Interaction between benzoylprop ethyl, flamprop methyl or flamprop isopropyl and herbicides used for broadleaved weed control

A marked antagonism of A. fatua control was found when 2,4-D, dicamba, 2,4-D-dicamba-mecoprop or bromoxynil were added to benzoylprop ethyl, flamprop methyl or flamprop isopropyl. Because of the much poorer control of A. fatua by these mixtures obtaining broad spectrum weed control with them in one spray operation would not be possible. The A. fatua herbicides did not affect the activity of the added herbicides on Fagopyrum lataricum. In eight out of nine mixtures tested there was no significant antagonism in the field conditions when MCPA or bromoxynil-MCPA was added to the A. fatua herbicides. There were significant antagonistic effects, however, in six out of nine such mixtures in glasshouse experiments. This suggests that under farm use conditions benzoylprop ethyl, flamprop methyl or fiamprop isopropyl can be mixed with MCPA amine or ester or with bromoxynil-MCPA to obtain broad spectrum weed control from one spray operation, though slightly poorer A. fatua control may be expected with these mixtures.     

Asulam mixtures for weed control in flax

Five field experiments were conducted from 1972 to 1975 to evaluate weed control in flax (Linum usitatissimum L.) using post-emergence treatments of asulam [methyl (4-aminobenzenesulphonyl) carbamatel alone and in combination with other herbicides. The ¹⁴C-asulam absorption by leaf segments and roots of glasshouse grown wild oats (Avena fatua L.) was also investigated. Asulam at 1.12 kg/ha gave good wild oat control and acceptable control of green foxtail (Setaria viridis (L.) Beauv.). However, wild oat control was poorer when asulam was combined with other herbicides: on a 3-year average, as compared with asulam alone at equal rates, the asulam+MCPA mixture resulted in a greater antagonism and a significant 6% reduction in flax seed yield, whereas the asulam+bromoxynil/MCPA mixture gave the least antagonistic effect, improved broadleaf weed control and increased yield by 13%. In mixtures, the potassium salt of MCPA was more compatible with asulam for weed control than the amine form. Both leaf segments and roots of wild oats absorbed and distributed less ¹⁴C-asulam from solutions containing MCPA than from those containing bromoxynil or bromoxynil/MCPA.     

Joint action of root-absorbed mixtures of auxin herbicides in Sinapis alba L. and barley (Hordeum vulgare L.)

The efficacy of some binary mixtures of MCPA, 2,4-D dichlorprop, mecoprop and dicamba was evaluated in water culture experiments. Within each of seven assays, two dose-response curves described the phytotoxicity of the herbicides administered separately and three curves described the phytotoxicity of mixtures in fixed ratios. A non-linear regression model of dry matter on the logarithm of the dose, assuming the five response curves to be mutually parallel, was fitted and a reference model, the Additive Dose Model (ADM) to assess the efficacy of the mixtures, was also incorporated into the regression. The biological interpretation of ADM is often associated with mixtures of compounds having similar mode of action. Mixtures of dicamba with dichlorprop or 2,4-D were more potent than expected under ADM in two Sinapis alba assays. In one assay with mixtures of MCPA and dichlorprop the parallel-line model fitted somewhat better than did the ADM regression. This also applied to a barley assay with mixtures of dicamba and MCPA. On the basis of the results obtained it is suggested that the analogy between the implicit assumptions of the parallel-line assay can be extended to the ADM. Parallel dose response curves are a necessary but not a sufficient condition for assuming similar mode of action Similarly, it can be argued that mixtures of compounds following ADM are a necessary but not a sufficient condition for assuming a similar mode of action for the herbicides in a mixture.     

An integrated approach to weed management practices in direct-seeded rice under zero-tilled rice–wheat cropping system

An increasing water crisis as well as shortage of farm labor farmers in many Asian regions is forcing a shift from puddled transplanted rice to direct-seeded rice. The weeds, however, are a major constraint to the production of direct-seeded rice. In this perspective, a field study was carried out to evaluate various pre- and post-emergence herbicides and different possible integrated weed management practices in zero-till direct-seeded rice. Weed infestation decreased the rice yield by near about 75%. Co-culture rice with Sesbania followed by (fb) pendimethalin fb 2,4-D effectively reduced the total weed population (65.1%) and biomass (86.7%) at 30 days after sowing (DAS). The lowest total weed dry biomass at 60 DAS was recorded from bispyribac-sodium+azimsulfuron-treated plot and such tank mix application of herbicide performed better against diverse weed flora as compared to a single herbicide. Higher yield and more profit from zero-till direct-seeded rice were obtained with the application of bispyribac-sodium+azimsulfuron herbicides as a tank mixture or an integrated approach through cowpea green-manuring fb 2,4-D+glyphosate fb bispyribac-sodium by effective management of versatile weed flora.     

