S-ethyl Dipropyl Carbamothioate Research Articles

Community air monitoring for pesticides. Part 3: using health-based screening levels to evaluate results collected for a year

The CA Department of Pesticide Regulation (CDPR) and the CA Air Resources Board monitored 40 pesticides, including five degradation products, in Parlier, CA, to determine if its residents were exposed to any of these pesticides and, if so, in what amounts. They included 1,3-dichloropropene, acrolein, arsenic, azinphos-methyl, carbon disulfide, chlorpyrifos and its degradation product, chlorthalonil, copper, cypermethrin, diazinon and its degradation product, dichlorvos, dicofol, dimethoate and its degradation product, diuron, endosulfan and its degradation product, S-ethyl dipropylcarbamothioate (EPTC), formaldehyde, malathion and its degradation product, methyl isothiocyanate (MITC), methyl bromide, metolachlor, molinate, norflurazon, oryzalin, oxyfluorfen, permethrin, phosmet, propanil, propargite, simazine, SSS-tributylphosphorotrithioate, sulfur, thiobencarb, trifluralin, and xylene. Monitoring was conducted 3days per week for a year. Twenty-three pesticides and degradation products were detected. Acrolein, arsenic, carbon disulfide, chlorpyrifos, copper, formaldehyde, methyl bromide, MITC, and sulfur were detected in more than half the samples. Since no regulatory ambient air standards exist for these pesticides, CDPR developed advisory, health-based non-cancer screening levels (SLs) to assess acute, subchronic, and chronic exposures. For carcinogenic pesticides, CDPR assessed risk using cancer potency values. Amongst non-carcinogenic agricultural use pesticides, only diazinon exceeded its SL. For carcinogens, 1,3-dichloropropene concentrations exceeded its cancer potency value. Based on these findings, CDPR has undertaken a more comprehensive evaluation of 1,3-dichloropropene, diazinon, and the closely related chlorpyrifos that was frequently detected. Four chemicals-acrolein, arsenic, carbon disulfide, and formaldehyde-sometimes used as pesticides were detected, although no pesticidal use was reported in the area during this study. Their presence was most likely due to vehicular or industrial emissions.

Potato and Weed Response to Postemergence-applied Halosulfuron, Rimsulfuron, and Eptc

Weed control and potato response to halosulfuron applied alone POST and with rimsulfuron or S-ethyl dipropyl carbamothioate (EPTC) were evaluated in 2004 and 2005 near Paterson, WA. Potatoes were injured and exhibited chlorosis and stunted growth after halosulfuron applications of 18, 26, and 35 g/ha. Potato height was reduced 33 and 20% in late May by halosulfuron at 18 or 26 g/ha in 2004 and 2005, respectively. Halosulfuron applied alone failed to control hairy nightshade and large crabgrass. Total tuber yield and U.S. no. 1 yield were reduced 10% in halosulfuron-treated plots because of poor weed control and possibly herbicide injury. Tank-mixing rimsulfuron with halosulfuron improved control of hairy nightshade and large crabgrass and increased potato yield. Tank-mixing EPTC at 2 kg/ha with halosulfuron improved early-season hairy nightshade control, but weed control was poor at row closure. Rimsulfuron applied alone at 18 or 26 g/ha controlled hairy nightshade and large crabgrass without potato injury and resulted in the greatest potato yields.Nomenclature: Halosulfuron; rimsulfuron; hairy nightshade, Solanum sarrachoides Sendt. SOLSA; large crabgrass, Digitaria sanguinalis L. Scop. DIGSA; potato, Solanum tuberosum L. ‘Umatilla’.

Effect of in-row cultivation, herbicides, and dry bean canopy on weed seedling emergence

