Studies on the selectivity of alloxydim-sodium in plants

Alloxydim-sodium, methyl 3-[1-(allyloxyimino)butyl]-4-hydroxy-6,6-dimethyl-2-oxocyclohex-3-enecarboxylate sodium salt, is a selective herbicide which controls grass weeds in a wide range of broad-leaf crops. Spray retention, tested at two growth stages, was generally greater for the broad-leaf crops (cotton, sugarbeet, flax, beans and peas) than for wild oat (Avena fatua L.), blackgrass (Alopecurus myosuroides Huds), barley and couch grass [Agropyron repens (L.) Beauv.], and did not contribute to selectivity between susceptible and tolerant species. Broad-leaf crops tolerated 2820 g alloxydim-sodium ha^?1, three times the recommended rate used to control annual grasses. Differential uptake and translocation were not factors contributing to selectivity. In wild oat, blackgrass and sugarbeet, uptake and translocation of ¹⁴C continued during a period of 14 days after treatment with [¹⁴C]alloxydim-sodium. Translocation in susceptible and tolerant species was predominately symplastic. Over 40% of the applied ¹⁴C was eliminated from treated wild oat, blackgrass and sugarbeet plants within 7 days, due to degradation and volatilisation. A greater proportion of the methanol-soluble radioactivity extracted from leaves and roots was present as water-soluble polar metabolites in sugarbeet, than in wild oats, 7 days after treatment. The proportion of unaltered alloxydim in the organo-soluble fraction of a methanol extract was greater in wild oat than in sugarbeet. Differential metabolism appears to be one of the factors contributing to alloxydim-sodium selectivity between sugarbeet and wild oat.     

Degradation of the herbicide flamprop-methyl in soil

The degradation of the wild oat herbicide flamprop-methyl [methyl DL -N-benzoyl-N-(3-chloro-4-fluorophenyl)alaninate] in four soils has been studied under laboratory conditions using ¹⁴C-1abelled samples. The flamprop-methyl underwent degradation more rapidly than its analogue flamprop-isopropyl. However, similar degradation products were formed, namely the corresponding carboxylic acid and 3-chloro-4-fluoroaniline. The latter compound occurred mainly as ‘bound’ forms although evidence was obtained of limited ring-opening to give [¹⁴C]carbon dioxide. The time for depletion of 50% of the applied herbicide was approximately 1-2 weeks in sandy loam, clay and medium loam soils and 2-3 weeks in a peat soil.     

The origins of selectivity and performance of a new pre-emergence bleaching herbicide,WL 110547. Part II: Plant and environmental factors influencing biological activity

Pre-emergence applications of the novel tetrazole herbicide WL 110547 control a number of economically important grass and broad-leaved weed species in small grain cereals. To assess the influence of plant and environmental factors on the biological performance of WL 110547, a series of tests were carried out under controlled conditions and, where appropriate, comparisons were made with field observations. When presented with the maximum opportunity for compound uptake in the absence of soil, differences in the degree of susceptibility to WL 110547 were observed amongst both monocotyledonous and dicotyledonous species, although the latter group generally showed higher levels of phytotoxicity. This species susceptibility to WL 110547 was unaffected by temperature. Increasing the sowing depth in soil decreased the level of effect of WL 110547 on a number of monocotyledonous species, although small-seeded species (e.g. blackgrass, annual meadow grass), emerging from deep in the soil profile, subsequently developed levels of phytotoxicity comparable to, or even greater than, shallow-planted seedlings. This was attributed to less vigorous seedlings, emerging from depth, that were unable to regenerate new tissue and grow away from a treated soil layer. Reduced growth rates of wild oat, blackgrass and speedwell, induced by low temperatures, also increased the phytotoxicity of WL 110547. Furthermore, applications of WL 110547 during seedling emergence maximised herbicide effect, as did seedling emergence through moist rather than dry soil. The results are discussed in relation to the mobility of the herbicide in soil, the mode of action of WL 110547, its availability to the plant and the duration of contact between emerging shoot and treated soil layer.     

