Pesticide Type Herbicide
Site of Action PPO Inhibitor (Group 14)
Chemical Class Diphenylether
Common Trade Names* Reflex, Flexstar, Rhythm, Rumble, Sinister, Topgun, numerous premixes
Registration Status EPA: Registered since 1987
MN: Registered

chemical structure diagram of Fomesafen

*No endorsement is implied in the referencing of trade names.

Fomesafen is a selective soil-applied and foliar herbicide for control of annual broadleaf weeds in soybeans1. Some fomesafen products are also labeled for use in dry beans, snap beans, and potatoes. It can be applied preplant, preemergence, or postemergence and is a component in several preemergence herbicide premixes. In addition to annual broadleaf weeds, fomesafen also suppresses some perennial broadleaf weeds. In 2017, approximately 820,000 pounds of fomesafen were sold in Minnnnesota2.

Mode of Action

Fomesafen is in the diphenylether chemical class of herbicides which controls weeds by inhibiting the protoporphyrinogen oxidase (PPO) enzyme in plants3. PPO inhibitors, herbicide site-of-action 14, interfere with an enzyme involved in chlorophyll and heme biosynthesis4. This inhibition leads to a chemical chain reaction that ultimately causes lipid peroxidation and membrane disruption resulting in rapid degeneration of plant tissues. Since this reaction is rapid, it results in contact activity and very little translocation occurs following foliar application. Soil-applied PPO inhibitors are absorbed by roots and have limited upward movement in plants. Fomesafen may cause chlorosis/necrosis of leaves of labeled crops at higher rates. Plants typically outgrow this injury with no negative impact on maturation or yield. Follow label “per application” and “per season” rate limits. Be sure to consider fomesafen from both preemergence and postemergence applications when determining total application rate for a season.

Due to its contact activity, thorough spray coverage of target weeds is needed for effective weed control. This is achieved by:

  • treating small weed sizes
  • utilizing spray nozzles and pressure that produce medium-size droplets (250-350 microns), course droplets can result in reduced weed control
  • apply with a minimum of 15 gallons of water per acre (increase to 20 gallons per acre for large weeds or dense foliage).

Fomesafen Carryover/Persistence

Fomesafen undergoes microbial degradation. Conditions which reduce this process have the potential to cause herbicide carryover and result in injury to rotational crops such as corn, canola, sunflowers, and alfalfa. Carryover potential is increased by5:

  • higher application rates
  • later application timing
  • cold and dry weather during the fall and winter following application.

View the map on the label of the product being used to determine the maximum annual application rate in various areas of Minnesota. In some areas of the state, application may not be allowed or restricted to alternate years. Be certain to consider fomesafen from both preemergence and postemergence applications, including premixes when determining annual maximum rates.

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. Fomesafen resistant tall waterhemp, Amaranthus tuberculatus, has been reported in Minnesota6. To prevent the development of resistant weeds, use practices such as rotating and combining herbicide sites-of-action and utilizing mechanical weed control4.

Fomesafen Movement in the Environment

Pesticide movement in soil depends on both soil and pesticides properties. Fomesafen has a high leaching potential with a Koc of 68 indicating that it does not attach strongly to soil particles. The National Pesticide Information Center rates fomesafen as likely to reach shallow groundwater7. Increasing soil clay and organic matter content reduces leaching potential. Fomesafen is broken down in the soil by microbial degradation. Its long soil persistence (63-527 days)1, increases the likelihood of reaching groundwater especially in coarse soils.

Movement of fomesafen to surface water can occur dissolved in runoff water or adsorbed to eroding soil. Runoff loss is greatest if a surface water runoff event occurs shortly after application.

Detection in Minnesota Waters

The MDA monitors groundwater and surface water for fomesafen through the Agricultural Chemical Monitoring and Assessment program. In 2019, fomesafen was detected in 14% of groundwater samples with a maximum detection of 2,960 ng/L. The health-based reference values is 20,000 ng/L 8. It was detected in 43% of river and stream samples with a maximum concentration of 2,130 ng/L. The lowest EPA Office of Pesticide Programs (OPP) benchmark for fomesafen is 92,000 ng/L.

Fomesafen and Non-target Organisms

Fomesafen is very toxic to non-target broadleaf plants and a small volume of drift can result in necrotic vegetation (localized dead plant tissue). Since it is a contact herbicide with limited translocation (movement through the plant), thorough spray coverage is needed for effective control. This is especially important if plants are larger in size.

Fomesafen has low toxicity to freshwater fish with an EPA aquatic benchmark of 63 mg/L9. Fomesafen is slightly toxic to mammals with an acute oral LD50 value of >5000 mg/kg3.

Fomesafen and Human Health

Besides the acute toxicity mentioned, fomesafen shows no evidence of neurotoxicity, carcinogenicity, mutagenesis, or cytotoxicity. However, it can cause sever eye irritation and moderate skin irritation10. Applicators and handlers are required to wear chemical resistant clothing, gloves, and footwear.


1USEPA. 2007. Registration Review - Ecological Risk Assessment problem formulation for fomesafen. 

2Minnesota Department of Agriculture Pesticide Sales Database. Accessed November 26, 2020.

3Tadayon, S. and K. M. Lowe. 2017. Fomesafen Sodium. Occupational and Residential Exposure Assessment for Proposed Uses on Tuberous and Corm Vegetables (Crop Group IC), Legume vegetable (Crop Group 6) and Low Growing Berry (Except Cranberry) (Crop Group 13-07G) and for Registration Review.

4Ohio State University Extension. 2020 Weed Control Guide for Ohio, Indiana and Illinois. Bulletin 789.

5Zollinger, R. 2014. Flexstar carryover to corn. NDSU Crop and Pest Report. .

6Heap, I. International Herbicide-Resistant Weed Database. Accessed November 26, 2020.

7National Pesticide Information Center. Herbicide Properties Tool. Accessed November 26, 2020.

8National Water Quality Monitoring Council. Water Quality Portal. Accessed March 25, 2020.

9 USEPA. 2015. Aquatic life benchmarks for pesticide registration.

10USEPA. 2011. Fomesafen; Pesticide Tolerances. Federal Register 76:12877-12882.