Field experiments were conducted in 1996 and 1997 to evaluate the effect of EPTC (S-ethyl dipropyl carbamothioate) plus ethalfluralin at 2.4 plus 0.83 kg ai ha−1, rotary hoeing, in-row cultivation, rotary hoeing plus in-row cultivation, and dry bean canopy on weed seedling emergence. Cumulative weed emergence in 1996 and 1997 was similar in cropped and noncropped areas. Herbicides were more effective than mechanical cultivation in reducing weed emergence 91% in 1996 and 88% in 1997. Weed emergence was similar in both rotary hoed area and cultivated area in 1996 but weed emergence was 44% lower in rotary hoed plots than in cultivated plots in 1997. The Gompertz equation did not adequately predict weed seedling emergence in the untreated control and with in-row cultivation in 1996. Initial weed seedling emergence was observed at about 120 growing degree-days with 3 to 9% cumulative emergence among treatments. In 1997, the Gompertz equation adequately described weed seedling emergence in plots with or without disturbed soil. Weed emergence was first observed at 80 growing degree-days with 6 to 16% cumulative emergence among treatments. Predicted percent weed emergence closely approximated observed emergence in 1996 and 1997. Rotary hoeing plus in-row cultivation reduced maximum percent emergence rate 37% on an average. The greater maximum percent emergence rate obtained with in-row cultivation suggests that this treatment increased weed seedling emergence in 1997. On average, weed seedling emergence in the untreated check was lower in cropped areas than in noncropped areas, implying a competitive effect by the dry bean crop. Although weed seedling emergence occurred throughout the growing season, more weed seedlings emerged in June and early July than in late July and August.

Preemergence Weed Control in Potato (Solanum tuberosum) with Ethalfluralin1

Field studies were conducted to assess weed control and potato (Solanum tuberosum) tolerance to ethalfluralin. Ethalfluralin applied preemergence (PRE) alone at 1.05 kg ai/ha generally did not control weeds adequately. However, ethalfluralin at 1.05 kg/ha combined with either metribuzin at 0.28 kg ai/ha or rimsulfuron at 0.018 kg ai/ha controlled common lambsquarters (Chenopodium album), redroot pigweed (Amaranthus retroflexus), and green foxtail (Setaria viridis) ≥ 98%, which was similar to control observed with several currently registered herbicide mixtures. Volunteer oat (Avena sativa) control with either ethalfluralin at 1.05 kg/ha plus EPTC at 3.4 kg ai/ha or ethalfluralin plus metribuzin was equal to registered two-way mixtures. Ethalfluralin plus metribuzin did not adequately control hairy nightshade (Solanum sarrachoides), but ethalfluralin mixtures with either rimsulfuron or EPTC controlled hairy nightshade equal to or better than the registered two-way mixtures evaluated. A sequential application of ethalfluralin PRE followed by rimsulfuron or rimsulfuron plus metribuzin postemergence (POST) did not improve hairy nightshade control compared to ethalfluralin plus rimsulfuron applied PRE. Potato tolerance to herbicide treatments applied PRE or POST to potato was evaluated in weed-free studies. Ethalfluralin alone or with metribuzin was compared to mixtures of metribuzin with either pendimethalin or EPTC. Initial visual injury with ethalfluralin PRE was ≤ 4% both years. In 1996, initial injury with ethalfluralin POST was ≤ 4% and U.S. No. 1 and total tuber yields were not affected by herbicide treatment or application timing. However in 1997, initial injury from POST ethalfluralin at 1.05 or 2.1 kg/ha was 2 or 8% and increased to 9 or 17%, respectively, at potato row closure. Averaged over all herbicide treatments, POST applications reduced U.S. No. 1 and total tuber yield 7% relative to PRE applications in 1997.Nomenclature: EPTC, S-ethyl dipropyl carbamothioate; ethalfluralin, N-ethyl-N-(2-methyl-2-propenyl)-2,6-dinitro-4-(trifluoromethyl)benzenamine; metribuzin, 4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one; pendimethalin, N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine; rimsulfuron, N-[[(4,6-dimethoxy-2-pyrimidinyl)amino] carbonyl]-3-(ethylsulfonyl)-2-pyridinesulfonamide; common lambsquarters, Chenopodium album L. #3 CHEAL; green foxtail, Setaria viridis (L.) Beauv. # SETVI; hairy nightshade, Solanum sarrachoides Sendter # SOLSA; redroot pigweed, Amaranthus retroflexus L. # AMARE; volunteer oat, Avena sativa L. # AVESA; potato, Solanum tuberosum L. ‘Russet Burbank.’Additional index words: Injury, tolerance, AMARE, AVESA, CHEAL, SETVI, SOLSA.Abbreviations: PNW, Pacific Northwest; POST, postemergence; PRE, preemergence; WAT, weeks after treatment.