Increased foliar activity of clodinafop‐propargyl and/or tribenuron‐methyl by surfactants and their synergistic action on wild oat (Avena ludoviciana) and wild mustard (Sinapis arvensis)

Surfactants can improve postemergence herbicide efficacy and reduce the amount of herbicide required to obtain weed control. The effect of surfactants on the efficacy of herbicides is complicated and depends on the interaction among the plant, surfactant, and herbicide. The effects of surfactants on the efficacy of clodinafop‐propargyl and/or tribenuron‐methyl on wild oat (Avena ludoviciana) and wild mustard (Sinapis arvensis) under greenhouse conditions were investigated. In addition, the surface tension of aqueous solutions of the surfactants and surfactants + herbicides was determined. Significantly lower surface tension values were obtained with the aqueous solutions of citofrigate (Citogate plus Frigate) alone and with the herbicides used in this study. The citofrigate surfactant lead to the greatest enhancement of clodinafop‐propargyl and/or tribenuron‐methyl efficacy and the effect was species‐dependent. The efficacy of clodinafop‐propargyl and/or tribenuron‐methyl in the presence of surfactants in controlling wild oat was higher than for wild mustard. The foliar activity of the tested herbicides rose with increasing surfactant concentrations. The tank mixture of clodinafop‐propargyl and tribenuron‐methyl showed a synergistic effect in controlling wild oat and wild mustard. The synergistic effect in controlling wild mustard was greater than for wild oat.     

Competition between vegetative plants of wild oat (Avena fatua L.) and wheat (Triticum aestivum L.)

Competition between wild oat (Avena fatua L.) and wheat (Triticum aestivum L.) was studied in two experiments; a replacement series model and a technique for separation of root and shoot systems. Wild oat and wheat in association resulted in a relative yield total very close to unity showing that the two species were‘crowding for the same space’(or competing for the same resources) and were‘mutually exclusive'. Wild oat was more competitive than wheat, as shown by its aggressivity relative to wheat, relative yields, shoot dry weights and other plant attributes. The greater competitive ability of wild oat was predominantly due to its greater root competitive ability, while the two species had similar shoot competitive ability. Root competition had a much greater effect on the relative performance of the two species than did shoot competition. The effects of root and shoot competition were additive.     

Volatile organic compounds (VOCs) from cereal plants infested with crown rot: their identity and their capacity for inducing production of VOCs in uninfested plants

When infested with Fusarium sp., the cereals Triticum aestivum L. emend. Fiori et Paol. cv. ‘Bombona’, Avena sativa L. cv. ‘Deresz’, and Hordeum vulgare L. cv. ‘Rastik’ can emit volatile organic compounds (VOCs). The VOCs differ both qualitatively and quantitatively from those emitted by non-infested wheat, oat, and barley plants. We detected increased amounts of VOCs released by green leaves (green leaf volatiles – GLVs): (Z)-3-hexenal, (E)-2-hexenal, (Z)-3-hexen-1-ol, (E)-2-hexen-1-ol, (Z)-3-hexen-1-yl acetate, 1-hexyl acetate as well as the other VOCs like (Z)-ocimen, linalol, linaloloxide, benzyl acetate, indole, and β-caryophyllene. The lipoxygenase pathway resulted in the highest release of GLVs in comparison to the other biochemical pathways of volatile production. As a result of Fusarium infestation, the amounts of VOCs varied between tested cereals. We also subjected uninfested wheat, barley, and oat plants to infested wheat plants, and found that these cereals released larger amounts of VOCs compared to control plants. Emitted amounts of VOCs were significantly higher the shorter the distance between uninfested and infested plants.     

Selectivity and mode of action of ethyl (±)-2-(N-benzoyl-3,4-dichloroamiino)propionate in the control of Arena fatua in cereals

The activity of foliar applications of ethyl (±)-2-(N-benzoyl-3,4-dichloroanilino)—propionate(benzoylprop-ethyl)to oat was dependent upon its degradation to the biologically active acid (benzoylprop). The acid, unlike benzoylprop-ethyl, showed some movement via the phloem from the foliage to the stem where it inhibited stem elongation through an effect on cell expansion. The resulting stunted wild plants oat were unable to compete successfully with the cereal crop. Selectivity of the herbicide was dependent on its relative rate of de-esterification, and the subsequent detoxication of the acid to inactive conjugates. De-esterification was fastest in oat and slowest in wheat. The rate of detoxication in wheat prevented the accumulation of the acid to phytotoxic levels but in oat, although the rate of detoxication was higher, it failed to prevent the occurrence of phytotoxic levels of benzoylprop. In barley de-esterification was slower than in oat, and detoxication again failed to prevent some accumulation of the acid.     

