# Pyrithiobac sodium
**Pyrithiobac sodium** is a selective, post-emergence herbicide of the pyrimidinylthiobenzoate chemical family that kills weeds by inhibiting the plant enzyme acetolactate synthase (ALS); it is used chiefly for broadleaf weed control in cotton and is sold under trade names including Staple (DuPont) and, in India, Provide and other 10% EC brands.[1](https://downloads.regulations.gov/EPA-HQ-OPP-2011-0661-0042/content.pdf) • [2](https://ordspub.epa.gov/ords/pesticides/f?p=CHEMICALSEARCH%3A3%3A0%3A%3ANO%3A21%2C3%2C31%2C7%2C12%2C25%3AP3_XCHEMICAL_ID%3A3681) • [3](https://www.piindustries.com/pi-products/provide/)
| Key fact | Detail |
|---|---|
| Chemistry | Sodium 2-chloro-6-(4,6-dimethoxypyrimidin-2-yl)sulfanylbenzoate; formula C\(_{13}\)H\(_{10}\)ClN\(_{2}\)NaO\(_{4}\)S; molecular weight 348.74 g/mol; CAS 123343-16-8[4](https://pubchem.ncbi.nlm.nih.gov/compound/23682187) |
| Mode of action | Inhibits acetolactate synthase (ALS), blocking the first common step in biosynthesis of the branched-chain amino acids valine, leucine, and isoleucine, so cell division and growth cease[5](https://downloads.regulations.gov/EPA-HQ-OPP-2011-0661-0021/content.pdf) |
| US registration | First registered September 29, 1995; EPA PC Code 078905; registered for cotton nationally and for wildflowers in North Carolina under a SLN registration, with no residential uses[2](https://ordspub.epa.gov/ords/pesticides/f?p=CHEMICALSEARCH%3A3%3A0%3A%3ANO%3A21%2C3%2C31%2C7%2C12%2C25%3AP3_XCHEMICAL_ID%3A3681) • [1](https://downloads.regulations.gov/EPA-HQ-OPP-2011-0661-0042/content.pdf) |
| Use scale | About 62,000 pounds applied annually to 1.1 million acres of cotton in the US; more than 95% of use is the liquid formulation[1](https://downloads.regulations.gov/EPA-HQ-OPP-2011-0661-0042/content.pdf) |
| Application rate | Cotton use limited to one or two pre- or post-emergence applications at a maximum of 0.1275 lb ai/A[6](https://downloads.regulations.gov/EPA-HQ-OPP-2011-0661-0004/content.pdf) |
| Soil behavior | Laboratory soil half-life 60 days with 60 to 73% mineralization; field half-lives 11 to 46 days; low adsorption (Koc 14.7 to 26.9) makes it mobile in soil[7](https://pubs.acs.org/doi/10.1021/bk-2002-0813.ch015) |
| EU status | Not approved as an active substance in the European Union (status pending under Regulation (EC) No 1107/2009)[4](https://pubchem.ncbi.nlm.nih.gov/compound/23682187) |
| Resistance | Widespread ALS-inhibitor resistance in Amaranthus weeds, including a tryptophan-to-leucine substitution at position 574 of the ALS enzyme[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC7323944/) |
## Chemistry and mode of action
Pyrithiobac sodium is the sodium salt of pyrithiobac, a benzoic acid bearing a chloro substituent and a 4,6-dimethoxypyrimidinylthio group.[4](https://pubchem.ncbi.nlm.nih.gov/compound/23682187) It belongs to the pyrimidinyl(thio)benzoate family of ALS inhibitors, one of several chemical classes (alongside sulfonylureas, imidazolinones, and triazolopyrimidines) that share this mechanism.[1](https://downloads.regulations.gov/EPA-HQ-OPP-2011-0661-0042/content.pdf) ALS (also called acetohydroxyacid synthase) catalyzes the first common step in the synthesis of the essential branched-chain amino acids valine, leucine, and isoleucine; inhibition stops cell division and growth, and susceptible plants die over days to weeks after uptake.[5](https://downloads.regulations.gov/EPA-HQ-OPP-2011-0661-0021/content.pdf)
The herbicide is weakly acidic and highly water soluble, giving it high mobility and leaching potential in soil; it is described as moderately persistent in soil systems but non-persistent in aquatic systems, with low mammalian toxicity.[4](https://pubchem.ncbi.nlm.nih.gov/compound/23682187)