Impact of Timing and Frequency of In-Row Cultivation for Weed Control in Dry Bean (Phaseolus vulgaris)

Effectiveness of rotary hoeing with cultivation and comparison of an in-row cultivator with a standard row-crop cultivator were determined in dry edible bean. The effectiveness of in-row cultivation conducted at various timings and frequencies was examined. The in-row cultivator was more effective in reducing weed populations than the standard cultivator, although at least two mechanical weeding operations were needed to reduce weed populations to levels of the herbicide check (EPTC [S-ethyl dipropyl carbamothioate] plus ethalfluralin). When the in-row cultivation was delayed until the second trifoliolate stage or later, weed populations were greater than those in the herbicide check. In situations with high weed populations, rotary hoeing prior to cultivation was required to reduce weed populations to levels similar to the herbicide check. An in-row cultivator has potential to improve mechanical weed control options in a crop such as dry edible bean. The types of adjustments made in combination with soil textures, soil moisture, and operator experience affect overall weed control. Thus, it is expected that the level of weed control will vary from year to year and even field to field for the same operator.

Degradation of the thiocarbamate herbicide EPTC (S-ethyl dipropylcarbamothioate) and biosafening by Rhodococcus sp. strain NI86/21 involve an inducible cytochrome P-450 system and aldehyde dehydrogenase

Determination of the N-terminal sequences of two EPTC (S-ethyl dipropylcarbamothioate)-induced proteins from thiocarbamate-degrading Rhodococcus sp. strain NI86/21 resolved by two-dimensional electrophoresis enabled the localization of the respective structural genes on two distinct DNA fragments. One of these strongly induced proteins is a NAD(+)-dependent dehydrogenase active on aliphatic aldehydes. The second protein was identified as a cytochrome P-450 enzyme. The cytochrome P-450 gene represents the first member of a new family, CYP116. Downstream of the cytochrome P-450 gene, two genes for a [2Fe-2S] ferredoxin (rhodocoxin) and a ferredoxin reductase are located. A putative regulatory gene encoding a new member of the AraC-XylS family of positive transcriptional regulators is divergently transcribed from the cytochrome P-450 gene. By hybridization, it was demonstrated that the aldehyde dehydrogenase gene is widespread in the Rhodococcus genus, but the components of the cytochrome P-450 system are unique to Rhodococcus sp. strain NI86/21. Overexpression in Escherichia coli was achieved for all of these proteins except for the regulatory protein. Evidence for the involvement of this cytochrome P-450 system in EPTC degradation and herbicide biosafening for maize was obtained by complementation experiments using EPTC-negative Rhodococcus erythropolis SQ1 and mutant FAJ2027 as acceptor strains. N dealkylation by cytochrome P-450 and conversion of the released aldehyde into the corresponding carboxylic acid by aldehyde dehydrogenase are proposed as the reactions initiating thiocarbamate catabolism in Rhodococcus sp. strain NI86/21. In addition to the major metabolite N-depropyl EPTC, another degradation product was identified, EPTC-sulfoxide.

Slow Release of S-Ethyl Dipropylcarbamothioate from Clay Surfaces

The adsorption/desorption behavior of the volatile herbicide S-ethyl dipropylcarbamothioate (EPTC) to/from montmorillonite and sepiolite was studied. The clays were used as such or with their surface properties modified by adsorption of the cationic dye methyl green (MG). At 30 degrees C, the half-life time (T1/2) of the herbicide in its free form was 10 h, whereas when adsorbed to montmorillonite the T1/2 was more than 5 days. When EPTC was incorporated into the soil, the T1/2 values were 4 and 9 days for the free and adsorbed forms, respectively. The interactions between EPTC and clay surface were studied by Fourier-transform infrared spectroscopy. Vibrational spectra revealed two different populations of EPTC in the clay-organic complexes: one in which the molecules are adsorbed, interacting with the clay surface, and another in which they do not interact with the clay, their spectrum being almost identical to that of free EPTC. Coadsorbed MG impairs the EPTC-clay interactions, causing an increase in free EPTC, indicating that free EPTC is released from the organoclay at a rate higher than that of adsorbed EPTC. Oat bioassays showed that the herbicidal activity of clay-complexed EPTC was extended by more than 1 week. These data indicate that clay-EPTC interactions can be employed to improve herbicidal performance of EPTC and to control its release to the environment.

