Effects of dose and application timing on the seed production of three weed species treated with MCPA or tribenuron-methyl

The seed production of Fallopia convolvulus (L.) A. Love, Galium spurium L, and Thlaspi arvense L., treated with MCPA or tribenuron-methyl, was studied in a pot experiment. The herbicides were applied in doses from 1/16 to 1/l of a full dose at five different growth stages. The largest reduction of seed production was observed on plants treated when one true leaf had developed (F. convolvulus, both herbicides) or at the beginning of the generative phase (G. spurium treated with tribenuron-methyl and T. arvense treated with either herbicide). Tribenuron-methyl (0.375 g a.i. ha^?1) or MCPA (0.1875 kg a.i. ha^?1) applied at bolting stage was enough to reduce the seed production of T. arvense by 83% and 100%, respectively. The seed production of F. convolvulus, treated with 0.75 kg MCPA at the cotyledon stage, was reduced by 70%. Plants of G. spurium with 5–9 axillary shoots, treated with 3 g tribenuron-methyl, produced 45% fewer seeds than the control. Both herbicides (at least at the highest doses) reduced seed production of all three species more than shoot dry matter.     

Seed production of Raphanus raphanistrum following herbicide application during reproduction and effects on wheat yield

The influence of weather and agronomic factors on the activity of six selective herbicides applied at reproductive stages of development for the reduction in seed production of Raphanus raphanistrum in wheat was evaluated. The herbicides used in this way generally reduced seed production by between 80% and 100%. Triasulfuron and mixtures of triasulfuron + MCPA consistently provided the greatest reduction in seed production. This was greater when herbicides were applied at the bud and early flowering stages of R. raphanistrum and the efficacy of the herbicides increased as maximum temperature on the day of spraying increased over the range 14–24°C. An applied model developed from these results predicts the reduction in seed production of R. raphanistrum, for each herbicide, given the stage of weed development and maximum temperature on the day of its application. Wheat yield was significantly reduced as densities of R. raphanistrum increased, with predicted losses at low densities being approximately half of those reported in the literature. There was no consistent evidence that the late application of herbicides had any negative effect on wheat yield through crop injury, nor was there any indication of yield improvement. It is concluded that certain herbicides applied during the reproductive phase of development have considerable potential to reduce R. raphanistrum seed production in wheat crops. As part of an integrated strategy, such late post‐emergence application of selective herbicides to regulate seed production has a likely role for managing weed seedbanks, but little or no value for counteracting weed competition.     

Are pre‐spraying growing conditions a major determinant of herbicide efficacy?

To evaluate the effect of pre‐spraying growing conditions on herbicide efficacy, two years of experimentation were conducted in which Persicaria maculosa plants were exposed to different light intensities for 1–4 days before metribuzin treatment. Specific leaf area, rather than plant growth rate or plant size, was the only parameter that correlated well with herbicide efficacy in both years of experimentation. The negative relationship between the ED₅₀ and the specific leaf area indicates that leaf characteristics might be an important determinant of herbicide efficacy, for instance through an effect on herbicide uptake. In the third year of experimentation this hypothesis was further investigated by raising six cohorts of weed plants at a 1‐week interval and thus exposing them to a range of weather conditions. Clear relationships between uptake and herbicide efficacy were found for a combination of four plant species (Solanum nigrum, Senecio vulgaris, Chenopodium album and Brassica napus) and two herbicides (phenmedipham and bentazone). For phenmedipham, uptake was negatively correlated with global radiation and positively correlated with relative humidity. For the herbicide bentazone the opposite was found. These results were not species‐specific. This study shows the importance of the sensitivity of herbicide × species combinations and indicates that pre‐spraying weather information is relevant for the development of reduced dose rate recommendations.     

Response of Brassica napus in tissue culture to metsulfuron-methyl and chlorsulfuron

A herbicide bioassay based on tissue cultures of Brassica napus L. was evaluated with selected sulfonylurea herbicides. Data were analysed by fitting the results to a log-logistic dose–response model. Within an experiment, the non-linear regression models were fitted simultaneously to the individual dose–response curves. The results obtained showed good response to even low concentrations of herbicide, with detection limits in the range 0.008–0.69 nmol L^?1 for chlorsulfuron and 0.02–0.13 nmol L^?1 for metsulfuron. The reproducibility of the assays, on the basis of coefficient of variation of the ED₅₀ values, was found to be 44% for chlorsulfuron and 48% for metsulfuron measurements. Assay of herbicide dissolved in aqueous soil extract showed significant interference from this matrix on the response, requiring a five times dilution of the extract to overcome this matrix effect.     

