Herbicide risk assessment for non-target aquatic plants: sulfosulfuron--a case study

Herbicides entering the aquatic environment by spray drift, run-off and leaching to field drains may cause adverse effects on non-target aquatic vegetation. The potential for such effects has typically been evaluated from tests with floating, monocotyledonous Lemna sp. However, concern has been expressed as to whether this species could be used to indicate potential effects on other vegetation types, particularly rooted, submerged, emergent or dicotyledonous species. In 1997, the Centre for Aquatic Plant Management undertook development of new tests based on the additional species, Glyceria maxima (Hartm) Holmb, Lagarosiphon major (Ridl) Moss and Myriophyllum spicatum L. The resulting methodology was used to assess the effects of the sulfonylurea herbicide, sulfosulfuron on these species. Data presented here demonstrate that exposure to initial sulfosulfuron concentrations of 3.33 microg litre(-1) for up to 21 days was tolerated by these species and that adverse effects were observed only when plants were exposed to initial concentrations of 3.33 and 10 microg litre(-1) for 70 days. As the occurrence of such high initial concentrations for long periods is unlikely in the aquatic environment, sulfosulfuron is not expected to have adverse effects on the growth of these species. This study has also demonstrated that G maxima, L major and M spicatum grown in small outdoor tanks can be used successfully to assess the effects of crop-protection products on non-target aquatic flora.     

Herbicide loss following application to a railway

Railways have been identified as a potential source of herbicides detected in surface and groundwaters, but there are few data to support this theory. Two studies were undertaken to investigate the fate of herbicides applied to railway trackbeds: a pilot study in a section of a disused, but intact, cutting where runoff and throughflow were sampled from trenches adjacent to the treated area, and a larger scale study on 0.75 km of embankment where surface water from the drainage ditch at the base of the embankment and groundwater were sampled. In the pilot study, peak concentrations of atrazine, diuron and glyphosate (1280, 210 and 15 microg litre(-1) respectively) were detected 6days after treatment (DAT). Oxadiazon, oryzalin and isoxaben were not detected above their limits of quantification. Lower concentrations were detected 81 DAT (10 and 0.8 microg litre(-1) of atrazine and glyphosate respectively). In the larger scale study, herbicides were not detected, in either the surface water or groundwater, at concentrations above the limit of detection that could be attributed to application to the railway. Rainfall volume and depth to sampling point may partly explain the different results obtained from the two studies. The findings are compared with herbicide losses from other 'hard surfaces'.     

Effects of low concentrations of vapour of the phenoxyalkanoic herbicide 2,4-D-butyl on growth of tomato plants

Tomato plants (5–7 weeks old)were exposed to a range of concentrations of vapour of 2,4-D-butyl (0.12–2.4 ng litre^?;1), for periods of 6 or 24 h, using an air-flow system. Net carbon dioxide exchange of the whole plant was measured for up to 2 days after exposure and was found to decrease in treated plants approximately in proportion to vapour concentration. Six weeks after exposure, shoot dry-weights of plants exposed to herbicide vapour were less than control plants in all cases, by about 20 % or more. All characteristics measured were affected more by a 24-h exposure period than 6 h, at the same vapour concentration. The amount of herbicide vapour, expressed as the product of exposure period and vapour concentration, gave a similar effect at a low range of vapour concentrations (≤1.0 ng litre^?;1)and air-flow rates of about 200 litre min^?;1, but not at higher concentrations at a flow rate of about 120 litre min ^?;1. Better agreement between the experiments carried out at the two concentration ranges was obtained by expressing the herbicide vapour as the total amount to which the plants were exposed (i.e. the product of air-flow rate, vapour concentration and duration of exposure)instead of concentration alone.     

Effects of chlorpyrifos in freshwater model ecosystems: the influence of experimental conditions on ecotoxicological thresholds