Effects of carbofuran and the corn rhizosphere on growth of soil microorganisms

Biodegradation of carbamates and carbamothioates by microorganisms in soil has been widely reported; however, the ecology of pesticidedegrading microorganisms in soils and plant rhizospheres containing these pesticides has received less attention. Some bacteria capable of carbofuran (2,3-dihydro-2,3-dimethyl-7-benzofuranyl methyl carbamate) biodegradation in soil have been isolated and identified (Felsot et al. 1981 ; Chaudhry and Ali 1988). Reed et a1.(1987) identified several fungi and bacteria from soils with histories of pesticide application and observed that bacteria were able to metabolize the pesticides in pure culture more efficiently than were fungi. Bacteria and actinomycetes from soils with histories of carbamothioate herbicide use were found to utilize carbamothioates more efficiently than were isolates from nonhistory soils (Mueller et al. 1989). Lee (1984) found that biodegradation of EPTC (Sethyl dipropyl carbamothioate) was significantly affected by soil fungi. Interestingly, after storage bacterial isolates lost the ability to degrade EPTC. He concluded that fungi able to degrade EPTC may retain this ability in soil longer than do bacteria. Recent studies demonstrated that fungi along with bacteria detoxify herbicide wastes during bioremediation of contaminated soils (Felsot and Dzantor 1990). Microbial communities composed of several to numerous species are more likely to be responsible for pesticide biodegradation in soil and rhizosphere environments than are single species. Pesticides applied to soil at planting should persist during the development of plant roots. Therefore, a portion of the pesticide likely interacts with microorganisms in the rhizosphere. Few previous studies have considered the effect of plant roots on the activity of microorganisms in the presence of pesticides. A five-member microbial community from a wheat rhizosphere was able to metabolize the herbicide mecoprop [2-(4-chloro2-methylphenoxy) propanoic acid] in liquid culture even though individual members could not (Lappin et al. 1985). Biodegradation of (2,4-dichlorophenoxy)acetic acid by microorganisms was found to be several times higher in sugarcane rhizospheres than in surrounding soils (Sandmann and Loos 1984). Send reprint requests to R.J. Kremer, 144 Mumford Hall, Columbia, Missouri 65211 USA

Timing of herbicide application and potato hilling

Weed management involving the time of hilling and various herbicide applications was compared. The best weed management program consisted of EPTC (S-ethyl dipropylcarbamothioate) at 3.3 kg ai ha-1 preplant incorporated, followed by metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] at 0.6 kg ai ha-1 applied delayed pree-mergence, or delayed preemergence applications of metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] plus linuron [N’-(3,4-dichlorophenyl)-N-methylurea] (2.2 + 1.1 kg ai ha-1) or metolachlor plus metribuzin (2.2 + 0.6 kg ai ha-1) regardless of the time of hilling. Alternatively, postemergence applications of sethoxydim [2-[1-(ethoxyimino) butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one] plus metribuzin plus crop oil concentrate at 0.2 plus 0.6 kg ai ha-1 plus 1.2 L ha-1 provided excellent season-long weed control regardless of the time of hilling. However, potato yield was reduced in 1 of 2 years by this treatment. Preemergence herbicides applied immediately after planting followed by an early hilling at ground cracking did not provide season-long weed control. The cultivar Atlantic was a better competitor with weeds than Russet Burbank in 1 of 2 years.

Kinetics of EPTC biodegradation and the effects of the inhibitor dietholate in solids

The mineralization kinetics and incorporation into microbial biomass of [ N- α propyl- 14C] and [carbonyl- 14C]EPTC ( S-ethyl dipropylcarbamothioate) were determined in a soil with no previous use of carbamothioate herbicides, and two soils with previous exposure to the herbicide EPTC. Previous exposure to EPTC resulted in microbial adaptation to EPTC, as evidenced by increased degradation and mineralization of EPTC. Amounts of 14CO 2 evolved from [ N- α propyl- 14C]EPTC in adapted soils were nearly twice that from nonadapted soil. The maximum recovery of EPTC in microbial biomass was 2.6% of added 14C. Dietholate ( O, O-diethyl O-phenyl phosphorothioate), which extends persistence and weed control of EPTC, appears to act by inhibiting metabolism of the carbonyl linkage of EPTC, based on [carbonyl- 14C]EPTC mineralization. Dietholate lost effectiveness after repeated applications with EPTC.