Efficacy of chemical weed control in spring cereals in Finland

A national survey was conducted in Finland from 1982 to 1984 to determine the main weed species in spring cereals and the efficacy of herbicides on Finnish farms. The most common dicotyledonous weeds producing the highest biomass in spring cereals were Chenopodium album L., Galeopsis spp. L., Stellaria media (L.) Vill. and Lapsana communis L. The main grass weed was Elymus repens (L.) Gould. The average reduction of weed biomass by herbicide treatment was 75%. More reliable efficacy was obtained with herbicide mixtures than with MCPA alone.     

Tolerance of two Bifora radians Bieb populations to ALS inhibitors in winter wheat

BACKGROUND: Bifora radians, an annual weed in winter wheat, is distributed mainly in the Mediterranean area, Asia Minor and the Caucasus. It infests winter-sown crops of the Central Anatolia and Middle Black Sea regions of Turkey. Field experiments in heavily B. radians-infested fields were conducted over 3 years in Samsun, Turkey, to determine the response of B. radians to ALS-inhibiting herbicides, because growers had complained of a decrease in herbicide effect. RESULTS: The efficacy of ALS inhibitors on a putatively tolerant population sprayed annually with ALS inhibitors and an adjacent allegedly sensitive population was estimated at the ED₅₀ and ED₉₀ response levels. The recommended rates of herbicides controlled 90% of the weed (ED₉₀) in the sensitive population at the early stage of B. radians development, but not in the tolerant population. The relative potencies (ED_x(tolerant)/ED_x(sensitive)) of herbicides on the two populations were estimated by assuming years as being random effects. The relative potency was on average about 1.7, irrespective of the ED_x levels. CONCLUSION: Although the relative potencies were not large, they were large enough to be noted by growers. In field experiments it would be important to establish tools demonstrating when farmers recognise loss of herbicide efficacy. There has been no indication of evolution of resistant biotypes so far, but continuous spraying favours biotypes with increased levels of tolerance. Copyright © 2011 Society of Chemical Industry     

Effect of herbicides and doses on short- and long-term control of Eleusine tristachya

In Argentina, Eleusine tristachya has been recently reported as a problematic weed that can occur at high densities in spring and summer in fallows and in maize and soyabean. The reason for the increase in E. tristachya populations is that once the weed is established, it is difficult to eliminate because it produces a high number of seeds and plant regrowth occurs after herbicide treatments. The aim of this study was to determine the effect of post-emergence application of herbicides (glyphosate, haloxyfop-methyl and clethodim) at the seedling, vegetative and reproductive stages on the short-term (biomass 30 days after treatment—30 DAT) and long-term (tiller number and height and seed production—regrowth at 330 DAT) control of E. tristachya selected biotypes in outdoor pot experiments. Data fitted to a log-logistic model. For all the herbicides considered, at the seedling and vegetative stages, short-term control was achieved with low ED₅₀ and ED₉₀ values, whereas at the reproductive stage, ED₉₀ values were three- to sixfold (glyphosate), six- to 52-fold (haloxyfop-methyl) and five- to 13-fold (clethodim) higher. Long-term control at the recommended dose or lower was not possible at the reproductive stage as tiller regrowth and seed production occurred with all herbicides. It is advisable to control Eleusine tristachya when plants are small at the beginning of the growing season using the recommended herbicide dose. A delayed application will produce tiller regeneration the following year, and consequently, control would only be achieved applying an overdose, which can cause risks to health and the environment. We concluded that a management programme based on the combination of glyphosate with post-emergence graminicides applied at early stages will be effective to control future infestations.     

Indices and criteria for comparing the tolerance of strawberries to herbicides in dose-response experiments

Three indices were used to assess the tolerance of strawberries to herbicides in different experiments. Tolerance was compared using a tolerance index. TI. (= ED value of the lest herbicide/ ED value of a standard herbicide). The degree of response to increases in dose was compared by calculating a dose-response index, RI (= ED₅₀/ED₂₀ value) for each herbicide and the speed of action by a speed of action index. SI (=time to minimum ED value, test herbicide'time to minimum ED value, standard herbicide). The optimum time of the final assessment varied according to the mode of action of the herbicides being tested. There was generally good agreement between ED values obtained from scores of plant condition and from fresh weight measurements. ED₂₀ values gave more useful estimates of relative tolerance than ED₅₀'s where the degree of response to increasing doses was different.     

