A biotype of rigid ryegrass (Lolium rigidum G. ♯ LOLRI) has become resistant to amitrole and atrazine after 10 yr of exposure to a mixture of these herbicides. Resistance has also been demonstrated to the chloro-s-triazines: simazine, cyanazine, propazine; the methylthios-triazines: ametryn, prometryn; the substituted ureas: chlortoluron, isoproturon, metoxuron, diuron, fluometuron, methazole; and the triazinone herbicide metribuzin. The biotype remains susceptible to chlorsulfuron, metsulfuron, sulfometuron, sethoxydim, diclofop, fluazifop, glyphosate, carbetamide, and oxyfluorfen. Inhibition of oxygen evolution by atrazine, diuron, and metribuzin was similar in thylakoids isolated from both resistant and susceptible biotypes. Therefore, resistance to the photosystem II inhibitors is not caused by an alteration of the target site of these herbicides. Resistant plants treated with a 3-h pulse of 0.12 mM chlortoluron recover photosynthetic activity more rapidly than susceptible plants. This suggests that the basis for resistance is enhanced metabolism or sequestration of the herbicide within the leaf.

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 influence of wheat variety on the dose-response of annual ryegrass to diclofop-methyl (POST) was examined in the field in 1992 and 1993 in southern New South Wales, Australia. The aim was to determine if planting a strongly competitive variety of wheat improved control of annual ryegrass at reduced doses of diclofop-methyl. Suppression of ryegrass was dependent on herbicide dose, season, and wheat variety. In the absence of herbicide, dry matter (DM) production of annual ryegrass at 300 plants m−2 at anthesis was 500 g ha−1 with Dollarbird and Katunga compared to 1000 g ha−1with Rosella or Shrike in 1992. In 1993, DM was approximately 150 g ha−1 with Dollarbird or Katunga, and 350 g ha−1with Shrike or Rosella. Ryegrass DM was reduced by diclofop-methyl to a greater extent, relative to the weedy unsprayed controls, with less competitive varieties Rosella and Shrike than with the more competitive Dollarbird or Katunga. Diclofop-methyl at 0.28 kg a.i. ha−1 reduced DM of ryegrass growing with Katunga to less than 100 g m−2 in 1992, compared to more than 200 g m2 with the other varieties. In 1993, diclofop-methyl was more effective on ryegrass, and the same dose reduced ryegrass DM to almost zero in all varieties. Grain yields in unsprayed weedy controls of Dollarbird and Katunga were reduced approximately 20% by annual ryegrass compared with yields achieved with herbicides in both years. Yields of Rosella and Shrike in the unsprayed controls were reduced about 40% in 1992 and 60% in 1993. Only small increases in grain yields of all varieties occurred from diclofop-methyl doses above 0.13 kg a.i. ha−1. Poorly competitive varieties were dependent on herbicides to achieve grain yield potential and had a greater risk of weed survival when herbicide efficacy was reduced. In contrast, strongly competitive varieties, likely to retard build-up of weed seed in the soil, are less dependent on herbicides to achieve grain yield potential, and therefore result in reduced weed control cost.

Rate of seedling emergence, relative growth rate (RGR), and phenological development were compared in several accessions of rigid ryegrass belonging to three distinct resistance classes. The aryloxyphenoxypropionate resistant (AOPP-R) class had a faster and less variable seedling emergence than the sulfonylurea resistant (SU-R) and susceptible (S) classes. However, even the fastest of the AOPP-R accessions was within the range of the S and SU-R classes. No significant differences were detected among the resistant classes in seed dormancy, RGR, and the rate of phenological development. The rate of spike emergence, irrespective of the resistance class, was related to the latitude of the origin of the accessions, suggesting adaptation to the local climates since introduction. Due to considerable variation among weed populations for most biological attributes, the need to include several R and S accessions, in studies similar to the one reported here, is of vital importance. Because of the means and variances of the three resistance classes, at least four accessions from each resistance class would have been required to detect the observed differences between emergence rates of the AOPP-R and S classes with a confidence of 95%.