The impact of uptake, translocation and metabolism on the differential selectivity between blackgrass and wheat for the herbicide pyroxsulam

BACKGROUND: Wheat shows selectivity to pyroxsulam, a new broad‐spectrum herbicide with high activity on blackgrass. Studies were performed to establish whether uptake, translocation or metabolism were responsible for the differential activity in wheat compared with blackgrass. In addition, the effect of the safener cloquintocet‐mexyl on metabolism was evaluated in wheat and blackgrass shoots. RESULTS: Root uptake of pyroxsulam in blackgrass was significantly higher than in wheat, suggesting a possible activity enhancement in blackgrass owing to root uptake. Translocation to foliage from root uptake as well as translocation out of treated foliage following foliar applications was low in wheat compared with blackgrass, likely owing to the rapid metabolism of pyroxsulam in wheat. Wheat metabolized pyroxsulam significantly faster than blackgrass to the less active O‐dealkylation product. Wheat shoots metabolized pyroxsulam faster when the safener cloquintocet‐mexyl was present, but cloquintocet‐mexyl did not increase the rate of metabolism in blackgrass. CONCLUSIONS: The selectivity of pyroxsulam to wheat relative to blackgrass was connected primarily with differences in the rate of metabolism and generation of an inactive metabolite. Metabolism in wheat restricted subsequent movement of radioactivity out of the treated leaf. The rapid metabolism in wheat was increased by the addition of cloquintocet‐mexyl. Copyright © 2010 Society of Chemical Industry     

Modelling the economics of herbicide treatment in wheat and barley using data on prevented grain yield losses

Losses in grain yield prevented by controlling weeds were measured in 59 fields of (southern hemisphere) spring-sown wheat (Triticum aestivum L.) (cv, Otane) and 45 fields of spring-sown barley (Hordeum vulgare L.) (cv. Corniche) in five consecutive growing seasons from 1988/89 until 1992/93 in the Canterbury region of New Zealand. The losses were measured as the differences in yield between weeded and non-weeded plots located in randomly positioned pairs in the fields. In the first 2 years, the weeding was by push hoe in‘organically grown crops. For the last 3 years, the fields were under prophylactic herbicide regimens with nonweeded plots created by excluding commercial herbicide applications (made mostly in October for wheat and November for barley) with polyethylene sheets placed temporarily over the plots. For each season the distributions of yield losses were modelled using the normal distribution and probabilities of ‘breaking even’ on herbicide use derived by substituting cumulative probability density functions into a simple break-even model for herbicide use. The model assumed that herbicide application in the current crop has no flow-on economic effect for succeeding crops. The mean annual yield losses prevented by herbicide application were positively correlated with September and October rainfall for wheat and bailey respectively. As a consequence, the probabilities of breaking even on herbicide use increased with increasing spring rainfall. Using historical rainfall records, probabilities of breaking even were estimated for each of the 48 years from 1947 to 1994. Averaging over these years, the analysis revealed that at current grain prices prophylactic use of the commonly applied herbicides is likely to be uneconomic in 24% (95% confidence limits 6% and 50%) of fields of average-yielding Otane wheat and in 26% (95% confidence limits 1% and 91 %) of fields of average-yielding Corniche barley in Canterbury. It was concluded that there is potential for withholding herbicide treatments without jeopardizing profitability in these crops, particularly in seasons with low spring rainfall.     

Reinvestigating adjuvants for the wild oat herbicide,flamprop-M-isopropyl. II: Field performance

