Herbicide Uptake Research Articles

Interaction of Surfactants with Plant Cuticles1

How surfactants modify the characteristics of a spray liquid is now reasonably well understood. Beneficial effects are primarily associated with reduction in surface tension. However, the mechanisms whereby surfactants enhance the diffusion of herbicides across the plant cuticle are less clear. Generally, hydrophilic surfactants with a high hydrophile/lipophile balance (HLB) are most effective at enhancing penetration of herbicides with high water solubility, whereas lipophilic surfactants with a low HLB are most effective for enhancing uptake of herbicides with low water solubility. Both high- and low-HLB surfactants are absorbed into the cuticle, but current theory suggests different mechanisms are involved in enhancing diffusion of hydrophilic and lipophilic herbicides across the cuticle. Surfactants having a high HLB are absorbed into the cuticle and enhance the water-holding capacity (hydration state) of the cuticle. With increased cuticle hydration, the permeance of hydrophilic herbicides into the cuticle is increased, which increases the herbicide diffusion rate at a constant concentration gradient. Surfactants having a low HLB are absorbed into the cuticle and increase the fluidity of waxes, as measured by a small reduction in melting point. This increased fluidity increases the permeance of lipophilic herbicides in the cuticle, which, in turn, increases their diffusion rate at a set concentration gradient.Additional index words: Herbicide diffusion, hydrophile/lipophile balance, surfactant mechanism of action, partition coefficient, permeance, postemergence herbicide absorption.Abbreviations: EO, ethylene oxide; HLB, hydrophile/lipophile balance.

Response of Sorghum (Sorghum bicolor) to Atrazine, Ammonium Sulfate, and Glyphosate1

Experiments were conducted to determine whether antagonism between atrazine and glyphosate on shattercane observed in field studies could be duplicated under greenhouse conditions on ‘Rox Orange’ forage sorghum and whether it could be overcome by the addition of ammonium sulfate, other adjuvants, or additional glyphosate. Atrazine or surfactant added to glyphosate did not significantly affect sorghum dry weights compared to glyphosate alone. The Colby equation for synergism indicated that atrazine did not antagonize sorghum control with glyphosate in the greenhouse. Glyphosate at 0.43 kg ae/ha plus ammonium sulfate provided greater control of sorghum than glyphosate at 0.43 kg/ha without ammonium sulfate; however, glyphosate at 0.84 kg/ha plus ammonium sulfate did not provide greater control of sorghum than glyphosate at 0.84 kg/ha without ammonium sulfate. Reduced activity of glyphosate at 0.43 kg/ha in the absence of ammonium sulfate was likely due to an abundance of calcium cations in the carrier water that associated with glyphosate molecules and subsequently reduced herbicide uptake by plants. Thus, antagonism observed under cool conditions in field studies was not evident in controlled-temperature greenhouse studies.Nomenclature: Atrazine, 6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine; glyphosate, N-(phosphonomethyl)glycine; ‘Rox Orange’ sorghum and shattercane, Sorghum bicolor (L.) Moench #3 SORVU.Additional index words: Antagonism, surfactant.

Effect of a vesicular-arbuscular mycorrhizal fungus ( Glomus epigaeus ) on herbicide uptake by roots

Vesicular-arbuscular mycorrhizal (VAM) fungi increase root uptake of P and other minerals, but their role in the uptake of herbicides has received far less attention. These experiments were conducted to determine the effect of the VAM fungus, Glomus epigaeus (Daniels and Trappe), on the absorption of atrazine and trifluralin by roots of corn and soybean. Atrazine uptake by excised corn-root segments was consistently increased by the VAM fungus, but VAM enhancement of atrazine uptake by soybeans was less than that observed for corn. Roots from corn grown for 8 weeks in the greenhouse prior to the experiment took up 25 pmol mm–3 root, whereas non-mycorrhizal roots took up only 11 pmol mm–3 root. Soybean and corn root uptake of trifluralin was also enhanced by VAM infection. Addition of P did not increase herbicide uptake by non-VAM plants. The direct role of VAM hyphae on atrazine uptake was demonstrated using a two-chamber system, where only the fungal hyphae had access to 14C-atrazine-treated soil. Hyphal systems of the fungus were able to remove and transfer 14C-atrazine residues from soil to corn plants, demonstrating direct uptake of the herbicide by the fungal hyphae.

