for this species from the seed bank in the field. Seed decay was much greater for seeds after-ripened on the soil surface compared with buried seed. In contrast, germination played an important role in the loss of buried seed as compared with surface seed. Regardless of the burial depth, the dormant seed component present at the end of the growing season was quite small (4 to 16%). Seedling emergence of rigid ryegrass in the field was greater (49%) for seeds buried at 1 cm than for those on the soil surface (16%). No seedlings emerged from seeds buried at 10 cm. Seed germination of rigid ryegrass was tolerant to a wide range of pH and showed some tolerance to salt and osmotic stress. The results indicate rigid ryegrass is unlikely to become a greater weed problem in no-till compared with cultivated seeding systems.

The widespread evolution of resistance in rigid ryegrass populations to the highly effective, in-crop, selective herbicides used within southern Australian grain-crop production systems has severely diminished the available herbicide resource. A new PRE grass-selective herbicide, pyroxasulfone, may offer Australian grain producers a new option for rigid ryegrass control in wheat crops. The efficacy and level of selectivity of rigid ryegrass control with pyroxasulfone was investigated for a range of annual crop species in potted-plant, dose–response studies. In comparison with other currently available PRE herbicides, pyroxasulfone provided effective control of both resistant and susceptible rigid ryegrass populations. Additionally, control of these populations was achieved at rates that had little or no effect on the growth and survival of wheat. This crop was also the most tolerant of cereal species, with triticale, barley, and oat being more injured at higher pyroxasulfone rates than wheat was. In general though, pulse-crop species were found to be more tolerant of high pyroxasulfone rates than cereal-crop species. There were subtle effects of soil type on the efficacy of pyroxasulfone, where higher rates were required to achieve effective control on soils with higher clay or organic matter contents. The ability of pyroxasulfone to selectively control resistant and susceptible rigid ryegrass populations as identified in these studies clearly indicate the potential for widespread use and success of this herbicide in Australian cropping systems.

Field experiments were conducted in 2008, 2011, and 2012 to investigate the interaction between seeding system disturbance and PPI herbicides on rigid ryegrass control in wheat. Of the herbicides examined, prosulfocarb + S-metolachlor, and pyroxasulfone provided ≥ 70% control of rigid ryegrass, irrespective of seeding system. In contrast, trifluralin was the least-effective herbicide against rigid ryegrass and was particularly ineffective when used with single disc (10% control) relative to the triple-disc seeding system (80%) in 2012. Trifluralin consistently reduced wheat density when incorporated using single discs (46 to 59%) but not do so with the triple disc or double-shoot knife-point systems. Although there were large differences in crop establishment because of herbicide phytotoxicity, that did not always translate into large differences in yield because wheat was able to recover from reductions in plant density by increasing the spike number per plant. Pyroxasulfone caused no damage to wheat and appeared to be the most suitable PPI herbicide for use with single-disc seeding systems.

In Australia, most wheat is sown in a no-till system without prior cultivation where herbicides are applied prior to sowing and incorporated by the planter. Trifluralin has been the most widely used PRE herbicide to control rigid ryegrass. The objective of this research was to determine crop safety and efficacy of alternative mechanism of action PRE herbicides for rigid ryegrass control in no-till wheat production. Pyroxasulfone achieved 98% control with PRE applications. The alternative PRE herbicides tested alone and in mixtures occasionally resulted in a significant reduction in wheat emergence but not crop yield. Trifluralin treatments failed at sites having trifluralin-resistant rigid ryegrass. Pyroxasulfone and prosulfocarb plus S-metolachlor were effective for control of rigid ryegrass across all trials with control ranging from 64 to 94%. This research demonstrated that PRE applications of herbicides other than trifluralin such as pyroxasulfone and prosulfocarb plus S-metolachlor can be safely and effectively used to control rigid ryegrass in no-till wheat.