Field and laboratory studies were conducted to examine effects of prior butylate use on biodegradation of subsequent applications of butylate and four other carbamothioate herbicides. Bioassays were used to demonstrate reductions of butylate and EPTC activity in four soils preconditioned by annual butylate applications. Combining these herbicides with dietholate, an enzyme inhibitor, prolonged persistence and restored normal herbicidal activity. Expected herbicidal efficacy occurred in adjacent plots with no history of carbamothioate use. Prior applications of butylate resulted in cross-adapted degradation of EPTC but not vernolate, pebulate, or cycloate. When samples of Cecil soil were treated in the laboratory with 14C-butylate, evolution of 14CO2 was significantly higher from soils that had received repeated field applications of butylate. The use of 14C-herbkides could be used to predict problem soils and potential herbicide failure.

The degradation of 14C-labeled metolachlor, acetochlor, and pretilachlor in control and soils with 10-yr metolachlor- history or 3-yr butylate history was studied by monitoring the evolution of 14CO2 in soil biometer flasks. The degradation rate of 14C-phenyl-labeled metolachlor was similar in soils with 0- and 10-yr metolachlor history over 52 d. 14C was released from carbonyl-labeled metolachlor about 40% faster than from phenyl-labeled metolachlor in both soils. Soil sterilization by autoclaving reduced significantly the metolachlor degradation rate in both soils. Degradation of 14C-labeled EPTC in soils with 3-yr butylate history was much faster than in soils with no history. Soil sterilization reduced the EPTC degradation rate, confirming microbial degradation. The degradation rates of acetochlor, metolachlor, and pretilachlor were similar in soils with and without butylate history. Most of the 14C that remained in history and nonhistory soils was extractable with ethyl acetate or sodium hydroxide. Three major metabolites of metolachlor and EPTC were detected by thin-layer chromatography (TLC) of extracts from both soils. In contrast to the situation with carbamothioate herbicides, soils exposed repeatedly to metolachlor or other acetanilides are not prone to become adapted to these herbicides. Soils with carbamothioate history did not exhibit any apparent cross-adaptation toward acetanilide herbicides.

Laboratory experiments were conducted to evaluate butylate and EPTC degradation in two butylate-history soils collected in 1983, 1984, and 1985, and stored at 4 C. No significant differences in biodegradation of butylate and EPTC were found due to length of cold storage of Varina or Wagram soils. Enhanced biodegradation of butylate and cross-adaptation for EPTC were not affected. Storage of soils up to 2 yr should not adversely affect results.

Greenhouse and field experiments were conducted to investigate degradation rates and efficacy of EPTC (S-ethyl dipropyl carbamothioate), butylate [S-ethyl bis(2-methylpropyl)carbamothioate], vernolate (S-propyl dipropylcarbamothioate), and cycloate (S-ethyl cyclohexylethylcarbamothioate) in soils with and without prior history of thiocarbamate use. Herbicide persistence was measured by an oat (Avena sativa L.) bioassay, and giant foxtail (Setaria faberi Herrm. # SETFA) control was visually estimated in field studies. Both greenhouse and field studies indicated that enhanced thiocarbamate degradation occurred when the same herbicide (EPTC, butylate, or vernolate) was applied over successive years. Addition of dietholate (O,O-diethyl O-phenyl phosphorothioate) or fonofos (O-ethyl-S-phenylethylphosphonodithioate) increased thiocarbamate persistence and giant foxtail control in soils with prior thiocarbamate exposure. Use of dietholate or fonofos two or three consecutive years with EPTC or butylate again resulted in enhanced degradation. EPTC persistence and giant foxtail control were not affected by prior treatments or tank mixes with carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate). Rotation away from successive applications of the same thiocarbamate herbicide (EPTC, butylate, or vernolate) improved herbicide efficacy.

Scanning electron micrographs that depict the surface and internal structure of several starch-encapsulated granular formulations of EPTC (S-ethyl dipropylthiocarbamate) and butylate (S-ethyl diisobutylthiocarbamate) relate well with herbicidal release rates previously reported. Fast-release formulations appear to have a porous surface with little distinctive internal structure. Diffusion would be expected to be more rapid as fewer diffusion barriers are present. Slow-release formulations have a smooth, hard surface with irregular surface pores and a honeycombed internal structure. The slower release characteristics are associated with stronger oxidants used for crosslinking xanthate in the entrapment of herbicides. The scanning electron microscope would be an excellent quality control technique in the preparation of starch-encapsulated herbicides.

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 reduced 14CO2 evolution 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-diethyl O-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.