Characteristics of seeds and seedlings from weeds treated with sublethal herbicide doses

Size, germination and viability of seeds as well as growth of seedlings derived from three weed species were studied in a pot trial. Fallopin convolvulus (L.) A. Loeve, Galium. spurium L. and Thlaspi arvense L. were treated with MCPA or tribenuron-methyl at four doses and at five growth stages, from seedling stage to flowering, In G. spurium subnormal doses of tribenuron-methyl, applied at intermediate growth stages, greatly reduced seed weight, gennination, viability, seedling shoot biomass and root biomass. Germination and viability, as well as the shoot biomass and root biomass of seedlings, were highly correlated with seed weight. In addition, but to a smaller extent, seed weight was reduced in F. convolvidus and T. arvense by tribenuron-methyl and in G. spurium by MCPA. Germination was reduced in F. convolvulus by MCPA and in T. arvense by tribenuron-methyl. However, the effects varied greatly depending on the growth stage at application.     

Variability of bioassays with metsulfuron-methyl in soil

A total of 74 independently run bioassays with soil incorporated metsulfuron-methyl from 12 different laboratories was analysed by a logistic dose-response curve to assess the precision of regression parameters and relate ED₅₀ to soil properties. The potency in terms of ED₅₀ of metsulfuron-methyl in Brassica rapa L., which was used by all laboratories, varied between 0.05 and 3.9 g a.i. ha^-1. ED₅₀ was negatively correlated with pH and positively correlated with organic matter. The majority of laboratories had ED₅₀ within the interval 0.1-1.0 g a.i. ha^-1. At one laboratory using three test species, the most sensitive species was Beta vulgaris L. followed by Brassica rapa L. and Lepidium sativum L. The coefficients of variation were smallest for the ED₅₀ and ED₉₀ response levels and largest for the ED₁₀. The slope of the response curves had considerably lower coefficients of variation than the EDs. The results are discussed in relation to a previous collaborative bioassay study. Finally it is suggested that standardization of bioassays with herbicides could be achieved in the same way as standardization of chemical analyses.     

Quantifying the detrimental effect of airborne dust on herbicide efficacy

One side effect of human activity and global climatic change is more airborne dust. This not only has undesirable effects on human health and the environment, but may also adversely affect many agricultural processes, including herbicide efficacy. This is particularly relevant in parts of the world where dust storms are common and limited rainfall means dust persists on plants for protracted periods. Accordingly, a series of greenhouse dose–response pot experiments was carried out to quantify the effect of a dust concentration of 1,500 µg/m³ on the performance of paraquat, glyphosate, sulfosulfuron and 2,4-D+MCPA. Dust-treated and untreated seedlings of Hordeum spontaneum and Sinapis arvensis were sprayed with sulfosulfuron and 2,4-D+MCPA, respectively, while paraquat and glyphosate were applied to both weed species. The efficacy of glyphosate and paraquat decreased significantly in the presence of dust on both species. Similar results were observed for 2,4-D+MCPA on S. arvensis. In contrast, the efficacy of sulfosulfuron on H. spontaneum was not affected by dust. In the presence of dust, the doses of glyphosate, paraquat and 2,4-D+MCPA required for a 90% effect were enhanced by a factor of 1.9 to 2.6. The negative impact on glyphosate efficacy was more pronounced in S. arvensis than H. spontaneum, while it was similar for paraquat. Future studies should consider the amount, duration and composition of dust as well as its impact on other weeds and herbicides. Potential approaches to overcome the detrimental influence of dust on the performance of herbicides were also suggested for future studies.     

UNTERSUCHUNGEN ÜBER AUFNAHME UND TRANSLOKATION VON 14C-MARKIERTEN HERBIZIDEN IN AGROSTEMMA GITHAGO L. UND TUSSILAGO FARFARA L.