A field trials programme was conducted in which the performance of a new emulsifiable concentrate formulation (ECI) of flamprop-M-isopropyl containing the adjuvant, ‘Dobanol’ 25-7, in a ratio of 2:1 (by weight) with the AI, was compared with the current commercial formulation of ‘Commando’, in combination with its recommended adjuvant, ‘Swirl’, for the control of wild oat (Avena fatua L.) in wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.). A further treatment, in which the ‘Dobanol’ 25-7: AI ratio was increased to 4:1 by the spray tank addition of the former, was also included. The mean results from six trials (five wheat, one barley) showed that the addition of ‘Swirl’ to ‘Commando’ was beneficial, increasing wild oat floret control from a mean value of 80% to 92% at current recommended rates (flamprop-M-isopropyl, 600 g ha^?1; ‘Swirl’, 2.5 litre ha^?1). However, combinations of flamprop-M-isopropyl and ‘Dobanol’ 25-7 gave superior levels of control even at lower AI application rates. For example, a mean level of 96% control of Avena spp. was obtained at 300 g AI ha^?1 with 1200 g ha^?1 ‘Dobanol’ 25–7; with even better control at higher rates of application of both components. This improvement in performance was accompanied by a higher risk of crop phytotoxicity than observed with the ‘Commando’/‘Swirl’ mixtures. Symptoms initially were scorch and subsequently growth depression, particularly of tillers. None of the mean values in the six ‘efficacy’ trials reached commercially unacceptable levels, but in a further six ‘crop effects’ trials (three wheat, three barley), in which double rates were applied, the levels of phytotoxicity did become unacceptable and subsequently reduced grain yields. In contrast, two barley ‘crop effects’ trials gave yields higher than the control plots, possibly through the effects of reducing stem length and lodging thereby enabling more efficient harvesting. Nevertheless, there were rates of application of flamprop-M-isopropyl in the range 300–400 g ha^?1 with ratios of ‘Dobanol’ 25-7 in the range 2:1 to 4:1 that would achieve high levels of control of Avena spp. without undue risk of crop phytotoxicity and further trials are planned to support this new adjuvant system.     

The response of resistant and susceptible plants to diclofop-methyl

Wild oat (Avena fatua L.) plants sprayed at the 2-or 3-leaf stages of growth with diclotop-methyl developed chlorosis over the entire leaf blade of all leaves. The leaves became necfrotic ⁷days after spraying Shool growth was inhibited. In wheat (Triticum aesicum L cv.Waldron) discrete chlorotic areas developed only where the herbicide convicted the 2nd or 3rd leaf with no visible injury so new growth uf'ter treutment. Growth inhibition of susceptible oat (Avena sativa L. cv. Garry) was sensitive to placement of diclutop-methyl near the upica and meristematic sites of the plant. Chlorosis and necrosis were independent of herbicide placement. Selective herbicide placement induced chlorosis only or both chlorosis and growth inhibition Root growth in wild oat and barley (Hordeum rulgare L. cv. Dickson) was strongly inhibited by 1–0 μM diclofop-methyl. Wild oat shoots were killed when seedlings were root-treated with 10 μM diclofop-melhyl. The 100 μM rool treatment killed barley shoots but only stunted the growth of wheat shoots by approximately 50%. In root-ireated wheat plants the shoots were turgid and developed a light purple colour, whereas in foliar-treated plants the shoots developed discrete chlorotic areas.     

A technique for mapping the spatial distribution of Elymus repots, with estimates of the potential reduction in herbicide usage from patch spraying

The spatial distribution of Elymus repens L. was mapped in five cereal fields during the 1994 season. Weed maps were created using a semi-automated system mounted on a vehicle that travelled up the tramlines, centred on a 12-m-wide spray boom. Two operators detected weeds visually and recorded their presence and density by means of a simple push-button system that recorded data from (2 × 1) m² areas, or cells, across the boom section on a portable computer. The position of the vehicle along the tramline was monitored by integrating output from a wheel sensor. Eight tramlines of one field were mapped three times consecutively, to assess detection and navigational error. Pair-wise comparisons of the three runs gave approximately 85% repeatability on presence/ absence data, 80% on zero/low- or high-density data and 85% on low/high-density data when weeds were definitely present. Simultaneous comparisons of all three runs gave 78%, 69% and 75% respectively. Repeated runs of tramlines up to 550 m long recorded mean differences of approximately 2 m with a maximum error of 14 m (2.5%). The spatial distribution of E. repens within the five fields was visibly patchy, but the size and morphology of patches varied both within and between fields. Thus, the potential reduction in herbicide usage as a result of patch spraying varied with patch morphology and infestation level. The effect of lowering the resolution from six (2 × 1) m² cells to three (4 × 1) m² cells across the 12-m boom width showed little change in potential reduction in herbicide usage.     

