Lower Herbicide Doses Research Articles

Effect of reduced doses of a post-emergence graminicide mixture to control Lolium rigidum G. in winter wheat under direct drilling in Mediterranean environment

A study was carried out over 2 years (2002/2003 and 2003/2004) on a private farm in the Alentejo (Évora), in the South of Portugal where rainfed wheat is sown after the beginning of the winter rainfall season. A no-till wheat crop was used as this technology provides the necessary soil bearing capacity for the post-emergence application at different weed development stages. A mixture of diclofop-methyl+fenoxaprop- p-ethyl+mefenepir-diethyl was used at three doses on two timings with three different application volumes. The results indicate that lower herbicide concentrations using higher water application volumes decreased weed control efficiency at both weed development stages. For every treatment, the first weed development stage (beginning of tillering) provided a higher crop grain yield. At this stage, both weeds and crop are more sensitive to the herbicide and as consequence the higher weed control efficiency was not reflected in a higher grain yield, due to some crop fitotoxicity caused by the herbicide when its concentration increased. Weed reinfestation was low for both application timings, indicating that lower herbicide doses than the recommended ones (2.5–31 ha −1) or those normally used by the Portuguese farmers (21 ha −1) can be used for a satisfactory control of Lolium rigidum G. In winter wheat post-emergence treatments can reduce doses if combined with adequate application volumes and early application timings. When herbicide application was delayed (complete tillering) higher herbicide doses were necessary to achieve the highest crop yield.

Effect of low doses of herbicides on weeds, nutrient uptake and yield of trans- planted rice (Oryza sativa)

A field experiment was conducted during the rainy season of 2001 and 2002 at Research farm of Institute of Agricultural Science, Banaras Hindu University, Varanasi. Pre-emergence application of mixture of almix + 2,4- DEE 15 + 500 g/ha recorded the minimum weed density and their biomass than rest of the herbicidal treatments. This treatment registered higher weed-control efficiency next to hand-weeding thrice. Hand-weeding and chemi- cal weeding treatment with almix + 2, 4 DEE 15 + 500 g/ha significantly increased the nutrient uptake by crop and gave higher grain yields. However, the minimum nutrient uptake by crop was noted under unweeded check. On an average, almix + 2, 4-DEE 15 + 500 glha registered 88 and 83% higher grain yield in 2001 and 2002, re- spectively, compared to weedy check.

Enhancement of Efficacy of Ascochyta caulina to Control Chenopodium album by Use of Phytotoxins and Reduced Rates of Herbicides

Abstract The use of toxins produced by Ascochyta caulina to enhance the efficacy of this mycoherbicide agent for the biological control of the noxious weed Chenopodium album was investigated. A method of purification of the fungal culture filtrate was developed to produce large amounts of a mixture containing three main toxins: ascaulitoxin, its aglycone, and trans -4-aminoproline. Greenhouse experiments showed that the use of toxin solutions (at 1 mg/ml) in conjunction with spores of A. caulina (at 10 6 /ml) strongly improved the biocontrol efficacy of this fungus by more than 30%. Furthermore, the simultaneous application of toxins or fungal spores, together with low doses of herbicides (metribuzin and rimsulfuron at 1/5 of the labeled rate), gave better results than single-agent treatments.

Biological control of Chenopodium album L. in Europe

Ascochyta caulina (P. Karst) v.d. Aa and v. Kest is aplant pathogenic fungus which is specific to Chenopodium albumL. It has been suggested as a potential mycoherbicide to this weed,which is important and wide spread in arable crops throughout Europe. Toinvestigate its potential as a biocontrol agent, the fungus has beentested in glasshouse and field experiments. Formulations containingdifferent combinations of A. caulina conidia, the phytotoxinsfrom the fungus and low doses of herbicides have been tested.Significant improvement in the efficacy of the fungus was achieved inglasshouse trials with an aqueous formulation containing PVA(0.1% v/v), Psyllium (0.4% w/v), Sylgard 309(0.1% v/v), nutrients and conidia (5 ×106/ml). The extracellular, hydrophilic phytotoxinsproduced by A. caulina were purified and their structuresdetermined. The main toxin, named ascaulitoxin, was characterised as theN2-β-D-glucopyranoside of the unusual bis-aminoacid2,4,7-triamino-5-hydroxyoctandioic acid. Two other toxins proved to betrans-4-amino-D-proline and the aglycone of ascaulitoxin. Thesetoxins have shown promising herbicidal properties. Field trials haveinvestigated the performance of A. caulina conidia applied atdifferent developmental stages of C. album either as a singletreatment or combined with sub-lethal doses of herbicides or with thefungal phytotoxins. With the available formulation, favourable weatherconditions are needed to obtain infection in the field. The efficacy ofthe strain of A. caulina used so far has proved to beinadequate to justify its development as a bioherbicide. This isprobably due to its low virulence.