Studies of the uptake and translocation of ¹⁴C-labelled 2, 4-D, MCPA and aminotriazole in Agrostemma githago L. and Tussilago farfara L. clarified the behaviour of the herbicides in both species. In A. githago, MCPA was more freely mobile than 2,4-D after application to the leaf; it was distributed in the plant more rapidly and in greater quantity. Similarly, following root uptake MCPA was transported in the shoot in greater amounts than was 2,4-D. There is a clear relationship between the susceptibility of A. githago to MCPA and the mobility of the herbicide in the plant. In T. farfara, 2,4-D and aminotriazole applied to the leaves were equally well absorbed and relatively rapidly translocated. During the period up to 72 h the amounts of herbicide in the plant increased to similar levels; after that, ¹⁴C activity in plants treated with 2,4-D fell slightly whereas there was further accumulation of aminotriazole. Following uptake through the roots, translocation and accumulation in the leaves were considerably greater with aminotriazole than with 2,4-D. The lack of accumulation of 2,4-D could be a factor in the resistance of T. farfara to this herbicidie. Recherches sur l'absorption et la migration d'herbicides marqués au ¹⁴ C dans Agrostemma githago L. et Tussilago farfara L.     

Mechanism of resistance to bensulfuron-methyl in Alisma plantago-aquatica biotypes from Portuguese rice paddy fields

Two Alisma plantago‐aquatica biotypes resistant to bensulfuron‐methyl were detected in rice paddy fields in Portugal’s Mondego (biotype T) and Tagus and Sorraia (biotype Q) River valleys. The fields had been treated with bensulfuron‐methyl‐based herbicide mixtures for 4–6 years. In order to characterize the resistant (R) biotypes, dose–response experiments, absorption and translocation assays, metabolism studies and acetolactate synthase (ALS) activity assays were performed. There were marked differences between R and susceptible (S) biotypes, with a resistance index (ED₅₀R/S) of 500 and 6.25 for biotypes Q and T respectively. Cross‐resistance to azimsulfuron, cinosulfuron and ethoxysulfuron, but not to metsulfuron‐methyl, imazethapyr, bentazone, propanil and MCPA was demonstrated. No differences in the absorption and translocation of ¹⁴C‐bensulfuron‐methyl were found between the biotypes studied. Maximum absorption attained 1.12, 2.02 and 2.56 nmol g⁻¹ dry weight after 96 h incubation with herbicide, for S, Q and T biotypes respectively. Most of the radioactivity taken up by the roots was translocated to shoots. Bensulfuron‐methyl metabolism in shoots was similar in all biotypes. The R biotypes displayed a higher level of ALS activity than the S biotype, both in the presence and absence of herbicide and the resistance indices (IC₅₀R/S) were 20 197 and 10 for biotypes Q and T respectively. These data confirm for the first time that resistance to bensulfuron‐methyl in A. plantago‐aquatica is target‐site‐based. In practice, to control target site R biotypes, it would be preferable to use mixtures of ALS inhibitors with herbicides with other modes of action.     