A biotype of rigid ryegrass (Lolium rigidum Gaudin biotype VLR69) resistant to some ALS inhibitors was characterized to determine the mechanisms of resistance to the sulfonylurea herbicides. The biotype had a high level of resistance to chlorsulfuron (20×) and triasulfuron (25×), and an intermediate level of resistance to imazaquin (7×) and sulfometuron (7.5×) but exhibited a low level of resistance to imazapyr (2.5×). At 60 to 90 g ai ha-1 sulfometuron, 4% of the population survived without apparent herbicidal effect The same response to sulfometuron was also observed when seeds of the resistant biotype VLR69 were germinated on agar containing sulfometuron. At 27 nM sulfometuron, 4% of the seeds germinated and grew normally while the growth of the bulk of the population was retarded. This differential response to sulfometuron at the seedling stage allowed selection of sulfometuron-resistant individuals from the population. Activity of ALS extracted from these sulfometuron-resistant plants was less sensitive to inhibition by chlorsulfuron than ALS extracted from plants considered sulfometuron susceptible in the same system. Unselected plants from the VLR69 population were able to detoxify chlorsulfuron more rapidly than susceptible VLR1 plants. It is apparent that there are at least two mechanisms of sulfonylurea resistance in the resistant biotype VLR69 which occur at different frequencies within the population. These data show that more than one mechanism or resistance may develop in response to herbicide selection pressure and that the resulting populations are not necessarily homogeneous.

This review considers the development of herbicide-resistant weed biotypes in Australia. Biotypes of the important annual weed species, capeweed, wall barley, and hare barley are resistant to the bipyridylium herbicides paraquat and diquat. These resistant biotypes developed on a small number of alfalfa fields that have a long history of paraquat and diquat use within a distinct geographical area in central western Victoria. The resistant biotypes are controlled by alternative herbicides and pose little practical concern. Some populations of wild oat are resistant to the methyl ester of diclofop. Of greatest concern is the development of cross resistance in biotypes of rigid ryegrass to aryloxyphenoxypropionate, cyclohexanedione, sulfonylurea, and dinitroaniline herbicides. The cross-resistant rigid ryegrass infests crops and pastures at widely divergent locales throughout the cropping zones of southern Australia. The options for control of cross-resistant rigid ryegrass by herbicides are limited. A biotype of rigid ryegrass on railway tracks treated for 10 yr with amitrole plus atrazine has resistance to amitrole and atrazine and other triazine, triazinone, and phenylurea herbicides. Management tactics for cross resistance are discussed.

Rigid ryegrass population VLR69 has become resistant to nine classes of herbicides after 21 yr of exposure to five herbicides in five different chemical classes. The population was exposed to diuron in 17 seasons and is resistant to diuron (4 fold) and chlorotoluron (8 fold) when compared with a reference susceptible population (VLR1). VLR69 had six seasons of exposure to chlorsulfuron and exhibits a high level of resistance to chlorsulfuron (> 20 fold) and triasulfuron (> 25 fold) and a lesser change in sensitivity to sulfometuron (7 fold); however, 4% of the population has a high level of resistance to sulfometuron. Resistance to atrazine (5 fold), simazine (6 fold), and ametryn (10 fold) was observed after five seasons of exposure to atrazine. There is a high level of resistance to all aryloxyphenoxypropionate herbicides after only two exposures to diclofop eight generations prior to testing the population. The population was cross-resistant to tralkoxydim (> 9.5 fold) and sethoxydim (1.8 fold). There was a small change in sensitivity to paraquat (1.4 fold) after three generations of exposure. The population displayed cross-resistance to: imazaquin (7 fold), imazapyr (2.5 fold), metribuzin (8.7 fold), and metolachlor (2 fold) but was susceptible to oxyfluorfen and dinitroaniline herbicides. There was also a small shift in sensitivity to tridiphane (1.6 fold).