Mechanism of Organic Anion Transport across the Apical Membrane of Choroid Plexus

The mechanism and membrane localization of choroid plexus (CP) organic anion transport were determined in apical (or brush border) membrane vesicles isolated from bovine choroid plexus and in intact CP tissue from cow and rat. Brush border membrane vesicles were enriched in Na(+),K(+)-ATPase (20-fold; an apical marker in CP) and demonstrated specific, sodium-coupled transport of proline, glucose, and glutarate. Vesicular uptake of the anionic herbicide 2, 4-dichlorophenoxyacetic acid (2,4-D) was markedly stimulated by an inward sodium gradient but only in the presence of glutarate, indicating the presence of apical dicarboxylate/organic anion exchange. Consistent with this interpretation, an imposed outward glutarate gradient stimulated 2,4-D uptake in the absence of sodium. Under both conditions, uptake was dramatically slowed and overshoot was abolished by probenecid. Likewise, apical accumulation of 2,4-D by intact bovine choroid plexus tissue in vitro was stimulated by external glutarate in the presence of sodium. Glutarate stimulation was abolished by 5 mM LiCl. Identical findings were obtained using rat CP tissue, which showed both sodium/glutarate-stimulated 2,4-D (tissue/medium (T/M) approximately 8) and p-aminohippurate (T/M = 2) transport. Finally, since the renal exchanger (rROAT1) has been cloned in rat kidney, a rROAT1-green fluorescent protein construct was used to analyze exchanger distribution directly in transiently transfected rat CP. As predicted by the functional studies, the fluorescently tagged transporter was seen in apical but not basolateral membranes of the CP.

Uptake and translocation of imazethapyr in peas as affected by parasitism of Orobanche crenata and herbicide application methods

[14C]‐Imazethapyr was applied as a seed treatment and at plant pre‐emergence and post‐emergence to peas (Pisum sativum L.) parasitized by Orobanche crenata Forsk. Herbicide uptake increased with time regardless of the application method. Uptake reached about 98%, 89%, 81% and 94% of the total herbicide applied for the seed coating, seed soaking, pre‐emergence and post‐emergence treatments respectively. Herbicide translocation within the host plants consistently differed between O. crenata‐infected and non‐infected plants. High levels of 14C activity were accumulated by parasitic plants from the host. In non‐infected pea plants, pods were stronger sinks for imazethapyr than the other parts of the plant, regardless of the application method. The herbicide distribution in the pea plant: O. crenata complex showed the same pattern regardless of the application methods. However, accumulation of radioactivity in the parasite was lower with pre‐emergence and post‐emergence application than with the seed treatments. In addition, radioactivity concentration in O. crenata plants was slightly higher when [14C]‐imazethapyr was applied to pea seeds by coating than by soaking.

Influence of allocation and detoxification of metribuzin in Chenopodium album on the reliability of prediction of the minimum lethal herbicide dose rate

A new method for application of minimum lethal herbicide dose (MLHD) rates is based on the assumption that a MLHD for a photosystem (PS) II herbicide can be predicted for weeds when the developmental stage of seedlings, herbicide uptake and herbicide action are taken into account. Chenopodium album

Absorption and efflux of glyphosate by cell suspensions

The absorption and efflux of [14C]-glyphosate (N-[phosphonomethyl]glycine) was studied in maize (Zea mays L. cv. Aussie) and soybean (Glycine max L. Merr. cv. Maple Arrow) cell suspensions. Glyphosate absorption was complex: at low external herbicide concentrations (3–250 (μM) there was evidence for a single active uptake system with an apparent Km of 31 μM and Vmax of 11 nmol g−1 fr. wt. 2 h−1. The system was inhibited by carbonylcyanide m-chlorophenyl hydrazone (CCCP), orthovanadate, diethylstilbestrol (DES), phosphate, and phosphonoformic acid (PFA) suggesting the glyphosate carrier to be a phosphate transporter energized by the plant plasmalemma ATPase. At higher external glyphosate concentrations the operation of this carrier was masked as passive diffusion became the dominant absorption mechanism. Any non-specific binding of glyphosate to the cell surface during absorption was low (0.01–0.02 nmol g−1 fr. wt). Efflux kinetics of [14C]-glyphosate suggests the herbicide to be located in the cells in three kinetically distinct compartments: after 24 h uptake of radiolabelled herbicide, 71% of absorbed glyphosate was found in the slow compartment (t½, 162 h), 19% in the medium (t½, 185 min) and 10% in the fast (t½, 27 min). The implications of these results in relation to the delivery of glyphosate to its subcellular target site and subsequent phytotoxicity are discussed.