Soil persistence studies with bromoxynil, propanil and [14C]dicamba in herbicidal mixtures

The persistence of bromoxynil (3,5-dibromo-4-hydroxybenzonitrile), [¹⁴C]dicamba (3,6-dichloro-2-methoxybenzoic-7-¹⁴C acid) and propanil [N-(3,4-dichlorophenyl)propionamide] at rates equivalent to 1 kg ha^?1, were studied under laboratory conditions in a clay loam, a heavy clay and a sandy loam at 85% of field capacity and at 20±1°C, both singly and in the presence of herbicides normally applied with these chemicals as tank-mix or split-mix components. The degradation of bromoxynil was rapid with over 90% breakdown occurring within a week in the heavy clay and sandy-loam soils, while in the clay-loam approximately 80% of the bromoxynil had broken down after 7 days. In all three soils degradation was unaffected by the presence of asulam, diclofop-methyl, flamprop-methyl, MCPA, metribuzin or propanil. Propanil underwent rapid degradation in all soil treatments, with over 95% of the applied propanil being dissipated within 7 days. There were no noticeable effects on propanil degradation resulting from applications of asulam, barban, bromoxynil, dicamba, MCPA, MCPB, metribuzin or 2,4-D. The breakdown of [¹⁴C]dicamba in a particular soil was unaffected by being applied alone or in the presence of diclofop-methyl, flampropmethyl, MCPA, metribuzin, propanil or 2,4-D. The times for 50% of the applied dicamba to be degraded were approximately 16 days in both the clay loam and sandy loam, and about 50 days in the heavy clay.     

Glasshouse trials with controlled drop application of some foliage-applied herbicides

MCPA, mecoprop, dichlorprop, dicamba, 2,3,6-TBA, bentazone, ioxynil/bromoxynil (a mixture), barban, difenzoquat and chlorfenprop-methyl were applied by spinning disc in controlled drop sizes from 150-350μm and at very low volume rates (5–45 1/ha), to the foliage of some dicotyledonous weed species or wild oats (Avena fatua L.). The same herbicides were also applied by means of conventional hydraulic nozzles at volume rates of about 200 1/ha. Most of these herbicides performed as well at very low volume rates as with the conventional application, the major exceptions being bentazone, ioxynil/bromoxynil (both of which have distinct contact effects) and dichlorprop. Comparisons between oil and water as the diluent at a very low volume rates suggest that there may be situations when oil is preferable and others when water is.     

Control of Avena fatua and Fagopyrum tataricum with tank mixtures of linuron or linuron + MCPA and sequential applications of linuron, and post-emergence A. fatua herbicides

Linuron (0.21 and 0.28 kg/ha) and linuron + MCPA (0.21+0.56 kg/ha) in a tank mixture with field rates of barban, difenzoquat and flamprop-methyl reduced the phytotoxicity of these herbicides to Avena fatua. When linuron was applied immediately following or 6 days after the A. fatua herbicides no reduction in phytotoxicity to A. fatua occurred, suggesting that the antagonism may be occurring as a result of physical or chemical incompatability when the herbicides are mixed together. The possibility of obtaining broad-spectrum weed control with one trip over the field by applying linuron and one of these wild oat herbicides separately but at the same time using a double-boom, double-tank system deserves evaluation. When linuron was applied in a tank mixture (0.21 and 0.28 kg/ha), immediately after, or 6 days after diclofop-methyl (0.70 kg/ha), there was no reduction of A. fatua control, and wheat tolerance to the tank mixture was good. This tank mixture offers potential for control under field conditions of A. fatua and some broad-leaved weeds in one spray operation. Linuron + MCPA (0.21+0.56 kg/ha) in a tank mixture severely reduced A. fatua control with diclofop-methyl. No loss of phytotoxicity to Fagopyrum tataricum occurred when the A. fatua herbicides tested were tank mixed with linuron or linuron + MCPA. Lutte contre Avena fatua et Fagopyrum tataricum avec des mélanges extemporanés de linuron ou de linuron + MCPA et des applications successives de linuron et d'herbicides de postlevée actifs contre A. fatua     