Three experiments were conducted to determine the impact of the insecticide chlorpyrifos (single applications of 0.01 to 10 microg AI litre(-1)) in plankton-dominated nutrient-rich microcosms. The microcosms (water volume approximately 14 litres) were established in the laboratory under temperature, light regimes and nutrient levels that simulated cool 'temperate' and warm 'Mediterranean' environmental conditions. The fate of chlorpyrifos in the water column was monitored and the effects on zooplankton, phytoplankton and community metabolism were followed for 4 or 5 weeks. The mean half-life (t1/2) of chlorpyrifos in the water of the test systems was 45 h under 'temperate' conditions and about 30 h under 'Mediterranean' environmental conditions. Microcrustaceans (cladocerans and copepod nauplii) were amongst the most sensitive organisms. All three experiments yielded community NOEC (no observed effect concentrations) of 0.1 microg AI litre(-1), similar to those derived from more complex outdoor studies. Above this threshold level, responses and effect chains, and time spans for recovery, differed between the experiments. For example, algal blooms as an indirect effect from the impact of exposure on grazing organisms were only observed under the 'Mediterranean' experimental conditions. The relatively simple indoor test system seems to be sufficient to provide estimates of safe threshold levels for the acute insecticidal effects of low-persistence compounds such as chlorpyrifos. The robustness of the community NOEC indicates that this threshold level is likely to be representative for many freshwater systems.     

The toxicity of herbicides to non-target aquatic plants and algae: assessment of predictive factors and hazard

Widely used herbicides sometimes inadvertently contaminate surface waters. In this study we evaluate the toxicity of herbicides to aquatic plants and algae and relate it to environmental herbicide concentrations and exposure scenarios, herbicide formulation and mode of action. This was done experimentally for ten herbicides, using the aquatic macrophyte Lemna minor L. and the green alga Pseudokirchneriella subcapitata (Korshikov) Hindak, supplemented with a database study comprising algae toxicity data for 146 herbicides. The laboratory study showed that herbicide formulations in general did not enhance herbicide efficacy in the aquatic environment. The Roundup formulation of glyphosate proved to be the only exception, decreasing the EC(50) of the technical product for both L. minor and P. subcapitata approximately fourfold. Comparison of the sensitivity of L. minor and P. subcapitata revealed up to 1000-fold higher sensitivity of L. minor for the herbicides categorized as weak acids (pK(a) < 5), emphasizing the importance of higher plants in hazard assessment. Database analyses showed that no herbicide group, categorized by site of action, was significantly more toxic than another. Synthetic auxins were the exception as they are virtually non-toxic to unicellular algae. There was no strong correlation between toxicity to algae and K(ow) of the herbicides, not even within groups having the same site of action. Evaluating all data, few herbicides were toxic at concentrations below 1 microg l(-1), which is the 99.9th percentile of the herbicide concentrations measured in the Danish surveillance programme. Joint action of several herbicides cannot however be excluded.     

Relationship between biochemical biomarkers and pre-copulatory behaviour and mortality in Gammarus pulex following pulse-exposure to lambda-cyhalothrin

The impact of the pyrethroid lambda-cyhalothrin was studied in an in-stream mesocosm placed in a natural riffle of a Danish stream. Twice during summer 2002, the natural macroinvertebrate community was exposed in situ to a 30-min pulse of lambda-cyhalothrin. During exposure, nets caught macroinvertebrates in drift. Exposed and unexposed individuals of the amphipod Gammarus pulex (L) were transferred to aquaria in the laboratory and monitored for biochemical changes (ie biomarkers), pre-copulatory behaviour and mortality. Biochemical biomarkers were identified by screening extracts of exposed and unexposed G pulex using high-performance planar chromatography (HPPC). Biochemical biomarkers were detected 3 h after pulse-exposure, and one biomarker was persistent up to 7 days after exposure. Pre-copulatory behaviour (ie pair formation) was significantly impaired up to 5 days after exposure, and had not fully recovered at the end of the observation period. EC10(0.5 h) and EC50(0.5 h) values for pre-copulatory behaviour were 0.04 and 0.20 microg litre(-1), respectively. Mortality was significant at 0.35 microg litre(-1) with an LC50(0.5 h) of 5.69 microg litre(-1). There was a significant relationship between two biomarkers and mortality. The study demonstrated that pulse-exposure at expected environmental concentrations can reduce local populations of G pulex, resulting in severe impact on populations with limited possibilities of re-colonisation.     

Sulcotrione versus atrazine transport and degradation in soil columns

A soil column experiment under outdoor conditions was performed to monitor the fate of 14C-ring-labelled sulcotrione, 2-(2-chloro-4-mesylbenzoyl)cyclohexane-1,3-dione and atrazine, 6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine, in water leachates and in the ploughed horizon of a sandy loam soil. Two months after treatment, the cumulative amounts of herbicide residues leached from the soil were 14.5% and 7% of the applied radioactivity for sulcotrione and atrazine, respectively. Maximum leachate concentrations for each herbicide were observed during the first month following application: 120 and 95 microg litre(-1) for sulcotrione and atrazine respectively. After 2 weeks, 78% of the sulcotrione and atrazine was extractable from the soil, whereas after two months only 10 and 4%, respectively, could be extracted. The maximum sulcotrione content in the first 10 cm of soil was identical with that of atrazine. For both molecules, the content of non-extractable residues was low, being around 15%. Sulcotrione seems to be more mobile than atrazine but the consequences for water contamination are similar since lower doses are used.     