Joint action of sulfonylurea herbicides and MCPA

The joint action of mixtures of the sulfonylurea herbicides chlorsulfuron, metsulfuron and tribenuron and MCPA as a dimethylamine salt was assessed on Stellaria media L. and Lamium purpureum L. Two statistical analyses, and appropriate experimental designs, were used—the additive dose model (ADM) and the principle of parallel dose-response curves. The ADM analyses revealed that mixtures of the sulfony-lureas and MCPA were less potent than expected on Stellaria while the parallel-line assay analyses showed that activity of the sulfonylurea herbicides on Stellaria and Lamium was reduced in mixtures with MCPA. The antagonism was found to be dependent on the ratio of the two herbicides. Activity decreased with increasing MCPA ratios in the mixtures. Sequential spraying and selective placement of droplets revealed that MCPA only antagonized the sulfonylureas when the herbicides were in physical contact with each other. The benefits of applying both statistical methods when studying the joint action of two herbicides both possessing activity on the test plants are discussed. Action de melanges d'herbicides sulfonylurées et de MCPA L'action de melanges des herbicides sulfonylurées chlorsulfuron, metsulfuron et tribénuron avec le MCPA formulé en sel de diméthylamine, a été etudiée sur Stellaria media L. et Lamium purpureum L. Deux analyses statistiques asso-ciées a deux systèmes expérimentaux appropriés ont été utilisés: le modèle à doses additives (ADM) et le principe du parallélisme des cour-bes dose-effet. Les analyses ADM ont montré que les melanges de sulfonylurées et de MCPA ont été rnoins efficaces qu'attendu sur Stellaria. Les analyses en courbes parallèles ont montré que 1'activité des herbicides sulfonylurées sur Stellaria et Lamium était réduite dans des mélanges avec le MCPA.L'antagonisme dépendait du rapport entre les doses des deux herbicides, 1'activité décroissant avec 1'augmentation du taux de MCPA. Des traitements séquentiels et des dépôts localisés de gouttelettes ont montré que le MCPA n'était an-tagoniste des sulfonylurées que lorsque les herbicides étaient physiquement en contact. L'intérêt est discuté, d'utiliser les deux méthodes statistiques de concert pour étudier les interactions entre deux herbicides actifs sur les plantes test. Gemeinsame Wirkung von Sulfonylharnstoff-Herbiziden und MCPA Die Wirksamkeit von Mischungen der Sulfonyl-hamstoff-Herbizide Chlorsulfuron, Metsulfuron und Tribenuron mil MCPA als Dimethylamin-salz wurde an Stellaria media L. und Lamium purpureum L. bestimmt. Die Versuche wurden so angelegt, um 2 statistische Analysen anzuwen-den: das Additiv-Dosis-Modell (ADM) und das Prinzip paralleler Dosis-Wirkungs-Kurven. Die ADM-Analyse ergab, daß diese Mischungen gegenüber Stellaria weniger wirksam als erwartet waren, während die Parallel-Analyse zeigte, daß die Wirksamkeit der Sulfonylharnstoff-Herbiz-ide in Mischungen mil MCPA gegeniiber den beiden Arten herabgesetzt war. Es ergab sich, daß der Antagonismus vom Verhältnis der Mischungspartner abhängt. Die Wirkung nahm mit steigenden MCPA-Anteilen in den Mischungen ab. In Spritzfolgen und bei separater Applikation zeigte sich, daß MCPA nur dann gegenüber Sulfonylharnstoffe antagonistisch wirkte, wenn sich die Herbizide vermischten. Die Vorzüge der Anwendung der beiden statistischen Methoden zur Untersuchung des gemeinsamen Effektes von 2 Herbiziden ähnlicher Wirkung werden diskutiert.     

Residualwirkung von Chlortriazin-Herbiziden im Boden an drei rumänischen Standorten. II. Prognose möglicher Folgekulturen bei Atrazinrükständen im Boden

Residual effects of chlorotriazine herbicides in soil at three Rumanian sites. II. Prediction of the phytotoxicity of atrazine residues to following crops Total and plant-available atrazine residues in the top 10 cm soil were measured 120 days after application of 3 kg ai ha^?1 to maize (Zea mays L.) at three sites in Rumania. At one site, similar measurements were made 3?5 years after application of 100 kg ai ha^?1. Plant-available atrazine residues were estimated by extraction of soil samples with water, and by bioassay using Brassica rapa as the test plant. It was calculated that between 30 and 120μg atrazine 1^?1 was potentially available to plants in the different soils. Dose-response relationships for atrazine and the most important rotational crops with maize in Rumania—sunflower, winter wheat, soybean and flax—were determined in hydroponic culture using herbicide concentrations corresponding with the plant-available fractions measured in the different soils. ED₅₀ values were determined by probit analysis and the results showed that sunflower (ED₅₀, 22μg 1^?1) was the most sensitive crop, and soybean (ED₅₀, 78μg 1^?1) was the least. The residual phytotoxicity of atrazine to succeeding crops in the different soils was predicted using the appropriate availability and phytotoxicity data, and the results showed good agreement with those observed. The results suggest that measurements of plant-available herbicide residues afford a rapid method of assessing possible phytotoxicity to following crops.     