Literature on the effectiveness of grazing animals (especially cattle, goats, and sheep) in controlling weeds is reviewed. Availability of animals and the ability to fence them onto or off weed infestations are essential. Weeds of pastures are the most suitable subjects for control, although weeds of arable crops, forestry, and waste places are sometimes amenable to control by grazing animals. Although grazing animals themselves often cause weed problems in pasture, adjusting grazing timing or intensity or both can sometimes redress the balance. Increasing sheep or cattle stocking rates prevents animals from grazing selectively and can help control some weeds. Adjusting grazing pressure can also improve the growth of desirable pasture species so that these are more competitive and able to resist invasion of annual or biennial weeds. Introducing a different class of stock, like sheep into a cattle system or goats into a sheep system can control many weeds. Goats are capable of browsing on and controlling spiny or poisonous brush weeds, including gorse and poison ivy, without suffering adverse effects. Examples are given of the use of grazing animals for weed control in crops and forestry.

Glyphosate is the world's most widely used herbicide. It is nonselective and has been used to control a broad range of weed species for the past 20 yr, without the appearance of resistant weed biotypes. However, a biotype of Lolium rigidum from a field in Northern Victoria, Australia, in which glyphosate had been used for the past 15 yr, failed to be controlled by label recommended rates. Based on LD50 values from pot dose-response experiments, this biotype exhibited resistance to glyphosate and was nearly 10-fold more resistant compared to the susceptible biotypes tested. The biotype was resistant to three different salts of glyphosate. The biotype was also nearly threefold more resistant to diclofop-methyl but was susceptible to other commonly used selective and broad-spectrum herbicides. Between the two-leaf and tillering stages of development, a susceptible biotype exhibited a small but significant decrease in tolerance to glyphosate, whereas tolerance of the resistant biotype remained unchanged with age. The resistant phenotype was verified in experiments in which seed was germinated in the presence of glyphosate. Observations on shoot and root growth of seedlings in these experiments suggested that the resistance mechanism might be associated more with the shoot than with the root.

Thirteen biotypes of rigid ryegrass were screened for trifluralin resistance. From these, the two most resistant biotypes, SLR 31 and SLR 10, were chosen for further studies involving exposure to other dinitroanilines, mitosis-inhibiting herbicides and 14C-trifluralin. SLR 31, and SLR 10 exhibited an approximate 10-fold reduced sensitivity to trifluralin in comparison to susceptible biotypes. Resistance to five other dinitroaniline herbicides was observed, with reduced sensitivity varying from 32-fold for ethalfluralin to 2.5-fold for isopropalin. The resistance in rigid ryegrass to other herbicides and drugs that affect mitosis were tested. Resistance comparable to that of trifluralin was recorded for the herbicides terbutol and DCPA, while low levels of cross-resistance to amiprophosmethyl was present. Trifluralin affected mitotic indices at a much lower level in the susceptible biotypes than in the resistant biotypes. No differences in the uptake and translocation of 14C-trifluralin were observed between resistant and susceptible biotypes. Most of the 14C detected in the plant material was in the root tissue. A small level of 14C was detected in the seeds, and no substantial increases were noted in coleoptile tissue. The resistance spectra in SLR 31 and SLR 10 were phenotypically similar to those occurring in an intermediate trifluralin-resistant goosegrass and trifluralin-resistant green foxtail.

