Herbicide Dissipation Research Articles

The Effect of Citrus Pulp Amendment on Sunflower Production and the Dissipation of the Herbicide Aclonifen

The research evaluated the effects that amendment with 3 kg m-2 and 9 kg m-2 of citrus pulp had both on the production of sunflowers and the dissipation of the aclonifen herbicide. At the same time any eventual effect of the use of the herbicide on sunflower production was verified. The use of the citrus pulp determined an increase in the height of the plants, the diameter of the flower-heads and their achenes production and a reduction in the sterile zone. The effect of amending was not proportional to the quantity of citrus pulp added: in fact, the maximum agronomic efficiency was reached with the lowest quantity of amendant (97 kg of achenes per ton of citrus pulp used, as against the 53 kg obtained with the higher quantity). The herbicide had no effect on sunflower production. The dissipation of aclonifen was not influenced by the addition of citrus pulp in field conditions but in laboratory conditions a faster degradation was found. The mean half-life time was 14 days in the field and 30 and 13 days respectively, in untreated soil and soil treated with citrus pulp, in laboratory conditions.

Persistence of the Sulfonylurea Herbicides Thifensulfuron‐Methyl, Ethametsulfuron‐Methyl, and Metsulfuron‐Methyl in Farm Dugouts (Ponds)

Sulfonylurea herbicides are applied at relatively low rates (3 to 40 g ha(-1)) to control weeds in a variety of crops across the Canadian prairies. Because of their high phytotoxicity and the likelihood of their transport in surface runoff, there is concern about their possible impact to aquatic ecosystems. Little is known, however, about their persistence and behavior in aquatic ecosystems. To assess persistence in aquatic ecosystems, three prairie farm dugouts (ponds) were fortified with either thifensulfuron-methyl {methyl 3-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]amino]sulfonyl]-2-thiophenecarboxylate}, ethametsulfuron-methyl {methyl 2-[[[[[4-ethoxy-6-(methylamino)-1,3,5-triazin-2-yl]amino]carbonyl]amino]sulfonyl]benzoate} or metsulfuron-methyl {methyl 2-[[[[(4-methoxy-6-methyl-1,3,5-triazinyl)amino]carbonyl]amino]sulfonyl]benzoate}. The decreasing order of persistence of environmentally relevant concentrations (1 to 4.6 microg L(-1)) of these herbicides in dugout water over the June to October period was metsulfuron-methyl>ethametsulfuron-methyl>thifensulfuron-methyl. The corresponding dissipation half-lives (DT(50)) of 84, 30, and 16 d, respectively, are in the same relative order as the recropping intervals for these herbicides. Thifensulfuron-methyl showed a biphasic dissipation with slower dissipation during the winter months. In contrast, the dissipation of metsulfuron-methyl, the sulfonylurea herbicide with the longest DT(50), was somewhat enhanced under winter conditions. One of the major routes of sulfonylurea herbicide dissipation was removal from the water column when dugout water was lost during hydrological discharge. The relatively long persistence of these herbicides in water indicates that partitioning into sediments was minimal, the sulfonylurea and methyl ester linkages in these compounds were resistant to hydrolysis in weakly alkaline waters, and that microbial and photolytic degradation in dugout waters were slow.

Pesticide exposure assessment in rice paddies in Europe: a comparative study of existing mathematical models