The control of Achillea millefolium in the Snowy Mountains of Australia

The response of Achillea millefolium L. to herbicides was measured to determine the effectiveness of the current recommendations and to test alternative herbicides. Five plots at each of the three replicate sites were selected and randomly treated with one of the four herbicides: dicamba/2,4‐D, glyphosate, metsulfuron‐methyl and triclopyr/picloram. After 12 months, the recommended treatment, dicamba/2,4‐D, did not cause a significant reduction in aerial biomass compared with untreated controls and the number of flowering stems was significantly increased in treated plots. Metsulfuron‐methyl and triclopyr/picloram caused a significant reduction in A. millefolium aerial biomass but did not affect the number of flowering stems. Glyphosate produced a significant reduction in aerial biomass and was less effective. These results suggest that the current recommendation for A. millefolium might be improved. Site‐specific effects such as aspect may also influence the effectiveness of herbicides on this species.     

Impact of volatility reduction agents on dicamba and glyphosate spray solution pH,droplet dynamics,and weed control

BACKGROUND

Regulations in 2021 required the addition of a volatility reduction agent (VRA) to dicamba spray mixtures for postemergence weed control. Understanding the impact of VRAs on weed control, droplet dynamics, and spray pH is essential.

RESULTS

Adding glyphosate to dicamba decreased the solution pH by 0.63 to 1.85 units. Across locations, potassium carbonate increased the tank-mixture pH by 0.85 to 1.65 units while potassium acetate raised the pH by 0.46 to 0.53 units. Glyphosate and dicamba in tank-mixture reduced Palmer amaranth control by 14 percentage points compared to dicamba alone and decreased barnyardgrass control by 12 percentage points compared to glyphosate alone 4 weeks after application (WAA). VRAs resulted in a 5-percentage point reduction in barnyardgrass control 4 WAA. Common ragweed, common lambsquarters, and giant ragweed control were unaffected by herbicide solution 4 WAA. Dicamba alone produced a larger average droplet size and had the fewest driftable fines (% volume < 200 μm). Potassium acetate produced a larger droplet size than potassium carbonate for D_v0.1 and D_v0.5. The addition of glyphosate to dicamba decreased droplet size from the entire spray droplet spectrum (D_v0.1, D_v0.5, D_v0.9).

CONCLUSION

A reduction in spray pH, droplet size, and weed control was observed from mixing dicamba and glyphosate. It may be advisable to avoid tank-mixtures of these herbicides and instead, apply them sequentially to maximize effectiveness. VRAs differed in their impacts on spray solution pH and droplet dynamics, but resulted in a minimal negative to no impact on weed control. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.     

Weed Control in Wheat and Barley in the Middle East

Abstract

Wheat and barley constitute the bulk of agricultural production in the Middle East. Unfortunately, these crops are grown on marginal soils which are infested with annual and perennial weeds. The principal weed species include Avena sterilis L., Convolvulus arvensis L., and Sinapis arvensis L. and it has been estimated that yield losses due to weeds are between 20 and 30%. Control is mostly by hand pulling, hoeing or mechanical tillage, the use of herbicides being limited to about 2–5%, of the cereal area. The chemical used is usually 2,4-D but there are a few applications of barban. With the increase in wages, and unavailability of agricultural labour it is suggested that herbicide application should be widely used in the Middle East. Mixtures with benzonitriles and dicamba should be used in addition to the economical phenoxy herbicides to widen the spectrum of control of broadleaved weeds. Glyphosate may also be used after grain harvest for the control of perennial weeds and tri-allate, chlortoluron and other chemicals for the control of wild oats and other grasses. Plant breeders should study the genetic response of newly developed varieties to the most widely used herbicides.     