Characterization of virulence reactions for Heterodera avenae populations from two localities in Algeria

Heterodera avenae is widely distributed in areas where most cereal crops are produced in Algeria. However, the virulence of the Algerian populations of this nematode on individual cereal species and cultivars has not been well documented. The virulence of H. avenae populations from Tiaret and from Oued Smar were tested under natural outdoor conditions and in an in vitro test to determine reactions of nine barley, oat and wheat cultivars selected from the International differential assortment for identifying pathotypes of H. avenae. All nine cereal differentials expressed the same reactions to both populations. The nematodes reproduced well on the barley cultivar ‘Emir’ and the wheat cultivar ‘Capa’. Resistant entries included the barley cultivars ‘Siri’, ‘Ortolan’ and ‘Morocco’, the oat cultivars ‘Nidar II’ and ‘A. sterilis I.376’, and the wheat cultivars ‘Loros’ and ‘AUS10894’. This matrix of reactions indicated that H. avenae populations from both locations were characterized as H. avenae Group 1 pathotypes but did not conclusively distinguish among pathotypes Ha21, Ha31 or Ha81. The Cre1 gene was identified as a potentially valuable source of resistance when developing wheat cultivars intended for release into these localities.     

Development and evaluation of a model for predicting Lolium rigidum emergence in winter cereal crops in the Mediterranean area

Lolium rigidum is an extremely competitive and prevalent grass weed in cereal fields of Mediterranean areas. The proper timing of control measures is a prerequisite to maximising herbicide efficacy, in terms of both improved control and reduced herbicide inputs. The development of models to predict emergence flushes will contribute to this goal. Pooled cumulative emergence data obtained during three seasons from a cereal field were used to develop a Gompertz model. This explained relative seedling emergence from crop sowing onwards as a function of: (i) standard soil thermal time accumulation (TT) with a base temperature of 1.8°C and (ii) soil thermal time accumulation corrected for soil moisture (cTT). For the latter, no thermal time accumulation was computed for days in which the soil water balance within the upper 10‐cm soil layer indicated no water available for plants, because evapotranspiration was greater than rainfall plus the stored water remaining from the previous day. The model was validated with six datasets from four different sites and seasons. Compared with TT, the model based on cTT showed better performance in predicting L. rigidum emergence, particularly in predicting the end of emergence. Complemented with in‐field observations to minimise deviations, the model may be used as a predictive tool to better control this weed in dryland cereal fields of Mediterranean climate areas.     