A comparative test was undertaken in order to identify the potential of existing mathematical models, including the rice water quality (RICEWQ) 1.6.4v model, the pesticide concentration in paddy field (PCPF-1) model and the surface water and groundwater (SWAGW) model, for calculating pesticide dissipation and exposure in rice paddies in Europe. Previous versions of RICEWQ and PCPF-1 models had been validated under European and Japanese conditions respectively, unlike the SWAGW model which was only recently developed as a tier-2 modelling tool. Two datasets, derived from field dissipation studies undertaken in northern Italy with the herbicides cinosulfuron and pretilachlor, were used for the modelling exercise. All models were parameterized according to field experimentations, as far as possible, considering their individual deficiencies. Models were not calibrated against field data in order to remove bias in the comparison of the results. RICEWQ 1.6.4v provided the highest agreement between measured and predicted pesticide concentrations in both paddy water and paddy soil, with modelling efficiency (EF) values ranging from 0.78 to 0.93. PCPF-1 simulated well the dissipation of herbicides in paddy water, but significantly underestimated the concentrations of pretilachlor, a chemical with high affinity for soil sorption, in paddy soil. SWAGW simulated relatively well the dissipation of both herbicides in paddy water, and especially pretilachlor, but failed to predict closely the pesticide dissipation in paddy soil. Both RICEWQ and SWAGW provided low groundwater (GW) predicted environmental concentrations (PECs), suggesting a low risk of GW contamination for the two herbicides. Overall, this modelling exercise suggested that RICEWQ 1.6.4v is currently the most reliable model for higher-tier exposure assessment in rice paddies in Europe. PCPF-1 and SWAGW showed promising results, but further adjustments are required before these models can be considered as strong candidates for inclusion in the higher-tier pesticide regulatory scheme.

Fate of the herbicides glyphosate, glufosinate‐ammonium, phenmedipham, ethofumesate and metamitron in two Finnish arable soils

The fate of five herbicides (glyphosate, glufosinate-ammonium, phenmedipham, ethofumesate and metamitron) was studied in two Finnish sugar beet fields for 26 months. Soil types were sandy loam and clay. Two different herbicide-tolerant sugar beet cultivars and three different herbicide application schedules were used. Meteorological data were collected throughout the study and soil properties were thoroughly analysed. An extensive data set of herbicide residue concentrations in soil was collected. Five different soil depths were sampled. The study was carried out using common Finnish agricultural practices and represents typical sugar beet cultivation conditions in Finland. The overall observed order of persistence was ethofumesate > glyphosate > phenmedipham > metamitron > glufosinate-ammonium. Only ethofumesate and glyphosate persisted until the subsequent spring. Seasonal variation in herbicide dissipation was very high and dissipation ceased almost completely during winter. During the 2 year experiment no indication of potential groundwater pollution risk was obtained, but herbicides may cause surface water pollution.

Conservation cotton production in the southern United States: herbicide dissipation in soil and cover crops

Soil and surface residues from cotton field studies in Stoneville, MS (1994 through 1996) and Florence, SC (1995 through 1996) were sampled to evaluate effects of cover crop and tillage on herbicide dissipation. Mississippi treatments included tillage (conventional [CT]; none [NT]) and cover crop (ryegrass; none [NC]). South Carolina treatments included tillage (CT; reduced tillage [RT]) and cover crop (rye; NC). Fluometuron was applied preemergence (PRE) in both Mississippi and South Carolina, and norflurazon was applied PRE in Mississippi. Soils were sampled various times during the growing season (depths: 0 to 2 cm, 2 to 10 cm). Cover crop residues were sampled from RT or NT cover crop areas. Soil and cover crop sample extracts were analyzed for herbicides. Soil organic carbon tended to increase with tillage reduction and presence of cover crop and was positively correlated with herbicide sorption, especially in the surface. Across locations, herbicide half-lives ranged from 7 to 15 d in the soil surface. Tillage had mixed effects on herbicide persistence in surface soil, with higher herbicide concentrations in CT at early samplings, but differences were insignificant later on. The most consistent effects were observed in RT/NT with cover crops, where cover crop residues intercepted applied herbicide, impeding subsequent movement into soil. Herbicide dissipation in cover crop residues was often more rapid than in soil, with half-lives from 3 to 11 d. Herbicide retention in cover crop residues and rapid dissipation were attributed to strong herbicide affinity to cover crop residues (e.g., fluometuron Kd= 7.1 [in rye]; Kd= 1.65 [in Mississippi Dundee soil CT, NC]) and herbicide co-metabolism as cover crop residues decomposed. A fluometuron metabolite, desmethyl-fluometuron, was observed in most soil and cover crop samples after 1 wk. Only minimal herbicide or metabolite moved into the subsurface, and little treatment effect could be ascribed to herbicide or metabolite movement below 2 cm.