Toxicity of hercides 2,4-D and MCPA for Rats and Rabbits

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Synthesis, Properties, and Use of Copper-Chelating Amphiphilic Dithiocarbamates as Synergists of Oxidant-Generating Herbicides

The effect of herbicides that act by generating active oxygen species is often mitigated by the enzymes of the Halliwell-Asada oxygen detoxification pathway. Use of compounds interfering with this pathway synergizes herbicides, allowing the killing of weeds at lower herbicide doses. We have used different dithiocarbamate chelators (Dtcs) capable of removing copper from superoxide dismutase, the first enzyme of the pathway, suppressing its action. Dtcs with different hydrophilic-lipophilic properties (sodium salts of ethyl (2-(2-ethoxyethoxy)ethyl)-, butyl (2-(2-ethoxyethoxy)ethyl) -, hexyl (2-(2- ethoxyethoxy)ethyl)-, and dibutyldithiocarbamic acids) were synthesized so that they might penetrate plant cuticles better than sodium diethyldithiocarbamate. Octanol-water distribution ratios and the stabilities of their copper complexes were determined. All Dtcs tested had a high affinity for copper. The Dtcs did not prevent iron-catalyzed Fenton reaction leading to hydroxyl radical productionin vitro. Therefore they would not protect plants from the oxidative damage caused by herbicide, as do compounds that complex iron and thereby prevent the phytotoxic iron-catalyzed Fenton reaction. These amphiphilic compounds show some synergistic activityin vivo, with paraquat as well as lactofen, herbicides that generate active oxygen species by different mechanisms. The results demonstrate that dithiocarbamates have potential as synergists for oxidant-generating herbicides.

Oxidative stress tolerance and longevity in Arabidopsis: the late-flowering mutant gigantea is tolerant to paraquat.

Recent genetic analyses of longevity in animals have revealed that long-lived strains are more tolerant to environmental stresses. To investigate whether extended longevity in Arabidopsis also correlates with an increase in stress tolerance, the response was tested of 11 late-flowering mutants to the superoxide radical-generating herbicide paraquat. A tight correlation between flowering time and paraquat tolerance was found when plants were exposed to low doses of herbicide. Furthermore, the mutant gigantea (gi-3) with the longest delay in flowering time had a high tolerance level to paraquat-induced oxidative stress. All the tested gi alleles had an increased tolerance to paraquat toxicity compared to wild-type, although the actual levels of tolerance differed. In addition, the gi-3 mutant was more tolerant to hydrogen peroxide. These results suggest that the link between longevity and oxidative stress resistance in plants is similar to that found in animals, implying that this phenomenon may be general for all aerobic organisms.

Inheritance of diclofop resistance in wild oat (Avena fatuaL.) biotypes from the Willamette Valley of Oregon

Inheritance of resistance to diclofop was studied in three wild oat biotypes (designated B, C., and H) from the Willamette Valley of Oregon. Cultivated oat (cultivar ‘Monida’) was crossed, including reciprocals, to three wild oat biotypes. Leaves of each F1plant were spotted with diclofop as a nondestructive test for resistance or susceptibility. All F1hybrids were resistant, indicating that resistance is dominant and is under nuclear control. The F2plants where Monida was the maternal parent were screened with diclofop, and F2plants of the Monida/C cross were screened with fenoxaprop because the parent C biotype was resistant to fenoxaprop. At lower doses, a 3:1 (R:S) segregation ratio in F2was observed and at higher doses a 1:3 (R:S) segregation ratio was often observed. The F2:3families segregated in a 1:2:1 (all resistant : segregating resistant and susceptible all susceptible) ratio when treated with a 1.1-kg ae ha−1dose of diclofop. This confirms that resistance to diclofop in the B, C, and H biotypes is primarily under monogenic control, with resistance being dominant to susceptibility at lower herbicide doses. At increased doses, susceptibility becomes dominant. Knowledge of the inheritance of resistance may help in the development of containment measures to prevent the spread of herbicide-resistance genes.

In vivo manipulation of cuticular water permeance and its effect on stomatal response to air humidity.