Rouging wild oats

Over a period of three seasons the effectiveness of rouging wild oats in cereal crops with a patented herbicide glove was compared with hand pulling. Time spent by one operator in searching for the wild oats was found to remain relatively constant at 1.25–1.5 h/ha over a wide range of wild oat populations. If this searching time was excluded, treatment of the actual wild oat panicles was about three times as fast with the glove (1450/h) as by hand-pulling (538/h). The implications of this and of the saving in time spent in carrying off and disposing of’pulled’ wild oats are discussed. Solutions of 5% and 10% w/v glyphosate applied to either the stem or the panicle of the wild oats prevented the formation of viable seed. It did not, however, prevent some inviable wild oat seed appearing in the harvested grain; without additional cleaning these would have prevented the sale of the grain for seed but not into E.E.C. intervention. Extirpation de la folle-avoine Durant one période de trios saisons, les auteurs ont compare l'efficacité de l'extirpation de la folle avoine dans des cultures de céréales au moyen d'un gant herbicide breveté et en pratiquant I'arrachage manuel. Le temps passé par un opérateur à la recherche des pieds de folle-avoine s'est révélé relativement constant: 1,25 à 1.5 h/ha pour un large échantillonnage de populations de folle-avoine. Si l'on enlève ce temps de recherehes, I’operation sur les panicles présentes de folleavoine a été environ 3 fois plus rapide avec le gant (1 450/h) quä la main (538/h). Les conséquences de ce résultat et de lëconomie de temps passéâ transporter et â détruire les plants de folleavoine arrachees sont discutées. Des solutions de glyphosate à 5% ou 10% (poids pour volume) appliquées soit sur la tige soit sur la panucle des follesavoines ont empèche la formation de semences viables. Toutefois, cette opération n'empeche pas la présence d'un certain nombre de semences non viables de folle-avoine dans le grain recolté; sans nettoyage supplémentaire, ces graines d'adventices auraient empéché la commercialisation du grain récolté comme semence, mais pas à I'intérieur de la C.E.E. Manuelle Bekämpfung von Flughafer über drei Vegetationsperioden wurde die effizienz der manuellen Flughaferbekämpfung in Getreidebestanden mit hilfe eines ‘herbizid-Handschuhs’ verglichen mit dem Aufwand beim Jäten von Hand. Die von einer Arbeitskraft benötigte Zeit für das auffinden der Flughaferpflanzen blieb über einen weiten Bereich der Besatzdichte an Flughafer mit 1,25–1,5 h/ha verhältnismässig konstant. Lässt man diesen Zeitaufwand uberücksichtigt, dann erfolgte die Behandlung der Flughaferrispen mit dem Handschuh etwa dreimal so schnell (1450/h) wie durch Jäten von hand (538/h). Die hieraus sowie aus der Ersparnis an Zeit, die sonst für Abtransport and Ablagerung der gejateten Flughaferpflanzen benötigt worden wäre, zu ziehenden Folgerungen werden diskutiert. Lösungen von 5% oder 10% (Gew./Vol.) Glyphosat entweder am Halm oder an der Rispe der Flughaferpflanzen angebracht. verhinderten die Bildung von keimfahigen Samen. Sie konnten aber nicht verhindern. dass dem Getreide nach der Ernte einige nicht keimfähige Flughafersamen beigemengt waren. Hierdurch bedingt wäre ohne zusätzliche. Reinigung ein Verkauf des Getreides als Saatgut nicht möglich gewesen, wohl aber ein Verkauf an die EG-Interventionsstelle.     

Compatible and antagonistic mixtures of diclofop-methyl and flamprop-methyl with herbicides used to control broad-leaved weeds

In glasshouse experiments, the addition of four ‘pyridine herbicides’ (substituted picolinic and pyridyloxyacetic acids) to either diclofop-methyl or flamprop-methyl had little effect on wild oat (Avena fatua) control. This contrasts with the serious antagonisms which occur with 2, 4-D and 2, 3, 6-TBA. With wild and cultivated oat, l'-methylheptyl (4-amino-3, 5-dichloro-6-fluoro-2-pyridyl)oxyacetate (Dowco 433) was completely compatible with diclofop-methyl and flamprop-methyl, and there was evidence that its presence improved the control of wild oats. Picloram, 3, 6-dichloropicolinic acid and triclopyr had only slight effects on phytotoxicity. The control of cleavers (Galium aparine) by picloram, triclopyr and Dowco 433 was not significantly reduced by addition of flamprop-methyl. Preliminary metabolism studies suggest that picloram does not greatly increase the rate of diclofop detoxification as do 2, 4-D and 2, 3, 6-TBA, and the observed compatibility could well be a direct consequence of this. The absence in these greenhouse experiments of serious antagonism between the pyridine herbicides and diclofop-methyl or flamprop-methyl suggests that ‘tank mixes’ of these herbicides could be used for the control of both broad-leaved weeds and wild oats.     

Comparison of crop and weed height,for potential differentiation of weed patches at harvest

Weeds and weed control are major production costs in global agriculture, with increasing challenges associated with herbicide‐based management because of concerns with chemical residue and herbicide resistance. Non‐chemical weed management may address these challenges but requires the ability to differentiate weeds from crops. Harvest is an ideal opportunity for the differentiation of weeds that grow taller than the crop, however, the ability to differentiate late‐season weeds from the crop is unknown. Weed mapping enables farmers to locate weed patches, evaluate the success of previous weed management strategies, and assist with planning for future herbicide applications. The aim of this study was to determine whether weed patches could be differentiated from the crop plants, based on height differences. Field surveys were carried out before crop harvest in 2018 and 2019, where a total of 86 and 105 weedy patches were manually assessed respectively. The results of this study demonstrated that across the 191 assessed weedy patches, in 97% of patches with Avena fatua (wild oat) plants, 86% with Raphanus raphanistrum (wild radish) plants and 92% with Sonchus oleraceus L. (sow thistles) plants it was possible to distinguish the weeds taller than the 95% of the crop plants. Future work should be dedicated to the assessment of the ability of remote sensing methods such as Light Detection and Ranging to detect and map late‐season weed species based on the results from this study on crop and weed height differences.     