Spatial variability in14C‐herbicide degradation in surface and subsurface soils

The spatial variability in mineralization of atrazine, isoproturon and metamitron in soil and subsoil samples taken from a 135-ha catchment in north France was studied. Fifty-one samples from the top layer were taken to represent exhaustively the 31 agricultural fields and 21 soil types of the catchment. Sixteen additional samples were collected between depths of 0.7 and 10 m to represent the major geological materials encountered in the vadose zone of the catchment. All these samples were incubated with 14C-labelled atrazine under laboratory conditions at 28 degrees C. Fourteen selected surface samples which exhibited distinctly different behaviour for atrazine dissipation (including sorption and mineralization) were incubated with 14C-isoproturon and 14C-metamitron. Overall soil microbial activity and specific herbicide degradation activities were monitored during the incubations through measurements of total carbon dioxide and 14C-carbon dioxide respectively. At the end of the incubations, extractable and non-extractable (bound) residues remaining in soils were measured. Variability of herbicide dissipation half-life in soil surface samples was lower for atrazine and metamitron (CV < 12%) than for isoproturon (CV = 46%). The main contributor to the isoproturon dissipation variability was the variability of the extractable residues. For the other herbicides, spatial variability was mainly related to the variability of their mineralization. In all cases, herbicide mineralization half-lives showed higher variability than those of dissipation. Sorption or physicochemical soil properties could not explain atrazine and isoproturon degradation, whose main factors were probably directly related to the dynamics of the specific microbial degradation activity. In contrast, variability of metamitron degradation was significantly correlated to sorption coefficient (K(d)) through correlation with the sorptive soil components, organic matter and clay. Herbicide degradation decreased with depth as did the overall microbial activity. Atrazine mineralization activity was found down to a depth of 2.5 m; beyond that, it was negligible.

A case study of molinate application in a Portuguese rice field: herbicide dissipation and proposal of a clean-up methodology

This study was designed to monitor molinate losses in surface and underground waters during Ordram application in a rice field situated in central Portugal. Water samples were collected from different sites, before, during and about one month and a half after Ordram application. Molinate quantification was based on a solid-phase microextraction (SPME) method followed by gas chromatography with flame photometric detector (GC–FPD) analysis, and led to the conclusion that the herbicide was dissipated in the environment, reaching levels as high as 3.9 μg l −1 in underground water and 15.8 μg l −1 in the river receiving tail waters. The feasibility of the application of treatment methodologies based on adsorption or biodegradation as processes to remove molinate from real-world waters was assessed. These methods seem suitable to reduce molinate concentrations to values in the range of the legally recommended limits (<0.5 μg l −1).

Measuring of some selected herbicides in paddy surface water in the Saijo Basin, Western Japan

An analysis of the dissipation of herbicides was done in paddy water and in drainage channels of farm paddies as well as in experimental plots. A field study was conducted in three selected paddy farms of Higashi Hiroshima and an experiment was carried out in the Hiroshima Prefecture Agriculture Research Center in the paddy season of the year 2002. The herbicides were mefenacet, thiobencarb, pyributicarb and cyhalofop -butyl. The sample water was analyzed by using gas chromatography after solid phase extraction. The maximum concentration of all herbicides was detected within 1 to 3 days after herbicide application and became trace level after 21 to 28 days. The highest amount of herbicide detected was 338 μg/L mefenacet in Farm B. No cyhalofop-butyl was detected in the experiment but was found in farm paddies until 7 days. Thiobencarb was detected until 21 and 28 days, respectively, in farm paddies and in the experiment. Pyributicarb was detected until 28 days in farm paddies and in the experiment. We found that the concentration of all herbicides in drainage channels was comparable with the paddy field at 1 day and 3 days and thiobencarb at 1 day.

