Control with EPTC

Grower complaints of poor shattercane control with EPTC became numerous by 1977. Loss of efficacy was initially attributed to misapplication, inadequate incorporation, and adverse environmental conditions. However, the continuing widespread reports of unsatisfactory control with EPTC suggested other unknown factors must be involved. Field and greenhouse studies were initiated in 1978 to identify the reasons for poor field performance (1-3). Interestingly the problem appeared to be associated with repeated annual application. Though the extent of EPTC failure was unknown at the time, a 1983-8A survey revealed that 60% and 45% of corn growers in south central Nebraska who used butylate [S-ethyl bis(2-methylpropyl)carbamothioate] or EPTC were dissatisfied with their weed control (4). This was probably representative of the situation in 1978. 

Table IV. Wild proso millet control with EPTC and EPTC with dietholate in a six-year herbicide rotation study...

In 1977, Rahman, et. al (2) noted that EPTC provided less than expected weed control at the Manutuke Research Station, near Gisborne, N.Z. They concluded that weed control with EPTC may decrease when used repeatedly on the same field. Further greenhouse studies linked the lack of weed control to increased microbial activity in the soil (3). 

From the commercial perspective, the rotational statement placed on the EPTC label in 1985 was based on experimental evidence and has been verified by independent research (4). Annual crop or chemical rotation program following EPTC use has been very successful. Nonperformance complaints have been reduced and there is minimal perception that enhanced biodegradation affects annual weed control with EPTC. 

Foxtail Control With EPTC and EPTC + Atrazine On Silt Loam and Silty... 

Roxorange Sorghum and Indigenous Weed Control with EPTC on a Silty Clay Loam Soil, Lisbon, IA. Field treated with EPTC in 1982 and 1983 Treatments applied June 28, 1984 and June 25, 1985... 

In corn, the catalyst for adoption of preplant incorporated applications was not the introduction of an herbicide, but the introduction of safeners. The thiocarbamate herbicide S-ethyl dipropylthiocarbamate (EPTC) for corn weed control was first used in Illinois as early as 1961 however, the propensity for EPTC to injure com limited its use to 1% or less of the Illinois crop up to 1970 (Table 4.1). Dichlormid, a safener later packaged with EPTC, greatly improved crop safety and resulted in nearly one-quarter of the com acreage being treated with EPTC throughout the Corn Belt by 1976. 

Coping with Enhanced Degradation. Combinations of atrazine with EPTC or butylate have been very effective for controlling grasses in corn... 

Microbial isolate JE1 in BSME (100 mg EPTC L 1) was grown on a rotary shaker at 27°C to midlog phase (2-3 days O.D. 600, 0.06-0.08). An aliquot of the culture was treated with EPTC (50 mg L 1), and EPTC remaining in solution was measured at 2 h intervals. An aliquot containing a similar amount of EPTC but no microbiol cells was used as a control. A midlog culture was also treated with uC-l-propyl EPTC (100 mg L 1) and, at 8 h intervals, the headspace above the culture was swept into NaOH and the uC-activity was measured by LSC. 

Location Not Previously Treated With EPTC. Studies conducted in 1978 and 1979 demonstrated that EPTC at both 4.5 and 6.7 kg ai/ha application rates provided effective wild proso millet control in sweet corn grown on soils not previously treated with that herbicide (Table I). Because of the prolonged period of millet germination and the short soil persistence of EPTC, millet control 95 days after treatment (DAT) was substantially less than 45 DAT. However, these late-season millet escapes did not cause sweet corn yield reductions. Applying EPTC with cyanazine slightly improved late-... 

Table II. Wild proso millet control and sweet corn yields from herbicide treatments on soil previously treated with EPTC...

Location Previously Treated With EPTC Plus Dietholate. When applied in 1983 to a field treated the previous year with EPTC plus dietholate, 6.7 kg/ha EPTC with and without dietholate resulted in poor millet control 40, 70 and 100 DAT, and extremely low sweet corn yields (Table III). In this soil, enhanced EPTC biodegradation occurred whether applied with or without dietholate (j3). Combining cyanazine with EPTC plus dietholate only increased millet control 40 DAT compared to EPTC plus dietholate without cyanazine, and did not improve sweet corn yield. Following EPTC plus dietholate with an early postemergence application of pendimethalin plus cyanazine resulted in millet control 40 DAT comparable to the handweeded treatment, and millet control 70 and 100 DAT and sweet corn yields greater than those resulting from EPTC plus dietholate plus cyanazine treatment. The contribution from the EPTC plus dietholate component of this sequential treatment was apparently small, however, since results were similar to those from pendimethalin plus cyanazine applied alone. 