Cuticular water permeance was manipulated in Corylus avellana L., Hypericum androsaemum L. and Populus tremula L. by (1) long-term application of low doses of various systemic herbicides inhibiting biosynthesis of cuticular waxes, (2) very short-term application of organic solvents to the leaf surface, and (3) exposure to natural strong winds. Treatment effects were very variable, but increased the natural range of permeances by a factor of 10 or so in undamaged leaves. All species had hypostomatous leaves. Relative change of leaf conductance (g) in response to stepwise increases of leaf-to-air water vapour pressure difference (VPD) was measured for individual leaves (Corylus) or groups of leaves at the shoot or branch tip. Adaxial cuticular water permeance (P) was determined for the same leaves after measurement of the VPD-response. A proportional measure of relative change of g with VPD, d(log e .g)dVPD, was then plotted against P. No increase in the strength of the closing response to increasing VPD was found with increasing P, as would have been expected if water loss through the cuticle was involved in stomatal response to changes in VPD via a direct effect on guard cell turgor. By contrast, high P coincided, most clearly in Corylus, with a reduced strength of the stomatal closing response to increasing VPD, i.e. less negative d(loge g)dVPD. As the responses were non-linear, the value of d(loge g)dVPD changed with VPD. With rising VPD, all three species and a fourth one previously studied showed a decline in the value of [d(loge g)dVPD]/d(log P), reaching negative values in one species. This is interpreted in terms of two independent and antagonistic effects of increased cuticular water permeance on guard cell response to VPD, one acting by reducing the backpressure exerted on guard cells by the epidermis, and the other one possibly causing greater depression of guard cell turgor through delivery of more chemical messengers (such as abscisic acid) to the guard cells with the cuticular transpiration stream.

Effects of four cultivation systems for maize on nitrogen leaching 1. Field experiment

An experiment was conducted for 3 years on the Venetian Plain (northeast Italy) to evaluate the environmental impact of four cropping systems at different input levels. The four systems were: (A) higher inputs with pesticides, chemical fertilizers and liquid manure; (B) higher inputs with pesticides and chemical fertilizers; (C) lower inputs with low doses of fertilizers, post-emergence herbicides and use of cover crops; (D) organic, with low inputs without artificial fertilizers or pesticides. To assess the environmental impact samples of groundwater, soil and crop were taken from one field in each system, cultivated with maize but subjected to various management strategies. The results showed large differences in the nitrate nitrogen (NO 3-N) concentration in the shallow groundwater, ranging from a mean value of 7.89mgl −1 in system A to 2.21 mg l −1 in system C. Less variability was observed for total Kjeldhal nitrogen (TKN): a maximum of 5.69 mg l −1 for system D and a minimum of 2.74mg l −1 for system B. Total annual nitrogen losses (TKN + N-NO 3) were 48, 19, 16 and 15 kg ha −1 year −1 in the four cultivation systems, respectively.

Prediction of a low dose herbicide effect from studies on binding of metribuzin to the chloroplasts of Chenopodium album L.

Summary Chenopodium album L. seedlings at the 4‐ and 8‐leaf stage were exposed to low concentrations metribuzin [4‐amino‐6‐(l, l‐dimethyl)‐3‐(methylthio)‐l,2,4‐triazin (4H)‐one] in nutrient solution to study herbicide uptake and the effects of low‐dose rates. Chlorophyll fluorescence was measured to relate the inhibition of photosynthesis to herbicide dose. The minimum rate at which metribuzin fully inhibited photosynthesis was less than 1 μM for seedlings at the 4‐leaf stage of development, and between 1 and 5 μM for the 8‐leaf stage seedlings. With isolated chloroplasts, experiments were conducted to establish the relationship between the amount of herbicide molecules bound to each chloroplast and the inhibition of photosynthesis. From the dose‐response curves obtained it was calculated that photosynthesis was fully inhibited when 7.5 105 molecules metribuzin were bound to each chloroplast. This amount of binding was used to estimate minimum‐lethal dose rates of metribuzin required for seedlings differing in fresh weight of leaves and amounts of chloroplasts present. It is suggested that prediction of a low dose herbicide effect from studies on binding of photosystem‐II inhibitors in combination with chlorophyll fluorescence measurements may lead to the development of a new weed management strategy.