Glutathione S-Transferase Activity in Nontreated and CGA-154281- Treated Maize Shoots

Abstract Fast protein liquid chromatography (anion exchange) was used to separate glutathione S-transferase isozymes in nontreated etiolated maize shoots and those treated with the herbi­cide safener CGA -1542814-(dichloroacetyl)-3,4-dihydro-3-methyl-2 H-1 ,4-benzoxazine. Non­treated shoots contained isozymes active with the following substrates: trans-cinnamic acid (1 isozyme), atrazine (3 isozymes), 1-chloro-2,4-dinitrobenzene (1 isozyme), metolachlor (2 isozymes) and the sulfoxide derivative of S-ethyl dipropylcarbamothioate (2 isozymes). Pre­treatment of shoots with the safener CGA -154281 (1 μM) had no effect on the activity of the isozymes selective for trans-cinnamic acid and atrazine but increased the activity of the constitutively-expressed isozymes that exhibit activity with 1-chloro-2,4-dinitrobenzene, metola­chlor and the sulfoxide derivative of S-ethyl dipropylcarbamothioate. The safener pretreat­ment also caused the appearance of one new isozyme active with 1-chloro-2,4-dinitrobenzene and one new isozyme active with metolachlor. The results illustrate the complexity of gluta­thione S-transferase activity in etiolated maize shoots, and the selective enhancement of gluta­thione S-transferase isozymes by the safener CGA -154281.

Involvement of microorganisms in accelerated degradation of EPTC in soil

Accelerated EPTC (S-ethyl dipropylcarbamothioate) degradation was confirmed in a mixed culture of microorganisms derived from a soil with enhanced degradation (history soil) by using {sup 14}C-labeled EPTC. The antibacterial agent chloramphenicol (D-({minus})-threo-2,2-dichloro-N-({beta}-hydroxy-{alpha}-(hydroxymethyl)-p-nitrophenethyl)acetamide) markedly suppressed {sup 14}CO{sub 2} evolution while the antifungal agent cycloheximide (4-((2R)-2((1S,3S,5S)-3,5-dimethyl-2-oxocyclohexyl)-2-hydroxyethyl)glutarimide) did not, suggesting that soil bacteria play a significant role in enhanced EPTC degradation. A fast EPTC bacterial degrader (FD1) strain and a slower one (SD1), which were isolated by a soil enrichment technique from a history soil, were capable of utilizing EPTC as a sole carbon source. Vernolate (S-propyl dipropylcarbamothioate), butylate (S-ethyl bis(2-methylpropyl)carbamothioate), or cycloate (S-ethyl cyclohexylethylcarbamothioate) were also degraded by these bacteria in a pattern similar to that in a soil with enhanced degradation. Inoculation of nonhistory soil with FD1 strain induced accelerated degradation of the herbicide in the soil at rates similar to those in field soils exhibiting EPTC accelerated degradation.

Reversal of EPTC inhibition of aryl acylamidase activity by oximes

Aryl acylamidase (aryl-acylamine amidohydrolase, E.C.3.5.1.13) isolated from lettuce ( Lactuca sativa L.) leaf and stem tissue was assayed using propanil [ N-(3,4-dichlorophenyl)propanamide] as substrate. EPTC ( S-ethyl dipropyl carbamothioate) inhibited aryl acylamidase activity 20 to 65% at concentrations of 10 −4 to 10 −3 M. Kinetic studies indicated a K i of about 4.0 μ M for EPTC. When supplied simultaneously with and at concentrations equal to EPTC, acetone oxime, pyridine-2-aldoxime methiodide, and dimethylglyoxime each reduced the EPTC inhibition of aryl acylamidase activity by about 50%. These oximes alone had little or no effect on aryl acylamidase activity, but another oxime, benzoin-α-oxime, was inhibitory. These results indicate that molecular substituents near the oxime group can alter oxime function related to this class of herbicide safeners.