Between 1976 and 1978 Stauffer Chemical Company researchers identified fields where EPTC failed to give expected herbicidal activity. The fields had a history of repeated annual applications of EPTC. Greenhouse bioassays with EPTC demonstrated reduced persistence as the cause of reduced weed control observed in the field. Sterilization of the soil with heat or chemicals restored herbicidal activity. This indicated that, in some soils, enhanced microbial degradation might be associated with the observed reduction in herbicidal activity (D.L. Hyzak, personal communication). 

Reduced weed control with butylate in butylate history fields was reported in 1983 (7,8). Laboratory, greenhouse and field trials demonstrated that repeated annual use of butylate may cause enhanced butylate biodegradation and result in reduced weed control. Reports of reduced weed control with repeated use of butylate were much less frequent than with EPTC (Stauffer Chemical Company unpublished). Since 1986, the potential for enhanced biodegradation of butylate has been documented and described more completely by researchers in the Southeastern U.S. (9,10,11,12). 

Enhanced biodegradation. Four annual applications of EPTC resulted in a significant reduction in the recovery of EPTC by GC analysis following the fifth annual application. At 5 and 7 DAT, EPTC recovery was 1.40 and 0.28 ppm, respectively. Adjacent plots which had been treated with EPTC for 2 years and rotated out to alternate herbicides for 2 years had 2.99 and 1.99 ppm at 5 and 7 DAT, respectively. These results demonstrate enhanced biodegradation of EPTC with repeated annual use (Table I). Many other researchers have demonstrated that repeated annual applications of EPTC may lead to reduced control of weeds (2-3,14-19) due to enhanced biodegradation. 

An adjacent plot which had been treated for two years, then left untreated for one year prior to subsequent buytlate application had 2.11 ppm butylate recovered at 7 DAT. This demonstrated enhanced biodegradation of butylate with repeated annual use (Table II). Repeated annual applications of butylate do not lead to reduced weed control as frequently as is the case with EPTC, although butylate persistence may be reduced by annual repeated applications (7-12). 

Elimination or Inhibition of Enhanced Biodegradation by Herbicide Tank Mixtures. In one field trial, tank-mixes of atrazine with EPTC significantly increased green and yellow foxtail control at 60 DAT. In the green foxtail field, which had been treated with EPTC for 4 years, the application of EPTC provided little weed control. When EPTC was applied at 4.5 and 6.7 kg ai/ha tank-mixed with atrazine at 1.7 kg ai/ha, green foxtail control increased from 15 to 52% and from 22 to 94%, respectively. EPTC had only been applied once before to the yellow foxtail field. EPTC alone provided 68 to 72% control and tank-mixtures with atrazine provided 83 to 91% control, respectively (Table V). Atrazine tank-mixes appeared to be more beneficial In the field with the longer previous history of EPTC use. 

Weed Control and EPTC Persistence from EPTC + Dietholate + Atrazine in Rotation with Various Herbicides on a Silty Clay Loam Soil, Otoe, NE Field history of EPTC in 1982 and EPTC + dietholate in 1983 EPTC + dietholate broadcast applied at 4.5 kg/ha on 5/8/85... 

Herbicides in tank-mixture with EPTC or butylate can increase weed control in fields with accelerated biodegradation and may reduce the impact of enhanced biodegradation on weed control. 

Tests established in fields with previous applications of EPTC + dietholate have demonstrated that the repeated use of EPTC + dietholate provided only 78, 40, and 80% of wild proso millet, woolly cupgrass [Eriochloa villosa (Thumb.) Kunth], or green foxtail Setaria viridis L.), respectively] (Table VII). Two other trials with EPTC + dietholate have confirmed that second year use at 6.7 kg ai/ha provided only 56 to 70% wild proso millet control at 42 DAT (Table VIII). These results agree with observations reported in Iowa, Minnesota, and Wisconsin (7,31-33). 

Wild Proso Millet Control in EPTC + dietholate History Fields with Silty Clay Loam or Loam Soils, Waterloo, WI Treatments applied May 4, 1988 and May 3, 1989 Both Fields treated one year previously with EPTC + dietholate... 