Integração de práticas culturais e redução da dose de bentazon na cultura da soja

O trabalho foi desenvolvido no município de Jaboticabal, SP, no ano agrícola de 1990/91, onde testou-se a integração de práticas culturais e menor dosagem de herbicida aplicado em pós-emergência na cultura da soja, cultivar Paraná. O delineamento experimental utilizado foi o de blocos ao acaso, com dezesseis tratamentos e quatro repetições, sendo quatro tipos de manejos de plantas daninhas: testemunha infestada, 50% da dose recomendada (360 g/ha do herbicida bentazon, dose recomendada do herbicida bentazon (720 g/ha) e testemunha capinada; dois espaçamentos entre-linhas: 30 cm e 60 cm; e duas densidades: normal e reduzida (em torno de 20 e 10 plantas por metro linear, respectivamente). Observou-se que a quantidade de matéria seca das plantas daninhas por época da colheita da soja, foi ligeiramente maior no tratamento com 50% do que no tratamento com 100% da dose do herbicida, porém a diferença não foi significativa. A produtividade no tratamento com 50% de redução na dose do herbicida foi 8,7% menor do que no tratamento com 100%. A redução de 50% na dose do herbicida testado, quando se interagiu com espaçamento e densidades adequadas (30 cm entre-linhas e densidade normal), é possível, considerando-se aceitável as perdas menores que 10% de grãos de soja em relação ao tratamento com dose normal e levando-se em conta os benefícios de tal redução da dose do herbicida.

Evolution of Herbicide Programs in Sugarbeet

During the early years of herbicide use, the total amount of ai applied per ha increased in attempts to obtain season-long weed control, peaking in the decade of the mid-1970's to mid-1980's. Since then, the chemical load applied for broadleaf weed control has shown a consistent, if not dramatic, decline. A much more significant reduction has occurred in grass weed control. Main reasons for the reduction are a move from PPI and PRE treatments to POST, the development of repeat low-dose herbicide techniques, and the introduction of more active postemergence grass herbicides. In general, this change has been achieved with a concomitant improvement in crop safety. These developments occurred as a coincident benefit in pursuing the target objective of giving growers more convenient and flexible weed control and not as a specific attempt to reduce chemical use. This paper discusses the evolution of weed control programs in the U.K., France, Germany, and the U.S.A. All show a similar trend, though the pace of change has been slower in the U.S.A.

Reducing herbicide use in spring cereal production

A survey was conducted in southern and central Finland from 1982 to 1984 to determine the main weed species affecting spring cereal production. The weed flora was dominated by broad-leaved species. The most common broad-leaved weeds were Chenopodium album L., Galeopsis L. spp., Viola arvensis Murr. and Stellaria media (L.) Vili., and the most common grass weed was Elymus repens (L.) Gould. The density of weeds averaged 170 plants m-2 (median 124), and the dry weight 320 kg ha-1 (median 183). Ordination analyses revealed that the species composition of weed populations varied regionally and was affected by soil characteristics and crop management practices, particularly by long-term use of herbicides. An additional aspect studied in the survey was herbicide efficacy in farmers’ fields. Phenoxy acid herbicides, MCPA, dichlorprop and mecoprop, were the most common active ingredients used in the 252 spring cereal fields surveyed. MCPA alone gave only a moderate control of 65%, determined as a reduction of weed biomass, whereas the efficacy of herbicide mixtures containing MCPA averaged 83%. Inadequate control was in most cases due to a wrong choice of active ingredient for the prevailing weed population. Reduction in the use of herbicides by applying lower doses than recommended was studied in field experiments. Herbicide formulations of MCPA/dichlorprop, MCPA/mecoprop and MCPA/fluroxypyr were screened in spring barley (Hordeum vulgäre L.) and spring wheat (Triticum aestivum L.) fields. The efficacy of herbicides, applied at the lowest recommended dose, averaged 85%. At a 30% lower dose the efficacy still reached 79%. Even lower herbicide doses were often adequate, depending on the herbicide, weed species and the crop. The production of weed biomass was adequately suppressed with reduced doses since the most common and aggressive species, such as Chenopodium album and Galeopsis spp., were efficiently controlled with low doses. Use of reduced herbicide doses for three years in the same field caused neither an increase in the subsequent weed infestation nor changes in the species composition of weed populations compared with the treatments at recommended rates of application. The percentage emergence of weeds averaged 70-75% at the time of herbicide application when the crop was at the 3-4 leaf growth stage. However, spraying during the early growth stages of those weeds that emerge in the main flush is recommended since the competitive ability of the crop is normally sufficient to suppress the growth of late-emerging weed seedlings. At harvest the proportion of weed biomass in unsprayed plots, as a proportion of the total vegetative biomass, averaged 3.1% in barley fields and 3.6% in wheat fields. The growth of weeds was more efficiently suppressed with reduced herbicide doses than by increasing the seeding rate of the crop. The mean yield gain remained below 5% at all rates of herbicide application. No reliable density-based threshold for chemical weed control was established. Instead, site-specific dose adjustment based on the composition and infestation level of the prevailing weed populations is suggested to reduce the total use of herbicides and to maintain the current low levels of weed infestation. Consequently, annual reductions of 30% in use of cereal herbicides are expected. Such a reduction corresponds to monetary savings of approximately FIM 20 million per annum at the national level.

