| Pesticide Type | Herbicide |
|---|---|
| Site of Action | Photosystem II Inhibitor (Group 6) |
| Chemical Class | Benzothiadiazinone |
| Common Trade Names* | Basagran, Bashazon, Bentazon, Broadloom |
| Registration Status | EPA: Registered since 1974 MN: Registered |
| Structure |  |
*No endorsement is implied in the referencing of trade names.
Bentazon is a selective contact (foliar) herbicide for postemergence control of annual broadleaf weeds and yellow nutsedge in a variety of crops including soybeans, alfalfa, beans, corn, peas, peppers, and sorghum.1 It also has activity on some perennial broadleaf weeds such as Canada thistle and field bindweed. Some bentazon products are labeled for use in turf, lawns, and noncrop sites. In 2019, approximately 158,000 pounds of bentazon were sold in Minnesota.2
Mode of Action
Bentazon controls weeds by inhibiting photosynthesis by binding to a protein in photosystem II, herbicide site-of-action 6.3 Herbicides in sites-of-action groups 5 and 7 bind to different sites on this same protein.
Issues with Resistance
Herbicide resistance is the inherited ability of a plant, such as weeds, to survive an herbicide application that the original population was susceptible to. The development of resistance is a growing concern for weed management because it can lead to the loss of herbicide options, which can have important economic and environmental consequences. Bentazon resistant weeds have not been reported in the United States.4 Resistant smooth pigweed (Amaranthus hybridus) and redroot pigweed (Amaranthus retroflexus) have been reported in Ontario, Canada. To prevent the development of resistant weeds, use practices such as rotating and combining herbicide sites-of-action and utilizing mechanical weed control.3
Bentazon Persistence and Movement in the Environment
Although bentazon does not have soil activity, it has low to moderate persistence with a soil dissipation half-life, DT50, of 7-33 days.5 The dissipation time, DT50 for foliar residues is 5.5 days.6 Bentazon does not adsorb strongly to soil, Koc = 3-176 mL/goc.5 It has a medium to high potential to leach to groundwater or move offsite to surface water. Monitoring studies have demonstrated that bentazon can leach to groundwater in certain vulnerable environments. Bentazon can also runoff fields dissolved in surface water.
Detection in Minnesota Waters
The MDA monitors groundwater and surface water for bentazon through its Agricultural Chemical Monitoring and Assessment Program. In 2020, bentazon was detected in 15% of the 223 groundwater samples analyzed.7 It was detected more frequently than its major degradate, bentazon AIBA. The maximum concentration detected was 2,780 ng/L, which is approximately 10% of the Minnesota Department of Health’s human health-based reference value of 30,000 ng/L.8 Bentazon was detected in 28% of the 386 surface water samples analyzed by MDA in 2020, with a maximum concentration of 834 ng/L.6 The lowest EPA Office of Pesticide Programs (OPP) aquatic life benchmark for bentazon is 4,500,000 ng/L (for non-vascular plants).9 The degradate, bentazon AIBA, was not detected in surface water samples.
Water Quality Portal data for the Upper Midwest indicated that 9% of groundwater sample between 1993 and 2020 had detections of bentazon10. Minnesota groundwater samples had 17% detection frequency for bentazon, including the maximum detection of 24,500 ng/L. This detection was 82% of the Minnesota Department of Health reference value (30,000 ng/L). Bentazon was detected in 17% of Minnesota surface water samples with a maximum concentration of 4,690 ng/L which is 0.1% non-vascular plant benchmark.