Four cropping systems were evaluated from 1975 to 1977 for the control of yellow nutsedge (Cyperus esculentus L.). Crops grown in 1975 and 1976 included alfalfa (Medicago sativa L. ‘Elcamino WL-600′), barley (Hordeum vulgare L. ‘CM-67′), corn (Zea mays L. ‘Dekalb T214′), and cotton (Gossypium hirsutum L. ‘Acala SJ-2′). Herbicides used included butylate (S-ethyl diisobutylthiocarbamate) in corn, EPTC (S-ethyl dipropylthiocarbamate) in alfalfa, glyphosate [N-(phosphonomethyl)glycine] in fallow plots, and MSMA (monosodium methanearsonate) in cotton. Cotton, which was grown continuously as one of the four cropping systems, was the only crop grown in 1977. Based on weed counts, crop yields, and the consistent decline in the number of tubers, all cropping systems provided substantial control of yellow nutsedge each year. Two years of either alfalfa treated with EPTC or double cropping barley with corn treated with butylate preceeding cotton reduced the number of viable yellow nutsedge tubers by 96%. Two years of chemically fallowing plots with glyphosate following barley and preceeding cotton was 98% effective in reducing viable tubers. Treating continuous cotton with MSMA, although somewhat inferior to the above systems, reduced the number of viable nutsedge tubers by 91% in 3 yr.

Thirty-six postemergence-directed (PDIR) herbicide treatments, applied with a precision PDIR sprayer, were evaluated for wild-proso millet (PANMI) control in sweet corn field trials in Wisconsin from 1987 to 1990. The performance of butylate and cyanazine (B + C), applied to suppress PANMI early in the season and to provide a height differential between corn and PANMI, greatly influenced PANMI control with PDIR treatments. PDIR treatments controlled PANMI greater than 90% when PANMI was effectively suppressed by B + C, but poor PANMI suppression reduced PDIR herbicide efficacy. Generally, PDIR treatments of paraquat or sethoxydim most effectively controlled PANMI. Paraquat alone at 545 g ha–1 or combined with simazine or sethoxydim at 57 and 114 g ha–1, and sethoxydim at 170 and 227 g ha–1 plus an adjuvant controlled PANMI greater than 95%. PDIR applications of ametryn, linuron, sethoxydim at 57 to 227 g ha–1 alone, sethoxydim at less than 114 g ha–1 plus an adjuvant, and sethoxydim at 114 g ha–1 tank mixed with either of four other herbicides controlled less than 86% of PANMI. Tank mixing a photosynthetic inhibitor or sethoxydim with paraquat did not improve PANMI control compared with paraquat alone. PANMI control with sethoxydim plus an adjuvant at 114 g ha–1 was similar to 227 g ha–1. There was no difference in PANMI control with sethoxydim applied with crop oil concentrate or BCH-815. With the exception of glyphosate at 318 g ha–1 and sethoxydim at 227 g ha–1 plus an adjuvant, PDIR treatments did not injure sweet corn in 1987, 1988, or 1990. However, sethoxydim in 1989 at all rates severely injured corn.

Laboratory and field studies were conducted to examine the influence of R-33865 (O, O-diethyl-O-phenol phosphorothioate) on the degradation rate of EPTC (S-ethyl dipropylthiocarbamate) in three soils that had a history of repeated EPTC application and exhibited accelerated degradation of EPTC. In field studies, addition of R-33865 to EPTC extended EPTC persistence in soil and provided increased shattercane [Sorghum bicolor (L.) Moench] control. In two soils that exhibited rapid EPTC degradation, addition of R-33865 to EPTC extended the half-life of EPTC from 9 to 18 days and from 6 to 15 days, respectively. Evolution of 14CO2 from 14C-carbonyl EPTC-treated soils with a prior history of EPTC treatment was significantly reduced when R-33865 was added; however, the initial lag period of EPTC degradation was not restored. Addition of R-33865 to EPTC on soils with no prior EPTC treatment did not extend persistence of EPTC in soil.

An increased rate of butylate (S-ethyl diisobutylthiocarbamate) degradation occurred on a Kennebec silt loam (sil) exhibiting rapid EPTC (S-ethyl dipropylthiocarbamate) breakdown in the field. Butylate degradation was not as rapid as EPTC degradation on field soils previously treated with EPTC. Addition of R-33865 (O, O-diethyl-O-phenol phosphorothioate) to butylate extended its persistence in field soils with and without prior EPTC application. Although microbial degradation of butylate, EPTC, and vernolate (S-propyl dipropylthiocarbamate) was increased on EPTC-pretreated soils, EPTC and vernolate degradation was much more rapid than butylate degradation. Addition of R-33865 to butylate, EPTC, or vernolate extended their soil persistence on soils pretreated with EPTC. In a laboratory experiment, a maximum rate of EPTC degradation resulted from one prior application. The rate of butylate degradation increased with each successive butylate application, but the degradation rate was not as high as those observed for EPTC on EPTC-pretreated soil.

