Pyrithiobac Sodium Research Articles

Metabolism of Pyrithiobac Sodium in Soils and Sediment, Addressing Bound Residues via Kinetics Modeling.

Degradation of pyrithiobac sodium (PE350) was examined in a number of soils and sediments using (14)C-PE350. It degrades primarily via microbial degradation which leads to the separation of the two rings of the molecule. Identification of several metabolites, many of which were minor products, helped to understand the formation of nonextractable residues (NER) and (14)CO2. In all studies, unextractable residues accounted for a large portion (20-60%) of the residues. Traditional kinetics modeling treats NER and CO2 as a single compartment, stated as sink, and formation mechanism of such components individually is ignored. Since studies conducted with radiolabeled test substance provides an accurate measurement of NER and CO2, we have demonstrated that kinetics modeling with these compartments separately can be used to clarify degradation pathways, including the origin of NER and CO2. This work demonstrated that overall metabolism in soils and sediments proceeded via similar pathways, and kinetics modeling was useful in clarifying the degradation route and formation of NER in all studies.

Resistance Mechanisms to an Acetolactate Synthase (ALS) Inhibitor in Water Starwort (Myosoton aquaticum) Populations from China

Overreliance on tribenuron has resulted in resistance evolution in water starwort. This study investigates the resistance mechanisms to tribenuron in water starwort populations from China. The cytochrome P450 monooxygenase (P450) inhibitor malathion increased tribenuron sensitivity in all populations. The decrease in the amount of herbicide dose that causes 50% growth reduction (GR50) for the sensitive (S) population JS24 and the resistant (R) populations JS16 and JS17 were 2.3-, 2.5-, and 4.1-fold, respectively. However, the GR50 values for the R populations were still much higher than those of the S population. This observation indicates that P450-mediated enhanced metabolism is one mechanism for resistance in water starwort. The glutathione-S-transferase (GST) activity could be induced by tribenuron for all tested populations. In particular, the GST activity of JS16 is inherently greater and is more rapidly induced than that of JS17 or JS24. Resistance attributed to mutant acetolactate synthase (ALS) alleles was identified by sequence analysis for each population. Pro197Ser substitution was detected in JS16 and JS17. Molecular markers were also developed to rapidly identify resistance as well as individuals carrying the specific Pro197Ser mutation in water starwort populations. The resistance patterns experiment revealed that the R populations exhibited different levels of resistance to pyrithiobac sodium salt, florasulam, pyroxsulam, and flucarbazone-Na; however, R populations were sensitive to imazethapyr, fluroxypyr-meptyl, 2,4-D butylate, isoproturon, and diflufenican. This study establishes that either one or at least two resistance mechanisms are involved in herbicide resistance in water starwort. Moreover, these mechanisms might contribute to the different levels of resistance to tribenuron among water starwort populations.

A novel Pro197Glu substitution in acetolactate synthase (ALS) confers broad-spectrum resistance across ALS inhibitors

Water chickweed (Myosoton aquaticum L.), a competitive broadleaf weed, is widespread in wheat fields in China. Tribenuron and pyroxsulam failed to control water chickweed in the same field in Qiaotian Village in 2011 and 2012, respectively. An initial tribenuron resistance confirmation test identified a resistant population (AH02). ALS gene sequencing revealed a previously unreported substitution of Glu for Pro at amino acid position 197 in resistant individuals. A purified subpopulation (WRR04) that was individually homozygous for the Pro197Glu substitution was generated and characterized in terms of its response to different classes of ALS inhibitors. A whole-plant experiment showed that the WRR04 population exhibited broad-spectrum resistance to tribenuron (SU, 318-fold), pyrithiobac sodium (PTB, > 197-fold), pyroxsulam (TP, 81-fold), florasulam (TP, > 36-fold) and imazethapyr (IMI, 11-fold). An in vitro ALS assay confirmed that the ALS from WRR04 showed high resistance to all the tested ALS inhibitors. These results established that the Pro197Glu substitution endows broad-spectrum resistance across ALS inhibitors in water chickweed. In addition, molecular markers were developed to rapidly identify the Pro197Glu mutation.