## Use in cotton
Pyrithiobac sodium was developed by DuPont as DPX-PE350 and marketed as Staple, described at its introduction as the first broad-spectrum, low use rate, over-the-top herbicide registered for cotton.[9](https://www.cotton.org/beltwide/proceedings/getPDF.cfm?paper=B011.pdf&year=1996) In the United States it is applied over the top or postdirected to cotton anytime after the first visible true leaf, targeting seedling weeds that are actively growing; over-the-top application can cause temporary yellowing, bronzing, or crinkling of cotton leaves, more evident in Pima than Acala cottons.[10](https://ipm.ucanr.edu/agriculture/cotton/herbicide-treatment-table/) It controls many broadleaf weeds, including pigweeds ([Amaranthus](https://www.edgepedia.org/amaranthus) spp.), morning glories (Ipomoea spp.), common cocklebur, and smartweeds, but is not effective against barnyard grass and other grasses.[11](https://cropcare.co.in/herbicides/pyrithiobac-sodium10-ec/) • [12](https://indianjournals.com/article/ijws-31-3and4-016)
Because grass control is a gap, pyrithiobac is often paired with graminicides. In Indian Bt cotton trials, pyrithiobac sodium at 62.5 g/ha followed by quizalofop-ethyl at 50 g/ha at the 2 to 4 weed-leaf stage plus one hoeing gave the best weed control and net returns.[13](https://epubs.icar.org.in/index.php/AAR/article/view/62784) A pre-mix of pyrithiobac sodium plus [quizalofop](https://www.edgepedia.org/quizalofop) ethyl at 100 to 125 g/ha gave over 88% control of Trianthema portulacastrum, Dactyloctenium aegyptium, [Digitaria sanguinalis](https://www.edgepedia.org/digitaria-sanguinalis), and Acrachne racemosa in two-season field studies in Punjab.[14](https://agris.fao.org/search/en/providers/122535/records/65df1ddf4c5aef494fdfe1ef) Registered Indian labels for the 10% EC formulation specify 62.5 to 75 g a.i./ha on cotton with a 160-day pre-harvest interval.[15](https://www.napanta.com/pesticide-products/pyrithiobac-sodium-10-ec/)
[Crop rotation](https://www.edgepedia.org/crop-rotation) restrictions reflect the herbicide's soil persistence: plantback intervals include 8 months for tomatoes, 6 months for wheat, and no interval for cotton, with all other crops requiring at least 10 months plus a field bioassay.[10](https://ipm.ucanr.edu/agriculture/cotton/herbicide-treatment-table/)
## Behavior in soil
Laboratory studies with radiolabeled material found that pyrithiobac sodium degrades microbially, with 60 to 73% mineralization and a half-life of 60 days; adsorption is low, with linear Koc values of 14.7 to 26.9, indicating mobility.[7](https://pubs.acs.org/doi/10.1021/bk-2002-0813.ch015) Under field conditions degradation is faster, with half-lives of 11 to 46 days, and field studies showed limited mobility, corroborated by a small-scale prospective groundwater monitoring study.[7](https://pubs.acs.org/doi/10.1021/bk-2002-0813.ch015) Leaching experiments through intact soil columns of four cotton-region soils confirmed high mobility under saturated flow, with movement influenced by soil type and preferential flow.[16](https://pubmed.ncbi.nlm.nih.gov/10564050/) A Telangana, India study found first-order dissipation with half-lives of 46.2 to 63.0 days depending on soil type and moisture, slower under low moisture and in black clayey soil, where higher sorption reduces bioavailability.[17](https://journalijecc.com/index.php/IJECC/article/view/5116) Biologically available residues generally persist less than one growing season, though residues leached deeper in the profile could affect rotational crops.[18](https://www.cambridge.org/core/journals/weed-science/article/abs/activity-adsorption-mobility-and-field-persistence-of-pyrithiobac-in-three-soils/D9B13C6CE63A941B461B3FB46E053BCC)