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.

Germination stimulants were tested for effectiveness on velvetleaf seed imbibition and germination and concurrent microbial attack of seed to deplete weed seed in soil. Several chemicals increased in vitro seed germination and decreased the numbers of hard, viable seed. The proportion of nonviable seed, 5% with Fusarium oxysporum (Schlech.), was enhanced to 40% by adding ethephon at 100 μg/ml. Fungal density on seedling roots and imbibed (nonviable seed) was increased by chemical treatment. Seedling emergence was reduced 15% when ethephon or carbofuran was applied to soil with the fungus. Shoot dry weight decreased and root infection increased with all treatments regardless of fungal inoculation. Butylate and carbofuran increased infection of imbibed seed in soil by F. oxysporum while ethephon and AC94377 increased infection by other soil fungi.

Efficacy of EPTC (S-ethyl dipropylcarbamothioate), butylate [S-ethyl bis(2-methylpropyl)carbamothioate], vernolate (S-propyl dipropylcarbamothioate), and cycloate (S-ethyl cyclohexylethylcarbamothioate) applied with dichlormid (N,N-diallyl-2,2-dichloroacetamide), and the former three herbicides applied with dichlormid and dietholate (O,O-diethyl-O-phenol phosphorothioate) for wild proso millet (Panicum miliaceum L. ssp. ruderale (Kitagawa) Tzevelev. # PANMI) control in corn (Zea mays L.) was evaluated in fields previously treated with EPTC + dichlormid or EPTC + dichlormid + dietholate. Cycloate + dichlormid usually controlled millet regardless of prior herbicide use. But EPTC + dichlormid was less effective than on soils not previously treated with the herbicides, suggesting enhanced soil biodegradation. The use of EPTC + dichlormid with row cultivation or applications of selected herbicides improved millet control. Following prior use of EPTC + dichlormid, wild proso millet control was improved by the addition of dietholate to EPTC + dichlormid and vernolate + dichlormid but not butylate + dichlormid. In dissipation studies using soils previously treated with EPTC + dichlormid, the relative order of herbicide persistence was EPTC + dichlormid < vernolate + dichlormid < butylate + dichlormid < EPTC + dichlormid + dietholate < vernolate + dichlormid + dietholate = butylate + dichlormid + dietholate < cycloate + dichlormid. When EPTC + dichlormid + dietholate was applied to soils previously treated with that mixture, wild proso millet control was again reduced. Dissipation studies confirmed that dietholate slowed biodegradation of EPTC in soils previously treated with EPTC + dichlormid but not with EPTC + dichlormid + dietholate.

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 miliaceum L. spp. ruderale (Kitagawa) Tzevelev. #3 PANMI] 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-diethyl O-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.

Cold stress and field studies were conducted to determine the effects of alachlor, metolachlor, butylate plus dichlormid, and EPTC plus dichlormid on germination and seedling vigor of 30 sweet corn cultivars with three commercially important endosperm mutants (su, se, and sh2). Seedling emergence from cool soils was best when treated with EPTC plus dichlormid and alachlor and was decreased 11% by metolachlor and butylate plus dichlormid. The se and sh2 endosperm mutants were injured the most by the herbicides. Plant size was reduced by the thiocarbamate herbicides but not by the chloracetamides.

Laboratory studies were conducted to evaluate the degradation of butylate (S-ethyl diisobutylthiocarbamate), EPTC (S-ethyl dipropylthiocarbamate), cycloate (S-ethyl N-ethylthiocyclohexanecarbamate), and vernolate (S-propyldipropylthiocarbamate) in soil with prior thiocarbamate herbicide exposure. EPTC degradation was accelerated in soil which had previously been treated with butylate, vernolate, or EPTC in 1979, 1980, and 1981. The evolution rate of 14CO2 from 14C-labeled vernolate was increased in soil which had previously been treated with vernolate or EPTC in 1979, 1980, and 1981. Cycloate degradation was not enhanced in soil with prior exposure to butylate, EPTC, or vernolate. Butylate breakdown was enhanced to the greatest extent in soil with previous exposure to butylate.