Laboratory assessment of atrazine and fluometuron degradation in soils from a constructed wetland

Constructed wetlands offer promise for removal of nonpoint source contaminants such as herbicides from agricultural runoff. Laboratory studies assessed the potential of soils to degrade and sorb atrazine and fluometuron within a recently constructed wetland. The surface 3 cm of soil was sampled from two cells of a Mississippi Delta constructed wetland; one shallow area disturbed only hydrologically, and the second excavated to provide greater water-holding capacity. The excavated area was more acidic on average (pH 4.85 versus 5.21), but otherwise the physical properties and general microbial enzyme activities in the two areas were similar. Soils were treated with 84 and 68 μg kg −1 soil 14C-ring labeled atrazine and fluometuron, respectively, and incubated under either saturated (88% moisture, w:w) or flooded (1 cm standing water) conditions. Soils were sampled over 32 days and extracted for herbicide and metabolite analysis. Under saturated conditions, fluometuron metabolized to desmethylfluometuron (DMF) with a half-life equal 25–27 days. However, under flooded conditions, the half-life of fluometuron was more than 175 days. Atrazine dissipated rapidly in saturated and flooded soil with a half-life of approximately 23 days, but only 10% of atrazine was mineralized to CO 2. The overall atrazine and fluometuron dissipation rates were similar between the two cells, but each area had a different pattern of metabolite accumulation. The major route of atrazine dissipation was incorporation of atrazine residues into methanol-nonextractable (soil-bound) components, with minimal extractable metabolite accumulation. A mixed-mode extractant (potassium phosphate:acetonitrile) recovered greater amounts of 14C-residues from atrazine-treated soils, suggesting that hydrolysis of atrazine to hydroxylated metabolites was a major component of the bound residues. These studies indicate the potential for herbicide dissipation in wetland soils and a differential effect of flooding on the fate of these herbicides.

Variability of herbicide losses from 13 fields to surface water within a small catchment after a controlled herbicide application.

Diffuse losses from agricultural fields are a major input source for herbicides in surface waters. In this and in a companion paper, we present the results of a comprehensive field study aimed at assessing the overall loss dynamics of three model herbicides (i.e., atrazine, dimethenamid, and metolachlor) from a small agricultural catchment (2.1 km2) and evaluating the relative contributions of various fields having different soil and topographical characteristics. An identical mixture of the three model herbicides as well as an additional pesticide for identification of a given field were applied within 12 h on 13 cornfields (total area approximately 12 ha), thus ensuring that the herbicides were exposed to identical meteorological conditions. After the simultaneous application, the concentrations of the compounds were monitored in the soils and at the outlets of three subcatchments containing between 4 and 5 cornfields each. Particular emphasis was placed on the two rain events that led to the major losses of the herbicides. The rank orders of herbicide dissipation in the soils and of the compound-specific mobilization into runoff were the same in all three subcatchments and were independent of the field characteristics. In contrast, the field properties caused the relative losses from two subcatchments to differ by up to a factor of 56 during the most important event, whereas compound-specific differences of the three neutral herbicides caused the losses to vary only by a factor of 2 during the same event. The enormous spatial variability was mainly caused by factors influencing the fraction of rain that was lost to surface water by fast transport mechanisms. Thus, the key factors determining the spatially variable herbicide losses were the permeability of the soils, the topography, and the location of subsurface drainage systems. These results illustrate the large potential to reduce herbicide losses by avoiding application on risk areas.

Spatial variability of atrazine and alachlor efficacy and mineralization in an eastern South Dakota field

Abstract Landscape-induced differences in soil variability and other parameters have potential effects on herbicide sorption, persistence, degradation, and, ultimately, efficacy. This study examined the spatial variability of herbicide efficacy across an eastern South Dakota field in continuous corn. Atrazine and alachlor had been applied for the previous 10 yr. The spatial variability observed in weed control was compared with herbicide sorption (Kd), mineralization rate, and first-order half-life (t1/2), and field herbicide dissipation rates (DT50). Spatial structure was present in atrazine mineralization, weed biomass, and corn biomass data. The amount of atrazine and alachlor sorbed to soil collected from the summit position of the field was 10 and 20% less, respectively, than the amounts sorbed to backslope or toeslope soils. Generally, both herbicides had faster mineralization rates and shorter t1/2 in summit than in backslope or toeslope soils. Weed biomass was correlated positively with elevation ...

Dissipation of nicosulfuron and rimsulfuron in surface soil.