Activity,adsorption, mobility and field persistence of sulfosulfuron in soil

Petri dish bioassays, based on root response of corn grown in soil or in perlite, were used to study the activity, adsorption, mobility and field persistence of sulfosulfuron in a silty clay loam and a sandy loam soil. Both bioassays indicated that activity of sulfosulfuron increased with increasing herbicide concentration, and to a slightly greater degree in sandy loam soil than in silty clay loam soil. More sulfosulfuron was adsorbed on the sandy loam (not biologically available) than on the silty clay loam soil. Consequently, slightly greater amounts of sulfosulfuron were leached through the silty clay loam than through the sandy loam soil. Biologically available sulfosulfuron was not detected at depths below 40 cm after application in sandy loam, but this was not the case for the silty clay loam soil. In 2002, all sulfosulfuron rates showed field persistence of less than 5 months. On the other hand, in 2003, biologically available sulfosulfuron was detected in the 0–10-cm soil depth 150 days after application. http://www.phytoparasitica.org posting May 6, 2004.     

Movement of pendimethalin,ioxynil and soil particles to field drainage tiles

Knowledge of the movement of herbicides and soil particles to sub-surface tile drainage may help to predict chemical leaching to surface waters and deeper groundwater systems. The movement of pendimethalin (2 years), ioxynil (1 year) and soil particles (3 years) to two tile drains was investigated on a sandy loam soil under natural weather conditions. Herbicide and particle concentrations in the drain water showed a very dynamic pattern. The largest herbicide concentrations were detected during the first tile drain flow events after application. Very little herbicide was lost with drain water later than 2 months after application. The turbidity, reflecting concentrations of soil particles, correlated positively and strongly with the pendimethalin concentration and negatively with the rate of drain water discharge, whereas it was uncorrelated with the ioxynil concentration. Peak turbidity values occurred during or shortly after rainfall events, either in break of frost situations, or on unfrozen soil coinciding with the occurrence of peak moisture contents in the topsoil well (3-7%) above field capacity. On average, 0.0013% of the applied pendimethalin and 0.0015% of the applied ioxynil were lost with drain water. The results suggest that preferential flow promotes the movement of all three substances to the tile drains but indicate somewhat different transport mechanisms for the two herbicides.     

Influence of topsoil tilth and soil moisture status on losses of pesticide to drains from a heavy clay soil

Twelve lysimeters with a surface area of 0.5 m² and a length of 60 cm were taken over mole drains from a Denchworth heavy clay soil and divided into two groups with either a standard agricultural tilth or a finer topsoil tilth. The influence of topsoil tilth on leaching of the herbicide isoproturon and a bromide tracer was evaluated over a winter season. The effect of variations in soil moisture status in the immediate topsoil on leaching of isoproturon, chlorotoluron and linuron was investigated in the following winter season. Here, water inputs were controlled such that lysimeters received 50 mm at a maximum intensity of 2 mm h^?1 over a 4‐week period with herbicides applied on day 15. Three treatments received the water either all prior to application, all after application, or evenly spread over the 4‐week period. Leaching losses of the three herbicides were monitored for a subsequent drainage event. Analysis of covariance showed a significant effect of topsoil tilth and total flow on both the maximum concentrations (P = 0.034) and total losses (P = 0.012) of isoproturon in drainflow. Both concentrations and losses were c 35% smaller from lysimeters with the finer tilth. However, generation of the fine tilth in the field was restricted by a wet autumn and this is not considered a reliable management option for reducing pesticide losses from heavy clay soils. In the second experiment, variation in soil moisture content prior to and after application did not have any significant effect (P < 0.05) upon subsequent losses of the three herbicides to drains. © 2001 Society of Chemical Industry     

Multiple herbicide resistance in littleseed canarygrass (Phalaris minor): A threat to wheat production in India