Because the repeated annual use of EPTC + dietholate resulted in reduced control of wild proso millet, woolly cupgrass, and shattercane and because conflicting results were obtained with giant foxtail, alternate extenders were evaluated which could increase the performance of EPTC and provide consistent weed control with repeated annual use. Several extenders were provided for evaluation by Stauffer Chemical Company in 1984 and 1985. A review of the efficacy of these extenders is contained in Harvey et.al (4). 

Further studies at the University of Wisconsin since 1985 demonstrated that 5 consecutive annual applications of EPTC + R251005 maintained wild proso millet control at acceptable levels. EPTC and EPTC + dietholate demonstrated reduced control with repeated use in the same experiment (R. G. Harvey, personal communication). 

In addition to efficient application and incorporation the following factors have significantly increased the weed control from EPTC, EPTC + dietholate, or butylate when reduced performance due to enhanced biodegration was observed 1) reduce weed pressure or establish conditions favorable for immediate, uniform weed emergence 2) immediate, uniform incorporation just prior to corn planting 3) tank mixtures of carbamothioate herbicides with atrazine, cyanazine, or metolachlor 4) rotation for one year to no herbicide, alachlor, cycloate, metolachlor, or trifluralin or, 4) formulations of EPTC plus R251005 at 6.7 + 1.1 kg/ha. 

This herbicide developed simultaneously with EPTC (Tilles and Antognini, 1956) is, like the other alkylthiocarbamates, a preemergence herbicide effective, incorporated in the soil at an application rate of 1.5-3.0 kg active ingredient/ha for the control of sprouting broad-leaved and grass weeds. 

The encapsulation of herbicides has received much attention. Encapsulated alachlor is a high volume herbicide product generally sold as a Hquid formulation, although a dry granule version is also available. The capsules, produced by interfacial polymeri2ation (11), are reported to be spherical with a diameter of 2—15 p.m (75). Two thiocarbamate herbicides, EPTC and vemolate [1929-77-7], were encapsulated by interfacial polymeri2ation because they are volatile compounds. When appHed in unencapsulated form, they must be incorporated in the soil within two hours in order to provide effective weed control. When appHed as a microencapsulated formulation, the rate of volatili2ation is lower and soil incorporation can be delayed 24 hours (76). 

Currently no herbicides are cleared for use in the U.S. to control weeds in Jerusalem artichokes. Preliminary tests with several herbicides have been reported (Table 12.1). For example, the cultivar Columbia displayed satisfactory tolerance to preplant incorporated treatments of chloramben, S-ethyl dipropylthiocarbamate (EPTC), ethalfluralin, pendimethalin, and trifluralin, although metribuzin resulted in considerable damage, manifested as chlorosis and necrosis of leaf margins, and reduced plant height. Tuber yield, however, was not increased by weed control, whether via herbicides or hand weeding, when compared to weedy control treatments (Wall et al., 1987). 

Table V. Control of large crabgrass in corn with rotations of butylate (B) or EPTC plus dietholate (E) with alachlor...

Metabolic Studies with Microbial Isolate JE1. EPTC was found to be efficiently metabolized by JE1 (Figure 3). Growth of JE1 was associated with the degradation of EPTC over an 8 h period. In contrast, EPTC levels remained constant over the same 8 hour period in the uninoculated control. Degradation of 14C-labelled EPTC and appearance of metabolites into an aqueous or organic soluble fraction was also measured. An initial rise in 14C-activity in the... 

Figure 5. Autoradiogram of metabolic products of EPTC produced by microbial isolate JE1. Thin-layer chromatograph was developed with hexane/ethyl acetate (3 2). A, 8 h incubation B, Oh incubation C, sterilized control D, E, F, m-chloroperoxy benzoic acid + EPTC G, H, I, 24 h incubation. Metabolites 1, EPTC-sulfoxide 2, 3, unknowns 4, EPTC-sulfone 5, N-depropyl EPTC 6, EPTC.

Soil disinfestation with methyl bromide, vapam, or by solarization controlled degradation of certain pesticides such as EPTC to various degrees (13). Apparently some of the degraders were not affected by these biocides, a fact that was reflected by partial reduction in degradation of EPTC, when compared with the stronger inhibitory effect in sterile soil. 

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