Environmental factors affecting the herbicidal activity of DPX-A7881

DPX-A7881, methyl 2-[(4-ethoxy-6-methyl-amino-1, 3, 5-triazin-2-yl)carbamoylsulphanoyl] benzoate, is a sulfonylurea herbicide being developed in Canada and Europe for post-emergence broadleaf weed control in spring and winter rapeseed. Growth room studies were conducted to determine the environmental factors affecting the herbicidal activity of DPX-A7881 applied post-emergence on winter rapeseed (Brassica napusu L. ‘Tandem’) and on a closely related weed species, Sinapis arvensis L. (wild mustard). Laboratory tests were carried out at tempera tures ranging from 2–26°C, at relative humidities from near 40% to >95%, with rain-free periods from 0.25–8 h after herbicide application, at soil moisture contents from 50–200% of field capacity, and with irradiances from 23–450 μEm^?2s^?1 Significant control of S. arvensis was demonstrated for all treatments except under poor growing conditions at the lowest temperatures and irradiances tested. After two weeks' exposure to each of the tem perature treatments, the herbicide maintained control of S. arvensis during a subsequent week of favourable growing conditions. Relative humidity, soil moisture or simulated rainfall did not significantly alter the herbicidal activity of DPX-A7881 on S. arvensis. DPX-A7881 showed a high degree of crop safety on B. napus. The herbicide did not significantly reduce the dry weights of B. napus grown under any of the environmental conditions tested.     

Physiological effects of methyl 2-[4(2,4-dichlorophenoxy)phenoxy] propanoate on oat,wild oat,and wheat

Methyl 2-[4-(2,4-dichlorophenoxy)phenoxy]propanoate (dichlofop-methyl) is a selective herbicide for wild oat (Avena fatua L.) control in wheat (Triticum aestivum L.). Dichlofop-methyl inhibited IAA-stimulated elongation of oat and wheat coleoptile segments by 51 and 13%, respectively, at 10 μM concentrations. Dichlofop-methyl alone had no auxin activity at concentrations of 0.1, 1.0, and 10 μM. The inhibitory effect of dichlofop-methyl was overcome partially by increasing the IAA concentration or by application of 3,6-dichloro-o-anisic acid (dicamba), a herbicide with weak auxin activity. The de-esterified free acid metabolite, 2-[4-(2,4-dichlorophenoxy)phenoxy]-propionic acid (dichlofop), at 10 μM inhibited auxin-stimulated oat coleoptile elongation by 23%, but it did not affect wheat coleoptile elongation at the same concentration. Both dichlofop-methyl and dichlofop inhibited root growth in excised shoots and seedlings of wild oat but had no effect on wheat. Dichlofop was a more effective inhibitor of root growth than dichlofop-methyl. The results suggest that dichlofop-methyl functions as a strong auxin antagonist, while the metabolite, dichlofop, inhibits root growth and development by another mechanism. The herbicidal effect of dichlofop-methyl may be the net effect of two biologically active forms of the compound each with a different mode of action acting at different sites within a susceptible plant.     

Herbicide-resistant crops: yield penalties and weed thresholds for oilseed rape (Brassica napus L.)

An expérimental procedure was designed to provide a simple model for types of analyses necessary to determine weed density thresholds for advantageous use of crop plants engineered for herbicide resistance. Oilseed rape (Brassica napus L., cv. Tower) biotypes resistant (RES) and susceptible (SUS) to atrazine were used as model crop plants, and wild oat (Avena fatua L.) was used as the model weed. Along a wild oat density gradient equivalent to 0–128 plants m^?2, RES plants consistently experienced biomass and yield reductions of approximately 10–20% compared to SUS plants. When atrazine was applied at 1.5 kg ha^?1 to control wild oats competing with RES plants, RES biomasses and yields were stabilized at the same level as that where 25–30 wild oats m^?2 reduce yields of SUS plants. This implies that with wild oat densities of 25–30 plants m^?2, it becomes agronomically advantageous to crop with RES plants plus atrazine rather than to crop with higher-yielding SUS plants.