Toxicity and selective mechanism of imazamox by weedy rice and imidazolinone-resistant rice in China

Weedy rice (Oryza spp.) is a notorious weed in direct-seeding paddy fields. Because it has anatomical and physiological traits similar to those of cultivated rice, no selective herbicide is effective in controlling weedy rice growing among conventional rice cultivars. Imidazolinone (IMI)-resistant rice lines JD372 and JJ818 have been planted with imazamox to control weedy rice in Jiangsu and Shanghai, China. Whole-plant dose–response analysis showed that imazamox exhibited high efficacy against three populations of weedy rice. The ED₉₀ of weedy rice populations FN-5, GY-8, and HY-3 were 46.87, 61.43, and 52.17 g a.i. ha⁻¹, respectively, close to the recommended field dose (50 g a.i. ha⁻¹) of imazamox. Conversely, the ED₁₀ values of JD372 and JJ818 were slightly lower than 50 g a.i. ha⁻¹. These findings indicate that imazamox can control weedy rice production in JD372 and JJ818 fields. The acetolactate synthase (ALS) sensitivity of JD372 in vitro was 1714.89-fold lower to imazamox than was that of FN-5. ALS gene sequencing revealed a Ser653Asn point mutation—a common mutation that confers resistance to IMI herbicides in JD372. In addition, higher ALS expression levels in JD372 were found at 24 and 72 h after imazamox treatment. ALS overexpression might partially compensate for the ALS activity of JD372 that was suppressed by imazamox.     

Efficacy of reduced herbicide doses in spring cereals of different competitive ability

The effect of dose reduction on the efficacy of MCPA/mecoprop and MCPA/fluroxypyr mixtures in the control of broad-leaved weeds was studied in spring barley (Hordeum vulgare L.) and spring wheat (Triticum aestivum L.), Five crop densities were sown to obtain crop stands of different competitive ability. Halving the highest recommended dose decreased the control efficacy by 5–15 percentage units, but still provided a good weed control. Further reduction of the dose resulted in high variation in efficacy. The level of weed suppression was mainly determined by the competitiveness of the crop, environmental conditions and weed spectrum with differential sensitivity of species to the herbicides. No changes in the weed infestation were detected after continuous use of low herbicide doses for 3 years. The seed bank of weeds in soil was higher after a wheat-dominated rotation than after a barley-dominated rotation. Efficacité de doses réduites d'herbicides chez des céréales de printemps d'aptitudes à la compétitivé différentes L'impact de la réduction des doses sur l'efficacité de melange MCPA/mecoprop et MCPA/ fluroxypyr en matiere de desherbage des dicotyledones a étéétudié chez l'orge de printemps (Hordeum vulgare L.) et le blé de printemps (Triticum aestivum L.) 5 densités de culture ont été semées pour obtenir des cultures d'aptitudes a la compétitivité différentes. La réduction de moitié de la dose recommandée la plus élevée a reduit l'efficacité herbicide de 5 à 15% mais a encore fourni un bon résultat. Des reductions plus élevées de la dose ont conduit à de fortes variations dans l'efficacité. Le niveau de suppression des adventices était principalement déterminé par la compétitivité de la culture, les conditions environnementales et le spectre des adventices avec une sensibilité differentes des espèces aux herbicides, Aucun changement dans l'infestation en adventices n'a été observé après une utilisation continue de faibles doses d'herbicide pendant 3 ans, Le stock grainier d'adventices du sol était plus élevé après une rotation à dominante blé, qu'après une rotation à dominante orge. Wirksamkeit reduzierten Aufwands von Herbiziden in Sommergetreide mit unterschiedlicher Konkurrenzkraft In Sommergerste (Hordeum vulgare L.) und Sommerweizen (Triticum aestivum L.) wurde untersucht, wie die Reduzierung des Aufwands die Wirksamkeit von MCPA-Mecoprop- und MCPA-Fluroxypyr-Mischungen beeinflußt, Die Getreide wurden in 5 Dichten gesät, um Pflanzenbestände unterschiedlicher Konkurrenzkraft zu haben. Die Halbierung des hochsten empfohlenen Aufwands verminderte die Wirksamkeit um 5 bis 15 Prozentpunkte, ergab dennoch eine gute Unkrautbekämpfung, Eine weitere Reduzierung des Aufwands brachte sehr unterschiedliche Wirksamkeit. Der Grad der Unkrautunterdrückung wurde überwiegend durch die Konkurrenzkraft des Getreides, die Umweltbedingungen und das Unkrautspektrum mit gegenüber den Herbiziden verschieden empfindlichen Arten bestimmt. Nach 3 Jahren niedrigen Herbizidaufwands ließen sich Änderungen der Verunkrautung nicht erkennen. Der Unkrautsamenvorrat im Boden war nach einer weizen-dominierten Fruchtfolge größer als nach einer gerstendominierten.     