Influence of Prior Pesticide Treatments on EPTC and Butylate Degradation

Field and laboratory studies were conducted to examine the influence of previous pesticide use, number of prior EPTC (S-ethyl dipropyl carbamothioate) and butylate [S-ethyl bis(2-methylpropyl)carbamothioate] applications, and pretreatment of soil with EPTC and butylate metabolites on EPTC and butylate degradation. EPTC degradation was enhanced in Clay Center and Scottsbluff soils following three annual EPTC, butylate, or vernolate (S-propyl dipropylcarbamothioate) applications, but was not affected in soils following three annual atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine], cyanazine {2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl] amino]-2-methylpropanenitrile}, metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide], alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide], or cycloate (S-ethyl cyclohexylethylcarbamothioate) applications. EPTC degradation rate was not affected in a Scottsbluff soil following three annual carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate) applications, but was partially enhanced in a Clay Center soil. Self- and cross-enhanced degradation of EPTC and butylate did not change regardless of the number of prior annual applications of each herbicide. The degradation rates of EPTC and butylate were not affected by soil pretreated with butylate sulfone, but the degradation rates-were fully enhanced by pretreatment with the respective sulfoxides of each herbicide. In cross-enhancement testing, EPTC and butylate degradation was partially enhanced in Clay Center soil pretreated with the sulfoxides of each.

Populations of EPTC-Degrading Microorganisms in Soils by Accelerated Rates of EPTC Degradation

Reduced effectiveness of carbamothioate (thiocarbamate) herbicides in certain soils has been attributed to rapid herbicide degradation by soil microorganisms. Studies were conducted to determine if greater populations of EPTC (S-ethyl dipropyl carbamothioate)-degrading microorganisms were responsible for increased rates of degradation observed following repeated applications of EPTC to a Grenada silt loam soil. EPTC-degrading microorganism populations, measured with a14C-MPN (most-probable-number) technique, were not larger in soils with accelerated rates of EPTC degradation, and degrader populations did not increase after application of 6 mg EPTC/kg of soil. Degrader populations increased after application of 60 mg EPTC/kg of soil only in soil previously treated for 6 yr with EPTC. Increased rates of metabolism of EPTC were apparently responsible for the increased rates of degradation, rather than increased populations of degraders.

Johnsongrass (Sorghum halepense) Management Systems

Crop-herbicide systems for johnsongrass [Sorghum halepense(L.) Pers. # SORHA] control were compared in a 5-yr study at two locations in central Ohio. Monocultures of corn (Zea maysL.) and soybeans [Glycine max(L.) Merr.] were compared with a 3-yr rotation of corn, soybeans, and winter wheat (Triticum aestivumL.). Johnsongrass was controlled most effectively (over 95%) when glyphosate [N-(phosphonomethyl) glycine] was applied to emerged johnsongrass, followed by moldboard plowing, preplant incorporated (PPI) application of trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine], then soybean planting (MS-G-T). The second best control was in corn (over 90%) treated PPI with EPTC (S-ethyl dipropylcarbamothioate) in the rotation (RC-E). Johnsongrass control was least in monocultured corn treated PPI with EPTC (MC-E) and in monocultured soybeans treated PPI with trifluralin (MS-T).

Accelerated Degradation Potential of Selected Herbicides in the Southeastern United States

The influence of previous herbicide applications on the degradation rate of butylate [S-ethyl bis (2-methylpropyl)carbamothioate], EPTC (S-ethyl dipropyl carbamothioate), alachlor [2-chloro-N-(2,6-diethyl-phenyl)-N-(methoxymethyl)acetamide], and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] was measured. Degradation studies were conducted on soils with zero to eight previous applications of butylate, zero and six consecutive annual applications of alachlor, and zero to seven previous applications of metolachlor. Previous applications of alachlor or of metolachlor did not affect the rate of degradation when the same herbicide was reapplied. Soils with previous butylate-use history had more rapid degradation of butylate or EPTC than soils with no previous butylate applications. Because soil sterilization reduced14CO2evolution to a low level, soil microorganisms could be the primary mechanism of butylate degradation. There was no difference between butylate or EPTC degradation rate in soils treated previously with butylate. Butylate could not be detected 28 days after treatment in soils previously treated with butylate, whereas soils not previously treated with butylate still contained biologically active butylate. Dietholate (O,O-diethylO-phenyl phosphorothioate) added to butylate generally reduced the rate of butylate degradation in soils previously treated with butylate, but the butylate concentration always was lower in soils treated previously with butylate than in soils without previous butylate applications. Weed control in the field showed rapid loss of butylate and EPTC in soils with previous butylate treatment and that dietholate reduced the rate of butylate and EPTC degradation.