Some physicochemical factors influencing foliar uptake enhancement of glyphosatemono(isopropylammonium) by polyoxyethylene surfactants

AbstractStructure‐concentration–foliar uptake enhancement relationships between commercial polyoxyethylene primary aliphatic alcohol (A), nonylphenol (NP), primary aliphatic amine (AM) surfactants and the herbicide glyphosatemono(isopropylammonium) were studied in experiments with wheat (Triticum aestivum L.) and field bean (Vicia faba L.) plants growing under controlled‐environment conditions. Candidate surfactants had mean molar ethylene oxide (EO) contents ranging from 5 to 20 and were added at concentrations varying from 0·2 to 10 g litre−‐1 to [14C]glyphosate formulations in acetone–water. Rates and total amounts of herbicide uptake from c. 0·2–μl droplet applications of formulations to leaves were influenced by surfactant EO content, surfactant hydrophobe composition, surfactant concentration, glyphosate concentration and plant species, in a complex manner. Surfactant effects were most pronounced at 0·5 g acid equivalent (a.e.) glyphosate litre−‐1 where, for both target species, surfactants of high EO content (15–20) were most effective at enhancing herbicide uptake: surfactants of lower EO content (5–10) frequently reduced, or failed to improve, glyphosate absorption. Whereas, at optimal EO content, AM surfactants caused greatest uptake enhancement on wheat, A surfactants gave the best overall performance on field bean; NP surfactants were generally the least efficient class of adjuvants on both species. Threshold concentrations of surfactants needed to increase glyphosate uptake were much higher in field bean than wheat (c. 2 g litre−‐1 and < 1 g litre−‐1, respectively); less herbicide was taken up by both species at high AM surfactant concentrations. At 5 and 10 g a.e. glyphosate litre−‐1, there were substantial increases in herbicide absorption and surfactant addition could cause effects on uptake that were different from those observed at lower herbicide doses. In particular, the influence of EO content on glyphosate uptake was now much less marked in both species, especially with AM surfactants. The fundamental importance of glyphosate concentration for its uptake was further emphasised by experiments using formulations with constant a.i./surfactant weight ratios. Any increased foliar penetration resulting from inclusion of surfactants in 0·5 g litre−‐1 [14C]glyphosate formulations gave concomitant increases in the amounts of radiolabel that were translocated away from the site of application. At these low herbicide doses, translocation of absorbed [14C]glyphosate in wheat was c. twice that in field bean; surfactant addition to the formulation did not increase the proportion transported in wheat but substantially enhanced it in field bean.

Chemistry and Structure-Activity Relationships of Herbicide Safeners

Abstract The discovery and commercial success of safeners against thiolcarbamate herbicide injury to corn has stimulated a rapid progress and opened new possibilities for further research and development in the last decade. Compounds with new chemistry, increased efficacy, and a broader selectivity spectrum were synthesized and developed for agricultural use. Structure-activity relationship studies helped to optimize their chemical properties and to understand their biological modes of action. Several examples indicate close similarity between chemical structures possessing herbicidal and safener properties. In some cases this differentiation may be marginal, as shown in crops pretreated with low herbicide doses leading to safening effects. In other examples, however, structural optima for safening and herbicidal efficacy can be clearly differentiated.

Increased Profitability Using Low Doses of Herbicide with Supplementary Weeding in Smallholdings

Abstract The application of pre-emergence herbicides at low dosage rates in combination with manual weeding reduces weeding costs for smallholders by about 40% as compared to hand weeding or to herbicide application at the recommended rate. Moreover, the combined method appears to be safer with respect to climatic influences and phytotoxic effects. Results obtained from field trials on experimental farms are discussed.

Observation et selection chez Chenopodium album L. d'individus resistants aux triazines

Resume The authors checked resistance of different populations of Chenopodium album L. to heavy doses of atrazine. In the presence of different triazines, diazine and urea, they compared the behaviour of susceptible and resistant populations. Measured by whole leaves fluorescence, electron transfer of susceptible and resistant plants is respectively stopped or not by the different herbicides. Furthermore, with low doses of herbicides, about 1 % of tolerant plants have been selected from susceptible plants. In the presence of the same herbicides, leaves of tolerant plants present intermediate and variable reactions.