Littleseed canarygrass (Phalaris minor Retz.), a troublesome weed of wheat in India, has evolved multiple herbicide resistance across three modes of action: photosynthesis at the photosystem II site A, acetyl‐coA carboxylase (ACCase), and acetolactate synthase inhibition. The multiple herbicide‐resistant (MHR) populations had a low level of sulfosulfuron resistance but a high level of resistance to clodinafop and fenoxaprop (ACCase inhibitors). Some of the populations had GR₅₀ (50% growth reduction) values for clodinafop that were 11.7‐fold greater than that of the most susceptible population. The clodinafop‐resistant populations also showed a higher level of cross‐resistance to fenoxaprop (fop group) but a low level of cross‐resistance to pinoxaden (den group). Although clodinafop and pinoxaden are from two different chemical families (fop and den groups), their same site of action is responsible for cross‐resistance behavior. The populations that were resistant to four groups of herbicides (phenylureas, sulfonylurea, aryloxyphenoxypropionate, and phenylpyrazolin) were susceptible to the triazine (metribuzin and terbutryn) and dinitroaniline (pendimethalin) herbicides. The P. minor populations that were resistant to the aryloxyphenoxypropionate and phenylurea herbicides were effectively controlled by the sulfonylurea herbicide, sulfosulfuron. In the fields infested with P. minor that was resistant to clodinafop, a sulfosulfuron application (25 g ha^?1) increased the wheat yield by 99.2% over that achieved using the recommended rate of clodinafop (60 g ha^?1). However, the evolution of multiple resistance against the four groups is a threat to wheat production. To prevent the spread of MHR P. minor populations, as well as the extension of multiple resistance to new chemicals, concerted efforts in developing and implementing a sound, integrated weed management program are needed. The integrated approach, consisting of crop and herbicide rotation with cultural and mechanical weed control tactics, should be considered as a long‐term resistance management strategy that will help to sustain wheat productivity and farmers' income.     

Contribution of non-agricultural pesticides to pesticide load in surface water

Two small creeks, tributaries of the River Ruhr near Schwerte, Federal Republic of Germany, were investigated to reveal the regional agricultural and non-agricultural sources of pesticide inputs and the main pathways to surface water. In addition, the receiving water was monitored for pesticides. The watersheds are situated at the northern margin of the Rhenian Schiefergebirge, a highland landscape in North-Rhine-Westphalia. Solid carboniferous shale is covered by a shallow layer of quaternary unconsolidated rock (porous aquifer thickness <5 m). Occurrence of herbicides such as chlortoluron, isoproturon and terbuthylazine in surface water could be due to their broad agricultural application in regional dominant crops, such as barley, wheat and maize. Occurrence of diuron and glyphosate results from their use in residential settlements and industrial areas as well as from weed control on railway tracks. Atrazine concentrations up to 0.8 microg litre(-1) indicated recent use of this herbicide, which has been banned since 1991, and was also the result of non-agricultural applications. Pathways for pesticide input to the receiving waters were related to both surface run-off and underground passage. Two-thirds of the observed diuron load in the surface water resulted from an input by run-off. This was expected as a result of total herbicide application targets to sealed surfaces infringing current regulations and recommendations. Diuron load varied between 0.6 and 1.2% of the estimated amount applied annually in the investigated catchments. Non-agricultural pesticide use contributed more than two-thirds of the whole observed pesticide load in the tributaries and at least one-third in the River Ruhr.     

A risk assessment of atrazine use in California: human health and ecological aspects