Field studies were conducted for 3 yr near Mead, NE, to evaluate the effectiveness of seed safeners CGA-92194 [N-(1,3-dioxalon-2-yl-methoxy)iminobenzeneacetonitrile], NA (1,8-naphthalic anhydride), and R-29148 (2,2-dimethyl-5-methyldichloroacetyloxazolidine) to reduce herbicide injury to big bluestem (Andropogon gerardii Vitman), indiangrass [Sorghastrum nutans (L.) Nash], intermediate wheatgrass [Agropyron intermedium (Host.) Beauv.], sideoats grama [Bouteloua curtipendula (Michx.) Torr.], and switchgrass (Panicum virgatum L.) from preplant-incorporated butylate [5-ethyl bis-(2-methylpropyl)carbamothioate] and metolachlor [2-chloro-N- (2-ethyl-6-methylphenyl) -N- (2-methoxy-l-methylethyl)acetamide], applied at 4.5 and 2.2 kg ai/ha, respectively. Big bluestem stands were satisfactory, regardless of herbicide or safener treatment, although stands were reduced by NA treatment with either herbicide. Indiangrass stands varied by year, with protection from both herbicides by R-29148 in 1984 and by NA in 1985. All safeners reduced injury to intermediate wheatgrass from metolachlor and to a lesser extent from butylate; acceptable stands were obtained with metolachlor treatment when unsafened. Sideoats grama was nearly eliminated with either herbicide, regardless of safener. Switchgrass treated with NA produced stands two- to threefold higher than other safened or unsafened seed in metolachlor plots and equal to unsafened seed in the weeded control plots.

Field experiments were conducted to compare herbicides applied preplant incorporated (PPI), preplant incorporated/preemergence (PPI/PRE), and preplant incorporated/early postemergence (PPI/early POST) to control woolly cupgrass in corn. Although good early-season control of woolly cupgrass from PPI cycloate plus cyanazine, EPTC plus dietholate or SC-0058, and butylate plus cyanazine sometimes was observed, middle- and/or late-season control was often limited. Generally, better woolly cupgrass control and higher corn yields were obtained from split PPI/PRE applications rather than from single PPI applications of alachlor, metolachlor, and acetochlor. The highest and most consistent full-season woolly cupgrass control resulted when cycloate or EPTC plus dietholate applied PPI was followed by cyanazine plus either pendimethalin, alachlor, metolachlor, or acetochlor applied early POST. However, in 1989 adverse weather conditions near the early POST application timing injured corn and reduced yields.

Laboratory studies enumerated butylate-, EPTC-, vernolate-, pebulate-, and cycloate-utilizing bacteria and actinomycetes in a Dothan loamy sand following repeated use of butylate, EPTC, or vernolate. Herbicide utilizers were isolated on a defined medium composed of minimal salts plus 200 mg/L herbicide as the primary carbon source plus 25 mg/L TTC as an indicator of organisms capable of metabolizing the herbicide substrates. Media inoculated with a butylate-history soil developed more TTC-red actinomycetes on the butylate-indicator agar than did media inoculated with a non-herbicide-history soil. Similarly, media inoculated with a vernolate-history soil developed more TTC-red bacteria on the vernolate-indicator agar. Cross-adaptation from vernolate to butylate and cycloate was evident, with inoculant from the vernolate-history soil exhibiting an increase in bacterial counts when butylate or cycloate was present as the primary carbon source. Likewise, previous butylate exposure corresponded with an increase in the number of vernolate- and pebulate-utilizing actinomycetes. Previous exposure to EPTC resulted in an increased population of butylate- and pebulate-utilizing actinomycetes.

In studies conducted from 1991 through 1994, researchers investigated the effects of several crop rotations and herbicide programs on crop yield and populations of common lambsquarters, common ragweed, Amaranthus spp., and jimsonweed at two sites. Crop rotations included continuous corn, continuous soybean, corn–soybean, and corn–tomato–soybean, and herbicide programs were the split-plots and included continuous use of acetolactate synthase (ALS)–inhibitor herbicides, continuous use of non–ALS-inhibitor herbicides, annual rotations between ALS- and non–ALS-inhibitor herbicides, combinations of ALS- and non–ALS-inhibitor herbicides in the same year, and no herbicide. Weed control and weed populations generally were affected by an interaction between crop rotations and herbicide programs. After 4 yr, common lambsquarters control was lowest, and populations were highest where fomesafen was used alone for four consecutive years or in rotation with other herbicides. Although common ragweed populations were low at site 2, control at both sites was generally lowest from treatments that included only ALS-inhibitor herbicides. Common ragweed populations were highest at site 1 in 1992 and 1993 following continuous applications of ALS-inhibitor herbicides. Jimsonweed populations were also low at site 2, but control at site 1 in tomato was low. Jimsonweed control from fomesafen and the combination of butylate plus atrazine in soybean and corn, respectively, was variable. Amaranthus spp. populations decreased as the study progressed, and in 1993, control was over 90% from all treatments, except in the case of the treatment combining butylate plus atrazine. Corn and soybean yields varied with year and site, and yields of these crops and tomato were related to rainfall and weed control.