Enhanced Biodegradation of Herbicides in Soil and Effects on Weed Control

Research conducted since 1979 in the north central United States and southern Canada demonstrated that after repeated annual applications of the same thiocarbamate herbicide to the same field, control of some difficult-to-control weed species was reduced. Laboratory studies of herbicide degradation in soils from these fields indicated that these performance failures were due to more rapid or “enhanced” biodegradation of the thiocarbamate herbicides after repeated use with a shorter period during which effective herbicide levels remained in the soils. Weeds such as wild proso millet [Panicum miliaceumL. spp.ruderale(Kitagawa) Tzevelev. #3PANMI] and shattercane [Sorghum bicolor(L.) Moench. # SORVU] which germinate over long time periods were most likely to escape these herbicides after repeated use. Adding dietholate (O,O-diethylO-phenyl phosphorothioate) to EPTC (S-ethyl dipropyl carbamothioate) reduced problems caused by enhanced EPTC biodegradation in soils treated previously with EPTC alone but not in soils previously treated with EPTC plus dietholate. While previous use of other thiocarbamate herbicides frequently enhanced biodegradation of EPTC or butylate [S-ethyl bis(2-methylpropyl)carbamothioate], previous use of other classes of herbicides or the insecticide carbofuran (2,3 -dihydro-2,2 -dimethyl-7-benzofuranyl methylcarbamate) did not. Enhanced biodegradation of herbicides other than the thiocarbamates was not observed.

Shattercane (Sorghum bicolor) Control with Alachlor where EPTC Failed

A location in east Central Kansas with a history of continuous corn (Zea maysL.), EPTC (S-ethyl dipropyl carbamothioate) plus dichlormid (2,2-dichloro-N,N-di-2-propenylacetamide) plus dietholate (O,O-diethylO-phenylphosphorothioate) application, and shattercane [Sorghum bicolor(L.) Moench. #3SORVU] was selected for herbicide efficacy investigations. Thiocarbamate herbicides applied before planting corn failed to control shattercane in 1 of 2 years. Adding dietholate did not improve EPTC efficacy in either year. Alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] applied preplant failed to control shattercane both years. Treatments which included applications of alachlor at 2.2 kg ai/ha 7 or 10 days after preplant herbicide applications were made provided significantly better shattercane control than did preplant treatments.

Herbicide Dissipation from Soils with Different Herbicide use Histories

Herbicide dissipation was monitored in soils differing in herbicide use histories. Repeated annual applications over 5 yr enhanced biodegradation of butylate [S-ethyl bis(2-methylpropyl)carbamothioate] and EPTC (S-ethyl dipropylcarbamothioate) but not alachlor {2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide}, atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine], cyanazine {2-[[(4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methylpropanenitrile}, or metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide]. Prior application of butylate or EPTC enhanced biodegradation of the other thiocarbamate herbicide but not alachlor, atrazine, cyanazine, or metolachlor. Prior application of alachlor, atrazine, cyanazine, metolachlor, or trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine] did not enhance biodegradation of butylate or EPTC. Dissipation of EPTC applied with dietholate (O,O-diethyl-O-phenolphosphorothioate) was not enhanced by prior application of alachlor, atrazine, or trifluralin. Prior use of EPTC, EPTC + dietholate, butylate, or cycloate, respectively, enhanced biodegradation of EPTC in 100, 100, 71, and 50% of the experiments, of EPTC applied with dietholate in 57, 100, 60, and 33% of the experiments, of butylate in 33, 40, 100, and 20% of the experiments, and of cycloate in 0, 0, 17, and 0% of the experiments. Prior thiocarbamate herbicide applications usually did not enhance cycloate biodegradation, but soils from two locations without prior pesticide use histories rapidly degraded the herbicide. Storage of soil samples at 25 C for 6 or 12 months before application of EPTC and EPTC + dietholate resulted in less herbicide degradation than storage at 15 C. Differences in prior environmental conditions such as temperature may explain why development of enhanced biodegradation varied between years even in the same field plots.