## Herbicide resistance
Because ALS inhibitors act at a single target site, resistance has evolved repeatedly in weeds exposed to pyrithiobac and related ALS herbicides. In [Mississippi](https://www.edgepedia.org/mississippi) populations of redroot pigweed and tall waterhemp, sequencing of the ALS gene identified a point mutation at position 574 replacing a TGG codon with TTG, substituting leucine for tryptophan (W574L); the resistant redroot pigweed population's ALS enzyme was 3,360-fold less sensitive to pyrithiobac in vitro, and whole-plant resistance indices were 1,476 for pyrithiobac, 3,500 for imazaquin, and 900 for [trifloxysulfuron](https://www.edgepedia.org/trifloxysulfuron).[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC7323944/)
Resistance extends beyond target-site mutations. A survey of 215 roadside Palmer amaranth populations in the [Mississippi Delta](https://www.edgepedia.org/mississippi-delta) of eastern Arkansas found that about 89% showed over 90% survival to pyrithiobac at the labeled field rate of 73 g ai/ha, and none of the populations was completely susceptible; 209 of the 215 populations showed multiple resistance to both pyrithiobac and glyphosate.[19](https://pubmed.ncbi.nlm.nih.gov/27071064/) In Georgia, a Palmer amaranth biotype resistant to both glyphosate and ALS inhibitors required pyrithiobac rates 151 times higher (for 50% injury) and 563 times higher (for 50% fresh weight reduction) than a susceptible biotype, and a glyphosate plus pyrithiobac tank mixture at 6,930 + 420 g/ha controlled it only 89 to 95% in the field.[20](https://www.cambridge.org/core/journals/weed-science/article/multiple-resistance-in-palmer-amaranth-to-glyphosate-and-pyrithiobac-confirmed-in-georgia/5FD07079561689A7CF86EE339DF1F604) Nontarget-site resistance can also confer cross-resistance: a fomesafen-resistant Palmer amaranth accession showed 94.7% survival to pyrithiobac sodium at field rates, along with resistance to other PPO, HPPD, and ALS inhibitors and glyphosate.[21](https://www.cambridge.org/core/journals/weed-technology/article/nontarget-site-resistance-in-palmer-amaranth-amaranthus-palmeri-s-wats-confers-crossresistance-to-protoporphyrinogen-oxidaseinhibiting-herbicides/B0B6144FA41763F412132D9E89C15B6A) Resistance management guidance therefore calls for integrated weed management combining chemical, mechanical, and cultural tactics.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC7323944/)
## Regulatory status
In the United States, pyrithiobac sodium is undergoing EPA registration review (Docket EPA-HQ-OPP-2011-0661), with an interim registration review decision addressing ecological risks and spray drift mitigation.[1](https://downloads.regulations.gov/EPA-HQ-OPP-2011-0661-0042/content.pdf) EPA's 1999 tolerance rule concluded that aggregate dietary exposure from food and drinking water uses less than 0.1% of the reference dose for the US population and less than 0.2% for children and infants, and that the only residue of regulatory concern is the parent compound.[22](https://www.govinfo.gov/content/pkg/FR-1999-10-20/pdf/FR-1999-10-20.pdf) • [23](https://www.govinfo.gov/content/pkg/FR-1997-07-11/pdf/FR-1997-07-11.pdf) In the European Union the substance is not approved, with a default maximum residue level of 0.01 mg/kg applying under Regulation 396/2005.[4](https://pubchem.ncbi.nlm.nih.gov/compound/23682187) • [24](https://new.pesticidi.org/eu-podaci/supstanca/1355)
## References
1. [Pyrithiobac-sodium Interim Registration Review Decision, US EPA](https://downloads.regulations.gov/EPA-HQ-OPP-2011-0661-0042/content.pdf)
2. [Pyrithiobac-sodium Chemical Details, US EPA Pesticide Chemical Search](https://ordspub.epa.gov/ords/pesticides/f?p=CHEMICALSEARCH%3A3%3A0%3A%3ANO%3A21%2C3%2C31%2C7%2C12%2C25%3AP3_XCHEMICAL_ID%3A3681)
3. [Provide (Pyrithiobac Sodium) product page, PI Industries](https://www.piindustries.com/pi-products/provide/)