Influence of weather on the efficacy of dichlorprop‐P/MCPA and trihenuron‐methyl

SummaryThe influence of weather on the efficacy of dichlorprop‐P/MCPA and tribenuron‐methyl on annual weeds in spritig barley was studied in the field during 4 years at six locations in southern Sweden. The herbicides were applied at one‐eighth to three‐quarters of the full dose at three application times with approximately 6‐day intervals. Weather stations, placed in the experiments, recorded climatic data, Maximum herbicide efficacy was obtained when spraying 1 week after the cotyledon stage with half or three‐quarters of the recommended dose. Dose‐response curves were estimated and the ED80 doses were calculated. On average, the herbicide dose required to obtain an effect of 80% was about 40% of the recommended dose. The influence of weather was analysed for seven different periods: 7 and 2 days before and after herbicide application, 1 day before and after application, and the day of herbicide treatment. The most pronounced eftects of weather were found for the day of and the day before herbicide application, revealing the strong influence of weather on herbicide uptake and plant metabolism. High air tetnperature and low global radiation during the day of treattnent reduced the ED80 dose of dichlorprop‐P/MCPA, whereas the ED80 dose of tribenuron‐methyl increased. For both herbicides, precipitation and high soil temperature increased the ED80 dose, which reflects the importance of rain on herbicide uptake and the effect of soil temperature on plant growth. The R2 values were higher in the analyses of dichlorprop‐P/MCPA than of tribenuron‐methyl, indicating that the effect of dichlorprop‐P/MCA was more weather‐dependent than that of tribenuron‐methyl. Separate analyses of the ED80 doses for Brassica napus L., Chenopodium, album L. and Stellaria media (L.) Vill. generally resulted in increased R2 values. but otherwise gave results similar to those for the total weed population. Although the analyses revealed significant effects of indivtdual weather factors on herbicide efficacy, it was not possible to discern a consistent and causal relationship between weather and herbicide performance.

Plasma Membrane Lipid Composition and Herbicide Effects on Lipoxygenase Activity Do Not Contribute to Differential Membrane Responses in Herbicide-Resistant and -Susceptible Wild Oat (Avena fatua L.) Biotypes

Plasma membrane lipid composition of herbicide-resistant (R) and -susceptible (S) wild oat biotypes was analyzed to determine the basis for the differential effect of diclofop on the transmembrane electrogenic potential between the two biotypes and reduced herbicide uptake into protoplasts of the R biotype. In addition, lipoxygenase (LOX) activity was examined in herbicide-treated and untreated R and S plants to determine its involvement in herbicide action and resistance. Overall, no significant differences in lipid composition were found between the two biotypes. Glycolipids represented 41 and 36%, phospholipids 29 and 37%, and free sterols 30 and 27% of the total plasma membrane lipid in the R and S biotypes, respectively. No differences in LOX activity were observed between the herbicide-treated and untreated wild oat biotypes. It was concluded that differences in membrane transport of diclofop and its effect on plasma membrane potential in the R and S biotypes are not related to differences in membrane lipid composition or to differential effects of herbicides on LOX activity in the two biotypes. Keywords: Wild oat; herbicide resistance; lipid composition; lipoxygenase; ACCase inhibitors

The biology of Canadian weeds. 106. Linaria dalmatica (L.) Mill.

Dalmatian Toadflax, Linaria dalmatica (L.) Mill. (Scrophulariaceae), is an important weed of rangelands, agricultural crops and waste areas in North America. The literature is less extensive than for the closely related yellow toadflax (Linaria vulgaris Mill.). Introduced from Eurasia as an ornamental plant into North America by 1894, it became naturalized in seven Canadian provinces and all of the United States of America west of the 100th meridian except for New Mexico. In North America it ranges from ca. 35° to 56°N latitude and it grows from near sea level to altitudes up to ca. 2800 m. Production of up to one-half million seeds per plant and its long-lived perennial nature make the species highly competitive and able to invade cropland and even stands of native ungrazed vegetation. Linaria dalmatica is a hemicryptophyte with strong vegetative reproduction and dormant seeds. Growth of creeping roots after removal of aboveground plant parts limits the effectiveness of control treatments such as grazing, clipping, mowing or burning. Several herbicides control the species for the short term; the smooth and waxy leaf surfaces may hinder herbicide uptake. Experimental biological control with insects since the 1960s shows promise. Key words: Linaria dalmatica, Dalmatian toadflax, Scrophulariaceae, weed biology, control, review