Experiments were conducted to determine potential mechanisms of glyphosate resistance in Lolium rigidum from Australia. 14C-Glyphosate uptake, translocation, and metabolism were compared between resistant (R) and sensitive (S) biotypes. The R seed (48118a) represented the F1 progenies of plants having survived a 1.73-kg ae ha−1 (4.8 L ha−1) application of a Roundup® formulation. The S seed was a sensitive biotype of L. rigidum from Australia. Plants (one to four tillers, 2 to 4 wk old) were presprayed with a high (1.26 kg ae ha−1) or a low (0.28 kg ae ha−1) dose of formulated glyphosate. The first leaf of the first tiller, which was shielded from the spray, was immediately treated with a 14C-glyphosate solution via manual application to the adaxial surface. Harvest was made 6 days after treatment (DAT), and glyphosate residues in the leaf wash, treated leaf, roots, and shoots were quantified based on radioactivity as percentage of applied dose. The overall radioactivity recoveries were very good (90.2 to 97.3% of applied dose). R and S plants showed comparable uptake at the high (79.2 vs. 78.0%) or the low (64.0 vs. 64.7%) doses of glyphosate. About one-half of the absorbed glyphosate in both R and S (32.9 to 38.3% appl.) was translocated into the plant and distributed almost equally into roots (13.6 to 16.0% appl.) and shoots (18.1 to 22.6% appl.). Autoradiography studies demonstrated no difference in tissue localization of glyphosate between the R and S plants. For metabolism studies, tissues from individual plants were homogenized in water, and extracts were analyzed by anion exchange high-pressure liquid chromatography (HPLC) with radioactivity detection. There was little to no metabolism of glyphosate in extracts from various tissues of either R or S plants. Based on these results, we conclude that neither uptake, translocation, nor metabolism play a major role in glyphosate resistance in L. rigidum.

Greenhouse and field experiments were conducted to study competition between wheat and rigid ryegrass or between wheat and cowcockle using additive series and growth analysis. Wheat was the dominant competitor with either weed. One wheat plant was as competitive as 1 1 or 19 rigid ryegrass plants in greenhouse and field experiments, respectively. One wheat plant was as competitive as three to 24 cowcockle plants, depending on environmental conditions. Dry weight of roots, leaves, stems, and spikes or capsules responded similarly to the effects of competition between wheat and rigid ryegrass or cowcockle. Shoot dry weight was the easiest, fastest, and least expensive component to measure competition. Growth analyses of individual plants showed that wheat had a greater leaf area, shoot and root dry weight, and absolute growth rate than rigid ryegrass or cowcockle, particularly early in the season. A range of 120 to 240 wheat plants m−2 can minimize rigid ryegrass or cowcockle competition and achieve an acceptable grain yield in semiarid areas in Morocco.

Aryloxyphenoxypropanoate (APP) herbicides, such as diclofop, depolarize membranes in parenchyma cells of coleoptiles and root tips, and isolated tonoplast or plasma membrane vesicles from a variety of plant species. Some APP-resistant biotypes of rigid ryegrass and wild oat repolarize membranes after removal of herbicide from a bathing medium. The repolarization ability does not require presence of either APP-insensitive acetyl coenzyme A carboxylase or an increased capacity for herbicide detoxification. The kinetics of depolarization and repolarization depend upon the herbicide, the herbicide concentration, the biotype, and the pH of the bathing solution. For rigid ryegrass, depolarization in the presence of diclofop acid is more rapid than in the presence of diclofop-methyl, and 50% depolarization required about 4 μM diclofop acid. Both the nonherbicidal S(–) and the herbicidal R(+) enantiomers of diclofop acid depolarized membranes in susceptible and resistant ryegrass. Susceptible biotypes regenerated transmembrane potentials following removal of the S(–) but not the R(+) enantiomer, whereas resistant biotypes repolarized following exposure to either enantiomer or a mixture of the two. The herbicide 2,4-D affected, in a complex manner, the ability of both susceptible and resistant ryegrass biotypes to depolarize and repolarize. It is postulated that the intracellular concentration of diclofop acid in susceptible and resistant plants is not the same due to differences in the partitioning of diclofop acid between the extracellular spaces and the cytoplasm. The mechanism producing the postulated difference is unknown, but observations on the proton extrusion capacity of both ryegrass and wild oats, the responses of ryegrass to [K+] and PCMBS, and the single-gene inheritance pattern of resistance in wild oats indicate that changes in the diclofop sensitivity of a plasma membrane protein involved in the generation of proton or ion gradients may be involved.