Bentazon and Non-target Organisms
Bentazon has low acute mammalian toxicity by oral (LD50=1,100 mg/kg, Toxicity Category III), dermal (LD50 > 2,500 mg/kg, Category III), and inhalation (LC50 > 5.1 mg/L, Category IV) exposures.11 It is moderately irritating to the eye (Category II) and slightly irritating to the skin (Category IV). Bentazon is a dermal sensitizer. It is practically nontoxic to nontarget insects (honeybee), LD50 > 100 ug/bee, and fish, LC50 > 100 ppm.5
Plant injury can occur due to misapplication, spray tank contamination, or spray drift. Affected foliage will become yellow or bronze in color and eventually necrotic.12 Injury is confined to foliage that has come in contact with the herbicide since there is limited translocation within plants after absorption.13 Injury symptoms can look similar to those caused by cell membrane disrupters herbicides. Crop oil concentrate and other spray additives may increase weed control and crop injury symptoms.
Bentazon and Human Health
Besides the acute toxicity mentioned, bentazon has not been shown to cause acute neurotoxicity, carcinogenicity, mutagenesis, or cytotoxicity.14 It is a skin sensitizer and causes eye irritation. Applicators and handlers are required to wear chemical protective clothing and eyeware.5
References
1United States Environmental Protection Agency. 2016. Bentazon Interim Registration Review Decision Case Number 0182. www.regulations.gov/document/EPA-HQ-OPP-2010-0117-0032.
2Minnesota Department of Agriculture Pesticide Sales Database. www2.mda.state.mn.us/webapp/lis/chemsold_default.jsp. Accessed September 1, 2021.
3Ohio State University Extension. 2020 Weed Control Guide for Ohio, Indiana, and Illinois. Bulletin 789. https://lorain.osu.edu/sites/lorain/files/imce/Program_Pages/ANR/2020%20Weed%20Control%20Guide%20Field%20Crops%20e789.pdf.
4Heap, I. The International Herbicide-Resistant Weed Database. www.weedscience.org. September 2, 2021.
5United Stated Environmental Protection Agency. 1994. Reregistration Eligibility Decision (RED), Bentazon. EPA 738-R-94-029. www.regulations.gov/document/EPA-HQ-OPP-2009-0081-0104.
6United Stated Environmental Protection Agency. 2017. Sodium Bentazon: Response to BASF’s Comments on the foliar dissipation half-life value used in the Preliminary Risk Assessment and Revised Risk Assessment for Birds and Mammals based on new data submissions. www.regulations.gov/document/EPA-HQ-OPP-2010-0117-0033.
7 Minnesota Department of Agriculture. 2020 Water Quality Monitoring Report. Minnesota Digital Water Research Library..https://wrl.mnpals.net/islandora/object/WRLrepository%3A3746/datastream/PDF/view.
8 Minnesota Department of Health. Rapid Assessments for Pesticides. https://www.health.state.mn.us/communities/environment/risk/guidance/dwec/rapidpest.html. Accessed July 27, 2021.
9United States Environmental Protection Agency. Aquatic Life Benchmarks and Ecological Risk Assessments for Registered Pesticides. https://www.epa.gov/pesticide-science-and-assessing-pesticide-risks/aquatic-life-benchmarks-and-ecological-risk#aquatic-benchmarks. Accessed September 7, 2021.
10National Water Quality Monitoring Council. Water Quality Portal. www.waterqualitydata.us/. Accessed June 21, 2021.
11Environmental Protection Agency. December 2, 2014. Preliminary Human Health Risk Assessment for Sodium Bentazon. www.regulations.gov/document/EPA-HQ-OPP-2010-0117-0017.
12Gunsolus, JL and WS Curran. 1999. Herbicide Mode of Action and Injury Symptoms. www.appliedweeds.cfans.umn.edu/sites/appliedweeds.cfans.umn.edu/files/herbicide_mode_of_action_and_injury_symptoms.pdf
13University of Illinois Extension. Sept. 2021. Herbicide Injury. https://web.extension.illinois.edu/hortanswers/detailproblem.cfm?PathogenID=198
14United States Environmental Protection Agency. 2009. Environmental Fate, Ecological Risk and Endangered Species Assessment in Support of the Registration Review of Fomesafen Sodium (PC123802)-2008. https://www.regulations.gov/document/EPA-HQ-OPP-2006-0239-0032.