Response of Arabidopsis thaliana to 22 ALS inhibitors: baseline toxicity and cross-resistance of csr1-1 and csr1-2 resistant mutants

The baseline toxicity of 22 acetolactate synthase (ALS)-inhibiting herbicides and the cross-resistance patterns of chlorsulfuron- and imazapyr-resistant (R) lines on these 22 ALS-inhibiting herbicides were investigated using the model species Arabidopsis thaliana. The 22 herbicides consisted of 18 sulfonylureas (SU), three imidazolinones (IMI) and one triazolopyrimidine (TP). The ED₅₀ values (doses of herbicides required to reduce dry matter by 50%) of the post-emergence-treated Col and Ler susceptible (S) lines ranged from 22 to 4822 mg ha⁻¹ and from 17 to 3143 mg ha⁻¹ respectively. The csr1-1 chlorsulfuron-resistant line (substitution of Pro₁₉₇ to Ser) conferred a high resistance to the only TP tested as well as to nine SU herbicides (R:S ratio ≥30), a low resistance to two SU herbicides (R:S≥5 and <30) and little or no resistance to the three IMI and seven other SU herbicides (R:S <5). This result contradicts the expectation that an ALS mutation selected by an SU herbicide confers high cross-resistance to other SU herbicides. We found that the efficacy of specific ALS inhibitors was different for different species and therefore could not be predicted from our results with A. thaliana; however, the cross-resistance patterns in A. thaliana were highly correlated with cross-resistance patterns in unrelated species with the same resistance mutation. These results have implications for resistance management.     

Intraregional and inter‐regional variability of herbicide sensitivity in common arable weed populations

The question on intraregional versus inter‐regional variability in herbicide sensitivity for weed populations is of major importance, both in extrapolation of model parameters and in herbicide zonal approval procedures. We hypothesised that inter‐regional variability in herbicide sensitivity for field populations would be the same as intraregional variability for regions with similar climatic conditions. Seeds of field weed populations were collected in a Danish, German and Polish region. Herbicide sensitivity was tested in dose–response experiments in the glasshouse with flufenacet and iodosulfuron (Apera spica‐venti), florasulam and tribenuron (Tripleurospermum inodorum), diflufenican, diflufenican + flurtamone and pendimethalin (Viola arvensis). ED₅₀ values and variance components of the ED₅₀ values were estimated to describe the influence of region, year and population. The regions accounted for a maximum of 26% of the variance and always less than the variance accounted for by individual populations. Sensitivity indices (SI₅₀) were calculated as the ratio between ED₅₀ of the test population and a reference population. There was considerable intraregional variability in SI₅₀ values and SI₅₀ values from a single region did not consistently differ from other regions. The large intraregional variability in herbicide sensitivity between populations, with no evidence of resistance, is of interest both for zonal evaluation of herbicides and resistance research. For practical weed management, we conclude that dose–response functions can be transferred between the study regions, for example for the common use in decision support systems with proper insurance for the control of less sensitive populations.     

An investigation of herbicide interaction with the H+-ATPase activity of plant plasma membranes

This study has investigated the activity of several herbicide classes at the plant plasma membrane. Two-phase partitioning was used to prepare highly purified plasma membrane vesicles from the monocotyledon weed black-grass (Alopecurus myosuroides Huds.) and the dicotyledon crop sugar beet (Beta vulgaris L. cv. Celt). The purity of the plasma membrane H⁺-ATPase activity was characterised with respect to inhibitors, pH and substrate specificity. In both species, contamination of the plasma membrane by tonoplast fragments was largely eliminated and chlorophyll was absent. In addition, the plasma membrane H⁺-ATPase from black-grass and sugar beet exhibited high vanadate sensitivity and a sharp pH profile around 6·5. Subsequently, H⁺-ATPase activity was assayed in the presence (100 μM ) and absence of four graminicide classes and auxin-type herbicides. Graminicides, including the aryloxyphenoxypropionate diclofop-methyl and the thiocarbamate triallate, inhibited H⁺-ATPase activity by 50–80% in both species. However, other graminicides, including the cyclohexanediones and the chloroacetamide alachlor, had no affect. Similarly, auxin-type herbicides such as 2,4-D and MCPA did not inhibit H⁺-ATPase activity. Results are discussed in relation to the proposed mode of action of these herbicides. © 1998 SCI