Managing runoff of herbicides under rainfall and furrow irrigation with wheel traffic and banded spraying

Herbicide runoff is of concern due to their possible effects on non-target organisms in receiving environments. Band spraying and controlled traffic have potential for managing pesticide runoff in situations where crop residue cover can not be retained. The effectiveness of these two management practices was studied for (simulated) rainfall and the effectiveness of band spraying for furrow irrigation. The site was on a hill-furrow layout, on a heavy cracking clay. Three times after pesticide application treatment (2–42 DAT) were studied. Herbicides with high (pyrithiobac sodium) to moderate solubility (diuron and metolachlor) were studied on simulator plots. Pyrithiobac sodium was studied alone for irrigations. Pyrithiobac sodium was transported >90% in the water phase in rainfall and irrigation runoff, with a sediment-water partition coefficient (Kp) of ∼10Lkg−1 at 2 DAT, increasing to 20–30Lkg−1 after 24 DAT, similar to metolachlor. In contrast, diuron was transported 55% in sediment at 2 DAT (Kp=43) and ∼85% in sediment at 34 DAT (Kp=204). Percent in the water phase decreased and Kp increased over time for all herbicides. The 10 and 14 fold greater application rates for diuron and metolachlor than for pyrithiobac sodium led to 120 and 46 times greater herbicide rainfall runoff losses. Rainfall runoff concentrations and losses of herbicides were reduced by >70% at 34 DAT compared with 2 DAT. Non-wheel traffic furrows gave rainfall runoff, soil loss and sediment concentrations 37%, 59% and 33% less than from wheel track furrows. Pesticide runoff concentrations were ∼30% less and losses ∼55% less from non-wheel tracks than from wheel track furrows. This occurred even though conditions were conducive to runoff (moist soil and crusted surface). Band spraying on the hills (∼40% band) reduced rainfall runoff concentrations (cf. blanket) by 41, 32 and 54% and losses by 38, 22 and 50%, for pyrithiobac sodium, metolachlor, diuron, respectively. Band spraying was more effective in reducing rainfall runoff losses for chemicals transported primarily in the sediment phase. Band spraying on the hills (∼40% band) for furrow irrigation reduced concentrations of pyrithiobac sodium by 75% and losses by 88%. Band spraying and non-wheel traffic combined gave greater reductions in pesticide runoff than either practice alone.

Incubation of alligator snapping turtle (Macrochelys temminckii) eggs in natural and agricultural soils

Physicochemical properties of incubation substrates may affect embryonic growth, development, and sexual differentiation in oviparous species with temperature-dependent sex determination. Thus, the physicochemical modification of incubation substrates associated with anthropogenic activities may pose a risk to normal embryonic and hatchling development in the alligator snapping turtle (Macrochelys temminckii), an oviparous species with temperature-dependent sex determination. The present study evaluated the development of alligator snapping turtle embryos and hatchlings incubated in vermiculite, and in Tunica Soils Series from a natural nesting area and from a cotton farm. Differences between agricultural and natural soils included particle size composition, concentrations of phosphorus, nitrates (NO3), magnesium, manganese, potassium, calcium, sodium, and copper concentrations, salinity, and cation exchange capacity (CEC). Soil pH, and iron, boron and sulfate (SO4) concentrations were similar. Agricultural soils received applications of herbicides (glyphosate, fluometuron, pyrithiobac sodium, 2,4-D, trifluralin) and insecticides (methyl parathion, dicrotophos), and natural soils did not. No significant differences were detected in incubation period, clutch success, hatchling mass, sex ratio, or frequency of abnormalities in offspring from the different incubation substrates. Increases over a four-month period in mass and tail length of hatchlings incubated in natural soils were, however, significantly greater than in hatchlings incubated in other substrates. We therefore suggest that future studies should examine how physicochemical factors of incubation substrate affect hatchling growth.

Validation of Pyrithiobac Sodium (Staple® Herbicide) ELISA for Australian Cotton Soils

An enzyme‐linked immunosorbent assay (ELISA) for pyrithiobac‐sodium (Staple®) produced by DuPont was validated in Australian soils. This pyrithiobac‐sodium ELISA was shown to be highly sensitive with the limit of detection of 4–5 ppt. Soil samples were extracted either in PBS buffer by shaking or by accelerated solvent extraction (ASE). While pyrithiobac sodium can be analyzed directly by ELISA after ASE extraction with 1/10 or more dilutions, the analysis of PBS extract required filtration and dilution 1/20 or more depending on the concentration. Immunoassay results compared favorably with GC‐MS results for both ASE and PBS extract of incurred residue of pyrithiobac sodium in soil samples, indicating that this ELISA can be an inexpensive and reliable alternative to conventional residue analysis methods for quantification of pyrithiobac‐sodium. This validation provided the basis for applying the ELISA to a field study of pyrithiobac‐sodium.