Uptake of the Herbicide Diuron as Visualised by the Fluorescence Imaging Technique

Abstract:A new fluorescence imaging system for monitoring the uptake of the PSII‐herbicide diuron (OCMU) was tested in tobacco leaves. UV‐laser‐induced (Λexc = 355 nm) fluorescence images were collected for blue fluorescence F440 (Λem = 440 nm), green fluorescence F520 (Λem = 520 nm), red chlorophyll fluorescence F690 (Λem = 690 nm) and for far‐red chlorophyll fluorescence F740 (Λem = 740 nm). Diuron‐treated leaf parts exhibited a higher red and far‐red chlorophyll fluorescence emission (F690 and F740) than untreated leaf halves, whereas the blue and green fluorescence, F440 and F520, remained unaffected. As a consequence, the fluorescence ratios blue/red (F440/F690) and blue/far‐red (F440/F740) significantly decreased in diuron‐treated leaf parts. The time course of diuron uptake into the leaf could be followed by fluorescence images taken 10 and 30 min after diuron application. The novel high resolution fluorescence imaging method supplies information on the herbicide uptake of each point of the leaf area. Its great advantage as compared to the point data fluorescence measurements applied so far is discussed.

Herbicide Uptake as Influenced by Plant Water Status

The effect of defined plant water status (relative water content) on the foliar influx of [14C]2,4-D [(2,4-dichlorophenoxy)acetic acid] was studied in soybean (Glycine maxL.). A positive correlation between relative water content and leaf water potential was established with a regression coefficientr= 0.978. The foliar influx of 4.45 × 10−4μmol 2,4-D cm−2hr−1was greatly decreased under plant water stress conditions. An approximately 40% decrease in 2,4-D influx was obtained when the relative water content decreased from −0.55 to −1.21 MPa.

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.

Mechanism of Resistance to Chlorotoluron in Two Biotypes of the Grass Weed Alopecurus myosuroides

Possible mechanisms of resistance to the photosystem II (PSII)-inhibiting herbicide chlorotoluron (3-(3-chioro- p-tolyl)-1,1-dimethylurea) were examined in two resistant biotypes (Peldon Al and Lines El) of Alopecurus myosuroides from the United Kingdom. The resistance indices (the ratio of ED 50 values relative to a susceptible standard, Rothamsted) were found to be 28 for Peldon A1 and 2.6 for Lines E1 biotypes, based on whole plant responses. There were no significant differences between resistant and susceptible biotypes in rates of herbicide uptake and translocation. Similarly, the herbicide target site, PSII, was found to be equally sensitive to inhibition by chlorotoluron in all biotypes, indicating changes at the herbicide active site were not responsible for resistance. Both resistant biotypes had a markedly enhanced ability to metabolize chlorotoluron. Following a 24-hr herbicide pulse, 81% of chlorotoluron was metabolized by resistant Peldon Al compared with 37% by Lines El and 6% by susceptible Rothamsted. Twenty-four hours later, 94, 48, and 14% of the chlorotoluron was metabolized by Peldon A1, Lines E1, and Rothamsted, respectively. Chlorotoluron metabolism was decreased, and herbicide phytotoxicity correspondingly increased by treatment in the presence of the cytockrome P450 inhibitor 1-aminobenzoiriazole, suggesting that increased activity of cytochrome P450 monooxygenses may be responsible for herbicide degradation and, therefore, herbicide resistance in two resistant biotypes.