Waterhemp (Amaranthus tuberculatus) control under drought stress with 2,4‐dichlorophenoxyacetic acid and glyphosate

Waterhemp (Amaranthus tuberculatus) is a common and troublesome weed in cropping systems throughout the United States. With the potential for future periods of low rainfall or drought, the need for improved weed control under drought stress is necessary. Drought stress typically reduces herbicide efficacy by reducing the foliar uptake of herbicides and their translocation. The objectives of this research were to determine the efficacy of 2,4‐dichlorophenoxyacetic acid (2,4‐D) and glyphosate, applied alone or when tank‐mixed, on waterhemp under varying levels of drought stress, the effect of the timing of drought stress in relation to herbicide application and the absorption and translocation of each herbicide in drought‐stressed waterhemp. At reduced herbicide rates, 2,4‐D had a greater level of control of waterhemp under drought stress, compared to glyphosate. The level of herbicide efficacy was lower when the amount of water that was applied to the plants was reduced. The level of waterhemp control was greatest when drought stress occurred before the herbicide application and when the plants were watered to saturation after the application, compared to when drought stress occurred after the herbicide application or restricted watering levels occurred throughout the entire study. Glyphosate absorption and translocation were reduced in the drought‐stressed plants, but 2,4‐D absorption and translocation were not altered. The absence of a reduction in 2,4‐D translocation in the drought‐stressed weeds has not been previously reported. Applying herbicides prior to a rainfall event could increase the weed control level, even if the weed is stressed. Determining how and why 2,4‐D absorption and translocation levels, compared to those of glyphosate, are unaffected by drought stress in waterhemp can aid in improving the control of drought‐stressed weeds with other postemergence herbicides.     

Response of winter wheat (Triticum aestivum L.) and weeds to tank mixtures of 2,4-D plus MCPA with clodinafop propargyl

Field and greenhouse experiments were conducted in 2004 and 2005 to study weed control and the response of winter wheat to tank mixtures of 2,4-D plus MCPA with clodinafop propargyl. The field experiments were conducted at Yazd and Oroumieh, Iran, with factorial combinations of 2,4-D plus MCPA at 0, 975, and 1300 g ai ha⁻¹ and with clodinafop propargyl at 0, 64, 80, 96, and 112 g ai ha⁻¹ in four replications. The greenhouse experiments further evaluated the effect of these tank mixtures on weed control, where each herbicide mixture was considered as one treatment and the experiment was established in a randomized complete block design with four replications. In the field experiments, the herbicides were applied at wheat tillering, while in the greenhouse experiments they were applied at the beginning of the tillering stage and at the four-leaf stage of the grass and broadleaf weeds, respectively. The results indicated antagonistic effects between 2,4-D plus MCPA and clodinafop propargyl. The best tank mixture with regard to weed control efficacy was 2,4-D plus MCPA at 975 g ai ha⁻¹ with clodinafop propargyl at 96 g ai ha⁻¹. The wheat grain yield was also increased by the tank mixture of clodinafop propargyl with 2,4-D plus MCPA. Generally, to inhibit clodinafop propargyl efficacy reduction due to tank-mixing with 2,4-D plus MCPA, it is recommended that the application dose of 64 g ai ha⁻¹ should be increased to 96 g ai ha⁻¹.     

Antagonistic effects of 2,4-D amine and bentazone on control of Avena fatua with tralkoxydim

Control of Avena fatua with tralkoxydim was significantly reduced in glasshouse experiments when the herbicide was tank-mixed with either 2,4-D amine or bentazone. Antagonism increased with increasing rates of these broadleaf herbicides in the tank-mixture and it could, in turn, be decreased by increasing rates of tralkoxydim. When herbicide solutions were applied to single leaves with a micropipette applicator, bentazone was antagonistic only when mixed in the same droplet with tralkoxydim and not when the two herbicides were applied adjacently in separate droplets. In contrast, both separate and combined applications of 2,4-D amine and tralkoxydim were antagonistic. There was no antagonism when mixtures with either 2,4-D amine or bentazone were applied between the leaf sheath and culm. Antagonism could be circumvented by sequential applications of the herbicides. When tral-oxydim was applied first, there was no loss of A. fatua control if bentazone application was delayed 0.5 h or if 2,4-D amine was delayed 24 h. Results suggest that bentazone affects only cuticular penetration of tralkoxydim, whereas 2,4-D amine also influences subsequent translocation.     