Field and soil fortification studies were conducted to evaluate the half-lives (DT(50)) of nicosulfuron and rimsulfuron in a Sequatchie silt loam surface soil. The dissipation of each herbicide was also evaluated with the two compounds applied simultaneously, which is a typical application method used in corn production. Field studies in two years indicated that both herbicides alone and in mixture disappeared quickly, with all DT(50) < 6 days. Environmental conditions including warm, moist soil, and a soil pH of 5.7 encouraged rapid herbicide dissipation. Rapid degradation was observed under laboratory conditions using this same soil, with all DT(50) < 3 days. This research indicated minimal risk of carry-over to subsequent rotational crops and minimal residual weed control from these herbicides when applied to a silt loam soil under ambient climatic conditions in Tennessee.

Dissipation of triclopyr herbicide applied in Lake Minnetonka, MN concurrently with Rhodamine WT dye.

Fluorescent dye was applied concurrently with triclopyr in two 6.5-ha treatment plots in Lake Minnetonka, MN for data collection to support full aquatic registration of this herbicide. The herbicide and dye mixture was applied by airboat to Phelps Bay with weighted, trailing hoses to maximize uniform distribution of these materials in the water column. A surface application was made to the Carsons Bay plot to attain theoretical triclopyr and dye concentrations of 2500 and 10 micrograms litre-1, respectively. Water samples collected at various times following application showed very little movement of the herbicide and dye out of the Carsons Bay plot. Triclopyr residues moved to a greater extent out of the Phelps Bay plot. The dye was easily tracked in real-time using field fluorometers, which allowed new sampling stations to be established to monitor this movement. Dye concentrations were strongly correlated to herbicide concentrations (r2 = 0.97 in both plots) but were less predictive of the triclopyr metabolite 3,5,6-trichloropyridinol (TCP; r2 = 0.82 in Phelps and r2 = 0.73 in Carsons), probably due to its differential metabolism and degradation. The inert dye can be used to compare the dissipation of herbicide residues by dilution versus microbial or other breakdown processes. Vertical sampling of dye in the water column showed that surface applications of aquatic herbicides can delay uniform mixing in the water column by several days. Although the dye aided the tracking of residues outside the treatment areas, predetermined sampling times and stations were still needed if very low concentrations of herbicide were to be detected at times and stations where the dye had been diluted below its limit of detection.

Biodegradation of atrazine in sand sediments and in a sand-filter.

The potential of a microbial consortium for treating waters contaminated with atrazine was considered. In conventional liquid culture, atrazine and its two dealkylated by-products were equally metabolised by the microbial consortium. Transient production of hydroxyatrazine was observed during atrazine catabolism, indicating that the catabolic pathway was similar to the one reported for isolates capable of atrazine mineralisation. This consortium was then inoculated to sediments sampled from an artificial recharge site. These sediments were contaminated by atrazine and diuron and exhibited only a slow endogenous herbicide dissipation. Inoculated microorganisms led to extensive atrazine degradation and survived for more than 10 weeks in the sediments. A rudimentary bioreactor was then setup using a soil core originating from the same recharge site. Degrading microorganisms rapidly colonised the core and expressed their degrading activity. The efficiency of the bioreactor was improved in the presence of spiked environmental surface waters. Atrazine degraders thus possibly benefited from the other organic sources in developing and expressing their activity. The microbial consortium did not initially exhibit the capacity to degrade diuron, which was used as reference compound. No change in this characteristic was detected throughout the study.