Phytotoxicity of Potential Postemergence Herbicides for Jalapeno Peppers

Large weeds, particularly amaranths, are a serious impediment to mechanical harvesting of jalapeno peppers. Several herbicides were applied in 1998 and 1999 postemergence topical (PT) to commercial fields when peppers had four to six leaves, or postdirected (PD) with a shielded sprayer ≈1 month later, and evaluated for crop injury, weed control, and effects on yield. Treatments were applied to four-row plots 9 m long with a CO<subscript>2 backpack sprayer. PT treatments included pyrithiobac sodium at 0.036, 0.053, or 0.071 kg·ha–1 a.i. with nonionic surfactant or crop oil concentrate, metolachlor at 1.68 kg·ha–1 a.i., and oxyfluorfen at 0.14 or 0.28 kg·ha–1 a.i.. PD treatments consisted of the same rates of pyrithiobac sodium with nonionic surfactant only, and the same rates of oxyfluorfen. Pyrithiobac sodium PT caused significant chlorosis (reduction in SPAD chlorophyll) in new foliage and reduction in plant height after 1 week, but plants recovered with no effect on final plant height, chlorophyll, or yield. No significant difference was observed between the two adjuvants. Metolachlor had no measurable effect on pepper growth or yield. Oxyfluorfen PT killed young apical tissue and caused chlorosis of immature leaves. Plants recovered, but plant height was reduced by 14% to 28% and yield by 11% to 43%. PD treatments had no effect on pepper growth or yield. All herbicides provided adequate weed control under light pressure. Pyrithiobac sodium appears to have potential as a postemergence herbicide for control of amaranth in jalapeno peppers.

Pyrithiobac sodium controls nightshade without long-term effect on cotton

Nightshades are some of the most difficult weeds to control in Upland Acala varieties of cotton. A herbicide in the newest class of acetolactase synthase inhibitors has been developed for use as a selective over-the-top broadleaf herbicide in cotton. Studies were conducted in Upland Acala cotton varieties in 1991, 1992 and 1993 to evaluate the efficacy of Staple (pyrithiobac sodium) in controlling nightshade. The herbicide was applied as early postemergence, mid-postemergence and sequential applications at rates of 0.25, 0.50, 0.75, 1.0, 2.0 and 3.0 oz of active ingredient per acre (ai/acre). Acceptable nightshade control was achieved at all rates, when applied as a single application or as sequential applications, except for the 0.25 and 0.50 oz ai/acre applications. The best control was achieved when the herbicide was applied at rates of 1.0 to 3.0 oz ai/acre over the top of cotton in the cotyledon to eight true-leaf stage, with nightshade in the cotyledon to six-leaf stage. Cotton injury symptoms were evident with all treatments at 7 days after application but were nonexistent by 90 days after application. There was no evidence to indicate that pyrithiobac sodium has any long-term effect on cotton growth and development or on cotton lint yield.

Influence of pyrithiobac sodium on purple (Cyperus rotundus) and yellow nutsedge (C. esculentus)

Greenhouse studies were conducted to determine purple and yellow nutsedge response to selective placement of a 5-cm layer of pyrithiobac sodium-treated soil above, below, or above and below nutsedge tubers. Pyrithiobac sodium at 36 or 72 g ae ha−1applied postemergence as foliar, soil, or foliar and soil treatments also was evaluated. Pyrithiobac sodium applied above, below, or above and below nutsedge tubers reduced yellow and purple nutsedge shoot number, shoot regrowth, and root-tuber dry weight at least 90%. Foliar-only treatment of pyrithiobac sodium was less effective at reducing emerged purple and yellow nutsedge numbers than application to soil only or to foliage and soil. Best reduction in yellow and purple nutsedge growth with pyrithiobac sodium was obtained with soil-incorporated treatments.

Weed Management in No-Tillage Bromoxynil-Tolerant Cotton (Gossypium hirsutum)

An experiment was conducted at four locations in North Carolina during 1994 and 1995 to evaluate weed control, cotton yield, fiber quality, and net returns in no-tillage bromoxynil-tolerant cotton. The experiment focused on using bromoxynil or pyrithiobac sodium applied early POST over-the-top as alternatives to fluometuron plus MSMA applied early POST directed. Fluometuron plus MSMA was more effective than bromoxynil or pyrithiobac sodium on tall morningglory, large crabgrass, goosegrass, and broadleaf signalgrass. Bromoxynil and fluometuron plus MSMA were similarly effective on common lambsquarters, common ragweed, and eclipta and more effective than pyrithiobac sodium. Pyrithiobac sodium and fluometuron plus MSMA were similarly effective on smooth pigweed and Palmer amaranth and more effective than bromoxynil. Prickly sida control by bromoxynil and pyrithiobac sodium was equal to or greater than control by fluometuron plus MSMA. All early POST herbicides controlled pitted morningglory similarly. Regardless of the early POST herbicides used, fluometuron applied PRE and cyanazine plus MSMA applied late POST directed increased control of most weeds and increased cotton yield and net returns. Bromoxynil and pyrithiobac sodium effectively substituted for fluometuron plus MSMA only in systems that included fluometuron applied PRE and cyanazine plus MSMA applied late POST directed. Effects of herbicide systems on cotton fiber quality were minor.