A risk assessment of the triazine herbicide atrazine has been conducted by first analyzing the toxicity database and subsequently estimating exposure. Margins of safety (MOS) were then calculated. Toxicity was assessed in animal studies and exposure was estimated from occupational and dietary sources. In acute toxicity studies, atrazine caused developmental toxicity in the rabbit [no observed effect level (NOEL) 5 mg kg(-1) day(-1)] and cardiotoxicity in a dog chronic study (NOEL 0.5 mg kg(-1) day(-1)); cancer (mammary glands) resulted from lifetime exposure. The mammary tumors, which occurred specifically in female Sprague-Dawley rats, were malignant, increased in a dose-dependent manner and were also observed with other, related triazines. Evidence for a genotoxic basis for these tumors was either equivocal or negative. Triazines have been shown to be clastogenic in Chinese hamster ovary cells, in vitro, but without showing a convincing dose/response relationship. Atrazine can be converted into genotoxic N-nitrosoatrazine in the environment or the digestive system, suggesting that N-nitrosamines derived from triazines could be oncogenic. However, it was concluded that N-nitrosotriazines are unlikely to play a significant role in triazine-induced rat mammary gland tumors. An endocrine basis for the mammary tumors, involving premature aging of the female SD rat reproductive system, has been proposed. A suppression of the luteinizing hormone surge during the estrus cycle by atrazine leads to the maintenance of elevated blood levels of 17beta-estradiol (E2) and prolactin. The mechanism for tumor development may include one or more of the following: the induction of aromatase (CYP19) and/or other P450 oxygenases, an antagonist action at the estrogen feedback receptor in the hypothalamus, an agonist action at the mammary gland estrogen receptor or an effect on adrenergic neurons in the hypothalamic-pituitary pathway. None of these has been excluded as a target because there has been a lack of a rigorous attempt to address the mechanism of action for mammary tumors at the molecular level. The potential occupational exposure to atrazine was assessed during mixing, loading and application. Absorbed daily dosage values were 1.8-6.1 microg kg(-1) day(-1). The MOS values (animal NOEL/human exposure) for short-term (acute) exposure were 820-2800. Longer-term occupational exposure and risk were also calculated. Detectable crop residues are generally absent at harvest. Theoretical calculations of acute dietary exposure used tolerance levels, along with secondary residues, and water, for which there is a maximum contamination level; atrazine plus the three main chlorotriazine metabolites were combined. MOS values were above 2000 for all population subgroups. Dietary exposure to atrazine is therefore extremely unlikely to result in human health hazard. Recent publications have reported a possible feminization of frogs, measured in laboratory and field studies. This is assumed to be due to the induction of aromatase, but no measurements of enzyme activity have been reported. In field studies, the water bodies with the greatest numbers of deformed frogs sometimes had the lowest concentrations of atrazine. Other studies have also cast doubt on the feminization theory, except perhaps at very high levels of atrazine. Epidemiology studies have investigated the possibility that atrazine may result in adverse effects in humans. Although some studies have claimed that atrazine exposure results in an elevated risk of prostate cancer, the published literature is inconclusive with respect to cancer incidence.     

Pesticides in surface water runoff in south-eastern New York State, USA: seasonal and stormflow effects on concentrations

Samples from two streams (Kisco River and the Middle Branch of the Croton River) in the Croton Reservoir system in south-eastern New York State, USA were sampled from May 2000 through to February 2001 in order to document the effect of land use, streamflow and seasonal patterns of application on pesticide concentrations in runoff from developed watersheds. Many of the pesticides detected most commonly in this study are generally used in developed areas, and particularly on turfgrass. Pesticide concentrations were generally higher, and the numbers of compounds were generally larger, in samples from the Kisco River than in samples from the Middle Branch, probably because the Kisco River drainage has a greater population density and is more extensively developed. Four pesticides (2,4-D, 2,4-D-methyl, dicamba and metalaxyl) were detected in at least one sample from the Kisco River at a concentration >1 microg litre(-1), and no pesticides were detected at concentrations >0.4 microg litre(-1) in Middle Branch samples. No human-health-based water-quality standards were exceeded by samples from either site in this study, but samples from the Kisco River contained four insecticides (carbaryl, chlorpyrifos, diazinon and malathion) and one herbicide (2,4-D) in concentrations that exceeded water quality criteria for the protection of aquatic life. The highest concentrations of most compounds occurred during stormflows in both streams in June, September and December, 2000. The lowest concentrations of most compounds at both sites occurred during baseflows from October 2000 through February 2001, even though the concentrations of many compounds increased substantially at the Kisco River site during stormflows in November and December. Detailed data on the variability of pesticide concentrations during stormflows indicate that there may be two sources of pesticides in the Kisco River watershed: (1) elevated concentrations of pesticides during peak flows that occur early in stormflows likely reflect runoff from paved areas, and (2) elevated concentrations during peak flows that occur later in stormflows from areas with lesser amounts of pavement. Data from the Kisco River indicate that the relation between storm discharge and pesticide concentrations varies among compounds, in part because of variation in seasonal application patterns. These variations in the timing of application result in not all stormflows producing increased concentrations of pesticides. Overall, these results indicate the importance of stormflow sampling throughout the year in assessing pesticide fate and transport in urbanized, developed areas.     