The first significant cases of herbicide-resistant weed populations were to the triazines in the 1970s. In the last 10 years there has been an increase in the number of weed populations that have become resistant to an array of herbicides. In some of these cases, like rigid ryegrass in Australia, a multitude of resistant biotypes has evolved with different mechanisms of resistance. If the present trend continues, the number of herbicides effective on certain weed species may diminish rapidly. To counteract this trend, industry has organized a number of intercompany working groups to specifically address the development of resistance and to implement plans to manage resistance. University and extension along with industry personnel across the world have begun educating growers on resistance management. However, this effort needs to be intensified to find new solutions for controlling weeds through the use of integrated weed management practices that incorporate new and established herbicides with cultural, mechanical, and biological control methods. The challenge is to develop cost effective, environmentally sustainable programs for weed control while maintaining the present efficiency in food and fiber production so that needs of an ever expanding human population can be met.

The aryloxyphenoxypropionate and cyclohexanedione herbicides, which inhibit acetyl-coenzyme A carboxylase (EC 6.4.1.2), have also been hypothesized to act at specific sites on the plasmalemma. An impermeant sulfhydryl binding agent was reported to block the diclofop acid-induced depolarization of the membrane potential (Em) in rigid ryegrass. A correlation between the antagonistic interaction with auxin herbicides both in the field and in the Em response, and the repolarization of Em in herbicide-resistant rigid ryegrass following removal of diclofop acid also provide support for this hypothesis. However, similar membrane responses in resistant grasses and broadleaf species suggest that the membrane response may not be important in the phytotoxic activity of the postemergence graminicides under field conditions. In addition, an antagonistic interaction was not observed in roots of susceptible grasses exposed to combinations of diclofop-methyl and 2,4-D. Furthermore, the repolarization of the Em in diclofop-resistant rigid ryegrass was correlated to differential acidification of the external solution and an increase in the protonated form of diclofop acid, rather than a site-specific interaction at the plasmalemma. Although the membrane response is probably not involved in herbicide phytotoxicity in agricultural systems, a higher extracellular pH in the resistant biotypes of rigid ryegrass may inhibit the movement of these weak-acid herbicides across the plasmalemma, and possibly contribute to increased herbicide tolerance.

Metolachlor has been evaluated both as a herbicide for use in cultivated oats (Avena sativa L.) and for its potential as an alternative herbicide for the control of herbicide-resistant rigid ryegrass. Eight herbicide-resistant and two susceptible biotypes of rigid ryegrass were tested for their susceptibility to metolachlor. Response to metolachlor was determined both in soil and an agar germination medium. The LD50 for metolachlor in agar for a susceptible biotype (VLR1) was 0.13 μM. Five biotypes, SLR5 (6.9 fold), SLR31 (5.2 fold), SLR10 (2.5 fold), NLR12 (2.1 fold) and VLR69 (1.9 fold), were cross-resistant to metolachlor when compared with VLR1. Relative response of the biotypes was similar in both soil and agar, validating the use of an agar germination test to determine the susceptibility of rigid ryegrass biotypes to metolachlor. Biotypes cross-resistant to metolachlor also were cross-resistant to alachlor (SLR5 6.7 fold, SLR31 5.9 fold, SLR10 2.4 fold, and VLR69 1.6 fold with the LD50 for VLR1 being 0.49 μM) and propachlor (SLR57.2 fold, SLR31 7.2 fold, SLR10 3.0 fold and VLR69 2.5 fold with the LD50 for VLR1 being 0.47 μM) indicating that cross-resistance extends to other members of the chloroacetamide group. Cross-resistance to chloroacetamides was observed in biotypes that previously had been reported as cross-resistant to other herbicides. In contrast, biotypes with limited herbicide histories were generally not cross-resistant to metolachlor. These results indicate that there is a high probability of chloroacetamide cross-resistance in populations of herbicide-resistant rigid ryegrass.