4. [Pyrithiobac sodium, CID 23682187, PubChem](https://pubchem.ncbi.nlm.nih.gov/compound/23682187)
5. [Pyrithiobac-sodium Human Health Assessment, US EPA](https://downloads.regulations.gov/EPA-HQ-OPP-2011-0661-0021/content.pdf)
6. [Pyrithiobac Sodium Registration Review Human-Health Assessment Scoping Document, US EPA](https://downloads.regulations.gov/EPA-HQ-OPP-2011-0661-0004/content.pdf)
7. [Degradation of Pyrithiobac Sodium in Soil in the Laboratory and Field, ACS Symposium Series](https://pubs.acs.org/doi/10.1021/bk-2002-0813.ch015)
8. [Resistance to ALS inhibitors due to a W574L substitution in redroot pigweed and tall waterhemp, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC7323944/)
9. [Staple: A New Cotton Herbicide from DuPont, Beltwide Cotton Proceedings](https://www.cotton.org/beltwide/proceedings/getPDF.cfm?paper=B011.pdf&year=1996)
10. [Herbicide Treatment Table, Cotton Pest Management Guidelines, UC IPM](https://ipm.ucanr.edu/agriculture/cotton/herbicide-treatment-table/)
11. [Pyrithiobac-Sodium 10% EC, Crop Care](https://cropcare.co.in/herbicides/pyrithiobac-sodium10-ec/)
12. [Evaluation of Pyrithiobac for Weed Control in Cotton, Indian Journal of Weed Science](https://indianjournals.com/article/ijws-31-3and4-016)
13. [Performance of pre and post-emergence herbicides in Bt-cotton crop, Annals of Agricultural Research](https://epubs.icar.org.in/index.php/AAR/article/view/62784)
14. [Weed control in Bt cotton with pre-mix of pyrithiobac sodium plus quizalofop ethyl, AGRIS](https://agris.fao.org/search/en/providers/122535/records/65df1ddf4c5aef494fdfe1ef)
15. [Pyrithiobac Sodium dosage and label claims, NaPanta](https://www.napanta.com/pesticide-products/pyrithiobac-sodium-10-ec/)
16. [Mobility of the herbicide pyrithiobac through intact soil columns, J Agric Food Chem](https://pubmed.ncbi.nlm.nih.gov/10564050/)
17. [Dissipation of Pyrithiobac-Sodium at Different Soil Moisture Regimes in Telangana, IJECC](https://journalijecc.com/index.php/IJECC/article/view/5116)
18. [Activity, adsorption, mobility, and field persistence of pyrithiobac in three soils, Weed Science](https://www.cambridge.org/core/journals/weed-science/article/abs/activity-adsorption-mobility-and-field-persistence-of-pyrithiobac-in-three-soils/D9B13C6CE63A941B461B3FB46E053BCC)
19. [Multiple-Herbicide Resistance Is Widespread in Roadside Palmer Amaranth Populations, PubMed](https://pubmed.ncbi.nlm.nih.gov/27071064/)
20. [Multiple Resistance in Palmer Amaranth to Glyphosate and Pyrithiobac Confirmed in Georgia, Weed Science](https://www.cambridge.org/core/journals/weed-science/article/multiple-resistance-in-palmer-amaranth-to-glyphosate-and-pyrithiobac-confirmed-in-georgia/5FD07079561689A7CF86EE339DF1F604)
21. [Nontarget site resistance in Palmer amaranth confers cross-resistance to PPO-inhibiting herbicides, Weed Technology](https://www.cambridge.org/core/journals/weed-technology/article/nontarget-site-resistance-in-palmer-amaranth-amaranthus-palmeri-s-wats-confers-crossresistance-to-protoporphyrinogen-oxidaseinhibiting-herbicides/B0B6144FA41763F412132D9E89C15B6A)
22. [Federal Register Vol. 64 No. 202 (October 20, 1999), pyrithiobac sodium tolerance rule](https://www.govinfo.gov/content/pkg/FR-1999-10-20/pdf/FR-1999-10-20.pdf)
23. [Federal Register Vol. 62 No. 133 (July 11, 1997), pyrithiobac sodium notice](https://www.govinfo.gov/content/pkg/FR-1997-07-11/pdf/FR-1997-07-11.pdf)
24. [Pyrithiobac sodium EU regulatory data, pesticidi.org](https://new.pesticidi.org/eu-podaci/supstanca/1355)
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*Topic: Encyclopedia › Life and health › Applied biology and nonhuman health › Plant disease and plant protection › Pesticides › Herbicides › ALS/AHAS inhibitor herbicides*
*Initially written Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license