Molecular characterisation of resistance to ALS-inhibiting herbicides in Hordeum leporinum biotypes

BACKGROUND: The acetolactate synthase (ALS)-inhibiting herbicide sulfosulfuron is registered in Australia for the selective control of Hordeum leporinum Link. in wheat crops. This herbicide failed to control H. leporinum on two farms in Western Australia on its first use. This study aimed to determine the level of resistance of three H. leporinum biotypes, identify the biochemical and molecular basis and develop molecular markers for diagnostic analysis of the resistance. RESULTS: Dose-response studies revealed very high level (>340-fold) resistance to the sulfonylurea herbicides sulfosulfuron and sulfometuron. In vitro ALS assays revealed that resistance was due to reduced sensitivity of the ALS enzyme to herbicide inhibition. This altered ALS sensitivity in the resistant biotypes was found to be due to a mutation in the ALS gene resulting in amino acid proline to serine substitution at position 197. In addition, two- to threefold higher ALS activities were consistently found in the resistant biotypes, compared with the known susceptible biotype. Two cleaved amplified polymorphic sequence (CAPS) markers were developed for diagnostic testing of the resistant populations. CONCLUSION: This study established the first documented case of evolved ALS inhibitor resistance in H. leporinum and revealed that the molecular basis of resistance is due to a Pro to Ser mutation in the ALS gene.     

Testing MACRO (version 5.1) for pesticide leaching in a Dutch clay soil

Testing of pesticide leaching models against comprehensive field-scale measurements is necessary to increase confidence in their predictive ability when used as regulatory tools. Version 5.1 of the MACRO model was tested against measurements of water flow and the behaviour of bromide, bentazone [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one-2,2-dioxide] and imidacloprid [1-(6-chloro-3-pyridylmethyl)-N-nitroimidazolidin-2-ylideneamine] in a cracked clay soil. In keeping with EU (FOCUS) procedures, the model was first calibrated against the measured moisture profiles and bromide concentrations in soil and in drain water. Uncalibrated pesticide simulations based on laboratory measurements of sorption and degradation were then compared with field data on the leaching of bentazone and imidacloprid. Calibrated parameter values indicated that a high degree of physical non-equilibrium (i.e. strong macropore flow) was necessary to describe solute transport in this soil. Comparison of measured and simulated bentazone concentration profiles revealed that the bulk of the bentazone movement in this soil was underestimated by MACRO. Nevertheless, the model simulated the dynamics of the bentazone breakthrough in drain water rather well and, in particular, accurately simulated the timing and the concentration level of the early bentazone breakthrough in drain water. The imidacloprid concentration profiles and its persistence in soil were simulated well. Moreover, the timing of the early imidacloprid breakthrough in the drain water was simulated well, although the simulated concentrations were about 2-3 times larger than measured. Deep groundwater concentrations for all substances were underestimated by MACRO, although it simulated concentrations in the shallow groundwater reasonably well. It is concluded that, in the context of ecotoxicological risk assessments for surface water, MACRO can give reasonably good simulations of pesticide concentrations in water draining from cracking clay soils, but that prior calibration against hydrologic and tracer data is desirable to reduce uncertainty and improve accuracy.     

Model Predictions and Field Measurements of Chlorsulfuron Leaching under Non-steady-state Flow Conditions

Model simulations of chlorsulfuron (1-(2-chlorophenylsulfonyl)-3-(4-methoxy-6-methyl-1,3,5-triazin-2-yl)urea) leaching in a loamy soil were made with the mechanistic dual-porosity model MACRO. Comparisons were made with a data set obtained in a lysimeter experiment in which leaching was measured during an 11-month period after applying chlorsulfuron at two rates (4 and 8 g ha⁻¹). In this experiment, peak concentrations appeared c.6 months after pesticide application, reaching levels of 14 and 21 ng litre⁻¹ in the low- and high-dose treatments, respectively. These peak concentrations appeared after c.70 mm of accumulated leachate, implying that some of the herbicide was displaced through the soil columns by non-equilibrium flow processes. Model calibration was limited to parameters related to evapotranspiration, water uptake by roots and degradation rates in the subsoil. With this minimum amount of calibration, the model successfully described the leaching pattern of chlorsulfuron, provided that the two-flow domain option in the model was used. Running the model in one-flow domain resulted in considerable underestimates of leaching of chlorsulfuron over the short-term (<1 year). The degradation rate in the subsoil was also found to be critical. It had to be increased about fivefold to match measured chlorsulfuron concentrations in leachate. At such concentrations, 0·012 g ha⁻¹ of chlorsulfuron (0·3% of that applied) was predicted to leach through the soil profile during the 11-month simulation period when the lower dose of the compound was applied.     