Dissipation of the triketone mesotrione herbicide in the soil of corn crops grown on different soil types

The new triketone herbicide mesotrione corresponds to the older sulcotrione in which the 2‐chloro benzoyl substituent is replaced by a nitro group, generating an herbicide of greater efficiency and a broader spectrum of activity. Mesotrione has been applied within the same 15 days period pre‐emergence at the rate of 150gha‐1 to four corn crops made at different sites located 40 km apart in Belgium and of soils of different textures, but similar pH and organic matter (old humus) contents. The mesotrione soil half‐life in the 0–10 cm surface soil layer (which contained more than 90% of the residue) was 50 days in loam soil (at Zarlardinge), 41 days in sandy loam soil (at Melle) and in clay soil (at Koksijde), and 34 days in sandy soil (at Zingem). The cumulative effects of the recent organic fertilizer treatments and of the soil texture could explain the differences between the soil half‐lives. The time for the 90% dissipation of mesotrione was between 3.6 (in the sandy soil) to 4.7 months (in the sandy loam, loam and clay soils). The low mesotrione soil residues remaining after the corn harvest should disappear with the usual heavy rains in autumn, and the tilling which precedes the following crop and dilutes the mesotrione soil residue. These low mesotrione soil residues thus should have no phytotoxicity toward the following crop, especially at the lower application dose of 100 g mesotrione ha‐1 used in practice.

Soil dissipation of diuron, chlorotoluron, simazine, propyzamide, and diflufenican herbicides after repeated applications in fruit tree orchards.

In a pear tree orchard planted on loam soil, each plot was treated in April 1998 with either one of the ureas diuron or chlorotoluron, or triazine simazine herbicides applied at 3, 4, and 2 kg AI ha(-1), respectively. Some plots had not been previously treated with one of these herbicides. Other plots had been treated annually during the past 12 years with the same herbicide. One herbicide, and always the same, was thus applied to each plot. In the plots treated for the first time with either diuron, chlorotoluron, or simazine, the soil half-lives of these herbicides in the 0-10 cm surface soil layer were 81, 64, and 59 days, respectively. In the plots treated with the same herbicide for 12 years, the corresponding soil half-lives were 37, 11, and 46 days. Diuron thus produced a moderately enhanced biodegradation, chlorotoluron a high one, and simazine a low but significant one. In another pear tree orchard planted on sandy loam soil, each plot was treated in April 1998 with one of the amide propyzamide (1.25 or 1.0 AI kg ha(-1)) or diflufenican (250 g AI ha(-1)) herbicides. In the plots not previously treated with propyzamide, the propyzamide soil half-life was the same for both doses, i.e., about 30 days. In the plots treated annually for 3 or 14 years with propyzamide, the soil half-life was 12 and 10 days, respectively. In the plots treated for the first time with diflufenican and in those treated annually with diflufenican for 3 years, the diflufenican soil half-life was the same, i.e., 65 days. Propyzamide thus already showed a highly accelerated biodegradation after 3 years of repeated annual applications. Diflufenican, however, did not show enhanced biodegradation after 3 years of repeated annual applications.

Dissipation of herbicides in soil and grapes in a South Australian vineyard

The persistence of herbicides applied in vineyards has become a concern in recent years due to their wide use. Investigations into the fate of herbicides in a vineyard in the Barossa Valley, South Australia, have been directed towards the dissipation of herbicides in soil and on grapes. Concentrations of the herbicides, norflurazon, oxadiazon, and oxyfluorfen in soil were monitored following their application in the vineyard. With the exception of oxyfluorfen, dissipation of these herbicides in the vineyard soil was characterised by fast initial loss followed by slow degradation. For oxyfluorfen the dissipation was slow throughout the period of the study. The half-life for norflurazon in surface soils varied from 50 days in 1996 to 22 days in 1997 while that for trifluralin was 27 days in 1996 and 30 days in 1997. Oxyfluorfen had a very low dissipation rate with a half-life of 119 days. Oxadiazon had a relatively short half-life of 14 days. Dissipation of the herbicide residues on grapes in the Roseworthy campus vineyard showed that trifluralin and oxyfluorfen were not detected 4 days after treatment, while norflurazon and oxadiazon remained on grapes 1 month after treatment. This study showed that the dissipation of herbicides in soil and on grapes was dependent on the physicochemical properties of the herbicides and environmental conditions. The use of these relatively persistent herbicides in vineyards has the potential to harm vines and to contaminate grapes and the wine made from them.