Chemical management of weeds in corn hybrids

Corn stands out among the cultivars because of its high importance in food, animal feed and raw materials for energy production. The quantity and quality of the harvested grain can be reduced, mainly due to inadequate weed management. Nowadays, weed control is accomplished with the use of herbicides as pre‐emergence and postemergence, applied alone or in a tank mix. This study aimed to evaluate the efficiency of the control of weeds and phytotoxicity of the herbicides that were applied in tank mix or isolated in the corn hybrids, Formula TL® and SYN 7B28 . Two experiments were conducted: in the 2012/2013 season (Formula TL®) and 2013/2014 season (SYN 7B28). There was a low initial phytotoxicity of all the tested herbicides in the initial evaluation; however, from 14 days after treatment, it was minimal. The weed control of Ipomoea indivisa, Urochloa plantaginea and Euphorbia heterophylla in both experiments was better, in general, when using the tank mixtures of atrazine + simazine + tembotrione and atrazine + simazine + mesotrione, respectively. The application of atrazine + simazine + tembotrione in the SYN 7B28 hybrids provided, with the exception of the thousand‐grain weight, the best expression of grain yield components, combining a low phytotoxicity and best control of the weeds. The Formula TL® hybrid presented the highest mass of a thousand grains and yield with the application of nicosulfuron + tembotrione. The yield average difference of the herbicide treatments and the unweeded control showed an increase of 27.66 and 34.59% in the grain yield of the corn hybrids, Formula TL® and SYN 7B28, respectively.     

Physiological basis for antagonism induced by mixtures of quizalofop-ethyl and bromoxynil in maize (Zea mays)

In this study, the physiological basis for antagonism induced by mixtures of quizalofop‐ethyl and bromoxynil was investigated in maize seedlings. In sequential applications, antagonism was observed when bromoxynil was applied before quizalofop‐ethyl or in a mixture with quizalofop‐ethyl, but was minimal when bromoxynil was applied afterwards. The degree of antagonism differed with application rates of bromoxynil and with the timing of the treatment. When test herbicides were applied locally to the second leaf, the inhibition of photosystem II (PS‐II) in the herbicide‐treated leaf was higher with the mixture than with bromoxynil or quizalofop‐ethyl alone. Subsequent growth of the untreated third leaf inhibited by quizalofop‐ethyl alone then recovered, depending on the dose of bromoxynil. There was no evidence that bromoxynil affected absorption of quizalofop‐ethyl. In local applications at different positions on the second leaf, antagonism was only observed when quizalofop‐ethyl was applied to the distal part of the leaf and bromoxynil applied to the proximal part. The antagonism of bromoxynil + quizalofop‐ethyl did not occur at the level of acetyl CoA carboxylase and Hill reaction, as revealed by in vitro assays. These results suggest that bromoxynil inhibits the phloem transport of quizalofop‐ethyl and thus antagonises its whole‐plant activity in maize.     

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.     

STUDIES ON HERBICIDE CONTENTS IN ROOTS OF SKELETON WEED (CHONDRILLA JUNCEA L.) FOLLOWING LEAF APPLICATIONS

Summary. The herbicides studied were 2,4-D, 2,4-DB, dicamba and orthoarsenic acid. Herbicide content in the roots was taken as an overall measure of penetration into and absorption by the leaves, and of translocation to the roots.
A significantly greater 2,4-D content resulted from foliar application at pH 3–5 than at higher values, though at pH 8–5 the inclusion of triethanolamine significantly increased the 2,4-D content. No evidence was obtained that a greater 2,4-D content should result from foliage applications of 2,4-DB than from 2,4-D. Dicamba gave a greater herbicide content than 2,4-D when applied at high concentration at 20° C but not at 25° C, probably because of less injury at the lower temperature.
Concentrations of Tween 20 up to 2% had no deleterious effect on the 2,4-D content; on the other hand 2,4-D content was lowered by 0–25% or more of cetyltrimethyl-ammonium bromide. Poor wetting is not the cause of the variable herbicide contents sometimes obtained.
Orthoarsenic acid, which has given better control of the weed than 2,4-D, was very poorly translocated; its effectiveness is due to its high intrinsic toxicity.
Etudes sur la teneur en herbicide des racines de Chondrilla juncea L. á la suite d'applications sur les feuilles     