Evaluation of uncalibrated preferential flow models against data for isoproturon movement to drains through a heavy clay soil

The uncalibrated predictive ability of four preferential flow models (CRACK‐NP, MACRO/MACRO_DB, PLM, SWAT) has been evaluated against point rates of drainflow and associated concentrations of isoproturon from a highly structured and heterogeneous clay soil in the south of England. Data were available for four plots for a number of storm events in each of three successive growing seasons. The mechanistic models CRACK‐NP and MACRO generally gave reasonable estimates of drainflow over the three seasons, but under‐estimated concentrations of isoproturon over a prolonged period in the first season and over‐estimated them in the two remaining seasons. CRACK‐NP simulated maximum concentrations of isoproturon over the first two events of each of the three seasons of 156, 527 and 24.4 µg litre^?1, respectively, and matched the observed data (465, 65.1 and 0.65 µg litre^?1) slightly better than MACRO (69.1, 566 and 58.5 µg litre^?1). Automatic selection of parameters from soils information within MACRO_DB reduced the emphasis on preferential flow relative to the stand‐alone version of MACRO. This gave a poor simulation of isoproturon breakthrough and simulated maximum concentrations were 0, 50.1 and 35.1 µg litre^?1, respectively. The capacity model PLM gave the best overall simulation of total drainflow for the first two events in each season, but over‐estimated concentrations of isoproturon (967, 808 and 51.3 µg litre^?1). The simple model SWAT represented total drainflow reasonably well and gave the best simulation of maximum isoproturon concentrations (140, 80.2 and 8.2 µg litre^?1). There was no clear advantage here in using the mechanistic models rather than the simpler models. None of the models tested was able to simulate consistently the data set, and uncalibrated modelling cannot be recommended for such artificially drained heavy clay soils. © 2001 Society of Chemical Industry     

Sensitivity analyses for four pesticide leaching models

Sensitivity analyses using a one-at-a-time approach were carried out for leaching models which have been widely used for pesticide registration in Europe (PELMO, PRZM, PESTLA and MACRO). Four scenarios were considered for simulation of the leaching of two theoretical pesticides in a sandy loam and a clay loam soil, each with a broad distribution across Europe. Input parameters were varied within bounds reflecting their uncertainty and the influence of these variations on model predictions was investigated for accumulated percolation at 1-m depth and pesticide loading in leachate. Predictions for the base-case scenarios differed between chromatographic models and the preferential flow model MACRO for which large but transient pesticide losses were predicted in the clay loam. Volumes of percolated water predicted by the four models were affected by a small number of input parameters and to a small extent only, suggesting that meteorological variables will be the main drivers of water balance predictions. In contrast to percolation, predictions for pesticide loss were found to be sensitive to a large number of input parameters and to a much greater extent. Parameters which had the largest influence on the prediction of pesticide loss were generally those related to chemical sorption (Freundlich exponent nf and distribution coefficient Kf) and degradation (either degradation rates or DT50, QTEN value). Nevertheless, a significant influence of soil properties (field capacity, bulk density or parameters defining the boundary between flow domains in MACRO) was also noted in at least one scenario for all models. Large sensitivities were reported for all models, especially PELMO and PRZM, and sensitivity was greater where only limited leaching was simulated. Uncertainty should be addressed in risk assessment procedures for crop-protection products.     

The use of cytochrome P450 genes to introduce herbicide tolerance in crops: a review

Mechanisms of herbicide resistance include (1) modified target site, (2) enhanced detoxification or delayed activation, and (3) alterations in the uptake, translocation, or compartmentalization of a herbicide. The first two mechanisms have mainly been identified in plants. Herbicide resistance genes were isolated for several herbicides of different modes of action. Genes that coded for herbicide target or detoxification enzymes were transferred into crop plants. The transgenic plants expressing these genes were tolerant of the active ingredients of herbicides. Before commercialization, the transgenic plants were tested in the field for risk assessment. In the case of crops with herbicide detoxification enzymes, including cytochrome-P450-species-metabolizing xenobiotics, the substrate specificity of the enzymes as well as the toxicological properties of the herbicide metabolites and the pattern of secondary metabolites in plants must be evaluated. © 1999 Society of Chemical Industry