Persistence and leaching of the herbicides acetochlor and terbuthylazine in an allophanic soil: comparisons of field results with PRZM-3 predictions

Rates of degradation and adsorption of acetochlor [2-chloro-N-ethoxymethyl-6′-ethylaceto-o- toluidide] and terbuthylazine [N 2-tert-butyl-6-chloro-N4-ethyl-1,3,5-triazine-2,4-diamine] in a Horotiu sandy loam soil (Typic Orthic Allophanic) were determined under controlled temperature and soil moisture regimes. These were then combined with site-specific soil properties and climatic conditions in the Pesticide Root Zone Model (PRZM-3) to predict dissipation and leaching of the herbicides in the field. PRZM-3 significantly under-estimated dissipation of both herbicides in the field using parameters derived from the laboratory incubation studies. When these parameters were derived from the field trials, PRZM-3 adequately predicted dissipation of both herbicides using a two-rate dissipation sub-model but under-predicted the dissipation when a simpler single-rate sub-model was used. Earlier-than-expected appearance of both herbicides in sub-soil layers were postulated to result from the non-equilibrium adsorption/transport of the herbicides and preferential flow, which cannot be simulated by PRZM-3. © 2000 Society of Chemical Industry

Persistence and leaching of the herbicides acetochlor and terbuthylazine in an allophanic soil: comparisons of field results with PRZM-3 predictions

Rates of degradation and adsorption of acetochlor [2-chloro-N-ethoxymethyl-6′-ethylaceto-o- toluidide] and terbuthylazine [N 2-tert-butyl-6-chloro-N4-ethyl-1,3,5-triazine-2,4-diamine] in a Horotiu sandy loam soil (Typic Orthic Allophanic) were determined under controlled temperature and soil moisture regimes. These were then combined with site-specific soil properties and climatic conditions in the Pesticide Root Zone Model (PRZM-3) to predict dissipation and leaching of the herbicides in the field. PRZM-3 significantly under-estimated dissipation of both herbicides in the field using parameters derived from the laboratory incubation studies. When these parameters were derived from the field trials, PRZM-3 adequately predicted dissipation of both herbicides using a two-rate dissipation sub-model but under-predicted the dissipation when a simpler single-rate sub-model was used. Earlier-than-expected appearance of both herbicides in sub-soil layers were postulated to result from the non-equilibrium adsorption/transport of the herbicides and preferential flow, which cannot be simulated by PRZM-3. © 2000 Society of Chemical Industry

Isoxaben and BAS 479 14H retention/loss from peat substrate of nursery plants

Weed management for nursery ornamental plants requires several applications of preemergence herbicides to control recurring weed emergence and to counter chemical dissipation and leaching losses. Herbicide losses from plant containers in runoff water may pose risks to surface and groundwater supplies. Sphagnum peat is a frequently used rooting medium. Limited information exists on the fate of herbicides applied to peat. The leaching of two commonly used chemicals in Europe by the containerized ornamental plant industry, isoxaben and BAS 479 14H, was measured. Herbicides were applied to 1-yr-old conifers (Juniperus communis, cv. Repanda [common juniper]) grown in pots containing sphagnum peat. The isoxaben-treated pots were placed on 1- by 1-m lysimeters that were buried in the center of replicate field plots. Pots were also placed in subirrigated sandbeds to determine herbicide dissipation and movement in irrigation water. With overhead irrigation, isoxaben had a half-life of 2 mo in the 0- to 5-cm depth of the peat container. More than 96% of applied isoxaben was recovered in the 0- to 5-cm depth. Less than 4% of the amount applied was found in the 5- to 10-cm depth, and none was detected in the 10- to 15-cm depth. No isoxaben residues were detected in water percolating through the field lysimeters. In subirrigated sandbeds, no isoxaben was detected in the water rising by capillary tension. Most of the applied isoxaben was found in the 0- to 5-cm layer of the peat container, where its half-life was also 2 mo. Similar results were observed in containers treated with BAS 479 14H, which had a half-life of 1.2 mo. The high organic matter content of the peat medium strongly sorbed both isoxaben and BAS 479 14H, and these herbicides remained primarily in the near-surface zone of the peat containers. No significant herbicide leaching or loss from the containers was found when subirrigated or irrigated by sprinkler.