Foliar absorption and translocation of nicosulfuron and rimsulfuron in five annual weed species

Summary Ambrosia artemisüfolia L. (common ragweed) and Digitaria ischaemum Schreb. (smooth crabgrass) are not controlled by nicosulfuron and rimsulfuron at the highest recommended application rates, whereas Panicum miliaceum L. (wild proso millet), Amaranthus retroflexus L. (redroot pigweed) and Avena fatua L. (wild oat) are susceptible. The foliar absorption and translocation of 14C‐nicosulfuron and 14C‐rimsulf uron were studied in these weed species up to 48 h after treatment (HAT). Differences in herbicide uptake and translocation were not correlated with the species susceptibility. By 48 HAT, more than 50% of both herbicides remained on the treated leaf surface. Foliar absorption of rimsulfuron was greater than that of nicosulfuron in A. retroflexus, P. miliaceum and A. artemisüfolia. Most of the absorbed herbicide remained in the treated leaf of each weed species. Export of 14C–nicosulfuron ranged from 28 to 54% of that absorbed, in contrast to 15 to 39% for 14C–rimsulfuron. The absorption and translocation rates of both herbicides were highest within the initial 6 HAT, and decreased thereafter. Both herbicides showed approximately the same distribution pattern within each weed species.

Foliar absorption of some glyphosate formulations and their efficacy on plants

AbstractUptake of various doses (0.27–4.06 g litre−1) of glyphosate acid formulated with various concentrations (0.27–5.42 g litre−1) of two polyethoxylated alcohol surfactants ‘Genapol’ T‐150, ‘Genapol’ T‐250) in wheat (Triticum aestivum L. cv. Ralle) 24 h after application was studied. Both surfactants greatly enhanced glyphosate uptake at concentrations up to 1.35 g litre−1. Uptake could be slightly further improved by ‘Genapol’ T‐150 up to 5.42 g litre−1 while ‘Genapol’ T‐250 at this high concentration antagonized uptake a little. Uptake of glyphosate‐monoammonium formulated with a quaternary ammonium surfactant (‘Trimao’ (8PO)) in wheat and white mustard (Sinapis alba L. cv. Alba) was also studied. Uptake of glyphosate‐monoammonium in wheat was enhanced at surfactant concentrations up to 1.35 g litre−1. Beyond this concentration uptake continued to increase slightly but not significantly. Generally, glyphosatemonoammonium formulated with ‘Trimao’ (8PO) was not taken up as extensively by mustard as by wheat, and the optimum surfactant concentration is probably higher on mustard. The optimal combinations of surfactant concentration and glyphosate dose based on uptake 24 h after application were used in a time‐course study.Results from uptake studies were compared with the efficacy of similar formulations on wheat. The surfactant concentration range which gave the most pronounced effect on glyphosate uptake was found to be similar (0–1 g litre−1) in both studies. However, the surfactant concentration providing maximum herbicide uptake was found to be higher in the uptake study compared to the surfactant concentration providing optimal efficacy in the dose‐response experiments. A dose rate of 0.5 g litre−1 of ‘Tween’ 20 (sorbitan monolaurate surfactant) improved efficacy of a formulation with an optimal mix of glyphosate‐monoammonium and ‘Trimao’ (8PO). Ammonium sulfate (5.0 g litre−1) did not improve this formulation further.

Uptake, Translocation, and Metabolism of Picloram and Metsulfuron Methyl by Two Locoweed Species

The locoweeds, woolly loco and silky crazyweed, contribute to livestock poisoning in the western United States. Differences in response of these two locoweeds to foliar-applied picloram and metsulfuron was investigated by evaluating differences in herbicide uptake, translocation, and metabolism. Silky crazyweed compared to woolly loco was more than 10 times as sensitive to increasing rates of either herbicide. The two species absorbed 8 to 15%, and 11 to 17% of applied picloram and metsulfuron, respectively. Translocation of picloram and metsulfuron out of treated leaflets of either species was less than 3% of that absorbed after 96 h. Approximately 70 and 100% of the absorbed herbicides remained as picloram and metsulfuron, respectively, in both species. Therefore, differences in picloram or metsulfuron uptake, translocation, and metabolism appear inadequate to account for the differential response to each herbicide by the two locoweed species. Selectivity differences between these locoweed genera to picloram and metsulfuron most likely are due to sensitivity differences at sites of action.

Sítios de penetração do quinclorac em sementes e plântulas de pepino e capim-arroz

Experimentos realizados em casa de vegetação ou em laboratório, em Viçosa (MG), visaram determinar os sítios preferenciais de penetração do quinclorac em sementes e plântulas de Echinochloa spp. e Cucumis sativus L. cv. Caipira AG 221. O quinclorac foi eficientemente absorvido por semente, radícula, coleóptilo ou hipocótilo do capim-arroz e do pepino. As maiores reduções no alongamento e na massa seca do sistema radicular e aéreo ocorreram quando o herbicida foi absorvido por radículas, em ambas as espécies, indicando ser o sítio preferencial de penetração em estruturas jovens. O quinclorac produziu sintomas visuais de fitotoxicidade ou alterações morfológicas nas plantas tanto de pepino quanto de capim-arroz.