赤霉素缓解Mn~(2+)对草甘膦拮抗效应的研究

为探索能够减轻或消除Mn2+对草甘膦拮抗效应的方法,以高羊茅为试材,将赤霉素(GA3)与草甘膦和硫酸锰(Mn2+质量分数为0.1%)混用,研究了赤霉素对Mn2+降低草甘膦药效的缓解作用。结果表明:赤霉素+草甘膦+硫酸锰处理组高羊茅比同剂量草甘膦+硫酸锰处理组叶色更黄,萎蔫更严重,与同剂量草甘膦单剂处理组比较接近,其中加入50 mg/L赤霉素处理组缓解草甘膦拮抗效应的效果最好。赤霉素+草甘膦+硫酸锰处理组高羊茅的干、鲜重及叶绿素含量均低于草甘膦+硫酸锰处理组,而丙二醛和莽草酸含量均明显高于草甘膦+硫酸锰处理组。处理后第6天,赤霉素+草甘膦+硫酸锰组莽草酸含量分别比草甘膦+硫酸锰组增加了49.8%(加入50 mg/L赤霉素)和28.8%(加入30 mg/L赤霉素),差异显著;处理后第2天,赤霉素+草甘膦+硫酸锰组丙二醛含量分别比草甘膦+硫酸锰组增加了54.1%(加入50 mg/L赤霉素)和52.9%(加入30 mg/L赤霉素),差异显著。研究表明,将赤霉素与锰肥和草甘膦混合喷施,将有可能在一定程度上缓解Mn2+对草甘膦的拮抗效应,保证草甘膦的除草效果。     

Diclofop-methyl antagonism by broadleaf weed herbicides: the importance of leaf expansion rate

Avena saliva cv. Amuri and A. fatua were sprayed with diclofop methyl (1.0 kg a.i. ha^?1) alone and in combination with 2,4-D (1.1 kg a.i. ha^?1), bentazone (1.0 kg a.i. ha^?1), chlorsulfuron (15 g a.i. ha^?1) or dicamba (0.3 kg a.i. ha^?1). Effects of the herbicides on leaf extension rate during the first 8 to 10 days after spraying and subsequent growth (dry weight) after 57–75 days were determined by comparison with unsprayed plants. Diclofop-methyl applied alone did not cause a decrease in leaf extension rate of A. saliva or A. fatua until at least 4 days after spraying. All broadleaf weed herbicides in combination with diclofop-methyl caused a decrease in leaf extension rate of both species within 2 days of spraying. Ten days after spraying, leaf extension rates for plants sprayed with a broadleaf weed herbicide plus diclofopmethyl (all combinations) were lower than for unsprayed plants but greater than for plants sprayed with diclofop-methyl alone. With the exception of A. fatua sprayed with bentazone, long-term growth of plants sprayed with a broadleaf weed herbicide plus diclofop-methyl (all combinations) was lower than for unsprayed plants but greater than for plants sprayed with diclofop-methyl alone. Bentazone applied with diclofop-methyl caused a substantial decrease in leaf extension rate of A. fatua within 24 h of spraying but at harvest, dry weight of plants from this treatment was similar to or less than that for plants sprayed with diclofop-methyl alone. Application of diclofop-methyl with bentazone at a rate of 0.3 kg a.i. ha^?1 also caused a reduction in leaf extension rate of A. fatua within one day of spraying. At this rate of bentazone, dry weight of plants at harvest was intermediate to that of unsprayed plants and those sprayed with diclofop-methyl alone. It is proposed that decreased leaf expansion rate during the first few days afte spraying is the cause of broadleaf weed herbicide antagonism of diclofop-methyl.     

Applying the double knock technique to control Conyza bonariensis

Sequential applications of glyphosate followed by another postemergent herbicide, known as the "double knock" technique, were trialled for their effectiveness in controlling Conyza bonariensis . Combinations of glyphosate with and without 2,4-D followed by paraquat plus diquat, paraquat, or 2,4-D were tested at a range of follow-up application times in two field and two pot experiments. The results showed that paraquat plus diquat or paraquat following glyphosate or glyphosate plus 2,4-D provided highly effective weed control compared to glyphosate alone. The optimum timing for follow-up applications of paraquat or paraquat plus diquat was between 5 and 7 days after the initial glyphosate application. Combined applications of glyphosate and 2,4-D, compared to split applications, were not significantly different. However, following glyphosate application with 2,4-D >1 day later considerably reduced the level of control. This study showed that the double knock technique is highly effective in controlling C. bonariensis and is rapidly becoming an important tool in the management of this problem weed.