Relationships between the herbicidal activity and foliar uptake of surfactant-containing solutions of glyphosate applied to foliage of oats and field beans

The enhancement effects of a C 13/C 14 series of aliphatic alcohol (A) surfactants on the herbicidal activity of glyphosate-(monoisopropylammonium) were examined using a model formulation system that enabled relative performance to be assessed at equivalent spray deposition rates. Surfactants with mean molar ethylene oxide (E) contents of 11, 15 and 20 were more effective in increasing glyphosate activity against oats ( Avena sativa L.) than AE6, when the herbicide was sprayed at 54 or 108 g acid equivalent (a.e.) ha −1. At the lower herbicide dose, these surfactants, when added at 1 and 5 g I −1 caused ∼70% more reduction in foliage weight than the herbicide alone, compared with a ∼50% greater reduction at 0.2 g l −1. However, there was no significant concentration effect on enhancement with the same three surfactants at the higher herbicide dose. On field bean ( Vicia faba L.), surfactant E content had little influence on herbicide enhancement compared with surfactant concentration. This effect was noticeable only with glyphosate at 54 g a.e. ha −1, with the highest surfactant concentration (5 g l −1) inducing a ∼35% greater reduction in foliage weight than herbicide without surfactant. At the higher glyphosate rate there were only modest improvements in efficacy on this species after surfactant addition. Results obtained from experiments using 0.2 μl droplet applications of formulations containing [ 14C]glyphosate at herbicide (0.35 and 0.7 g l −1) and surfactant concentrations equivalent to those of the spray solutions, showed that the corresponding enhancements in uptake and translocation of the radiolabel correlated well with the observed improvements of herbicidal activity on oats but not on field bean. On both species, AE6 was antagonistic to uptake of radiolabelled herbicide. Increased herbicide absorption or biological activity did not appear to be related to the spreading properties of the surfactant-containing formulations on the two target surfaces. The implications of these findings are discussed with reference to the problems of predicting and optimizing the performance of agrochemical formulations with surfactant adjuvants.

Assessment of exposure to 2,4-dichlorophenoxyacetic acid in the chemical industry: results of a five year biological monitoring study.

Data on individual exposure to 2,4-dichlorophenoxyacetic acid (2,4-D) in herbicide production plants are limited. Hence, the urinary excretion of this herbicide was measured during a five year (1985-1989) biological monitoring study of 27 men and 18 women employees exposed during the production and formulation of 2,4-D and related sodium and dimethylamine salts. In separate studies, specimens of urine were collected in the morning, or during the last three hours of a working shift, or over a 24 hour period (1200 to 1200 or 0800 to 0800) and were analysed by an immunochemical method (2,4-D radioimmunoassay (RIA)). Urinary 2,4-D concentrations varied within a large scale from only a few micrograms/l to several 10s of mg/l. During a week, herbicide excretion increased, culminating on Friday. At the weekend, when no work was done, 2,4-D elimination decreased but did not return to zero in any case. After an interruption of exposure for about three weeks, urinary 2,4-D was no longer detectable. About five days after restarting work, body concentrations had built up again. Measurements of 2,4-D concentrations in air at different work-places showed that herbicide concentrations did not exceed 0.5 mg/m3. As well as inhalation, dermal 2,4-D absorption seemed to play an important part in total uptake of herbicide. Furthermore, a strong correlation was found (r = 0.9628) between 2,4-D urinary concentration, adjusted for endogenous creatinine, and the estimated amount of absorbed herbicide. Estimated absorbed doses were, in most cases, well below 0.1 mg 2,4-D/kg body wt/day. Sometimes this concentration was greatly exceeded. Thyroid hormone concentrations in blood were measured as well. No notable abnormalities were found. Exposed subjects were also typed for histocompatibility locus antigens (ABC antigens). The immunochemical determination of 2,4-D in specimens of urine proved to be a simple, cost effective, and non-invasive method to measure human exposure.