Glufosinate Herbicide

Glufosinate

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Glufosinate is a nonselective foliar herbicide used for broadcast burndown application before planting or prior to emergence of canola, corn, sweet corn, soybean, and sugar beet. It can also be applied post-emergence to glufosinate-tolerant canola, corn, sweet corn, and soybean; however, spray contact with non-tolerant crops can result in injury. Other labeled uses of glufosinate include directed postemergence applications around trees, vines, and berries, as well as potato vine desiccation. In addition to agricultural uses, some glufosinate products can be used to control plants along landscape borders and around ornamental trees.

Glufosinate provides control of many annual broadleaf and grass weeds; however, control of large or well-tillered annual grasses, such as yellow foxtail, wild oat, or volunteer cereals, can be marginal. Glufosinate also provides suppression of some perennial weeds. Glufosinate has no soil activity.

Approximately 800,000 pounds of glufosinate were sold in Minnesota in 2017.¹ A 2013 USDA:NASS Survey, indicated that glufosinate was applied to 1% of corn acres, and 2% of soybean acres in the state.² It was applied on up to 11% of the acres in specific counties (Wilkins). As glyphosate resistant weeds become more prevalent, glufosinate use may increase as an alternative herbicide option.³

Mode of Action

Glufosinate controls weeds by inhibiting glutamine synthetase (herbicide site of action 10), an enzyme involved in the incorporation of ammonium into the amino acid glutamine. Inhibition of this enzyme causes a buildup of phytotoxic ammonia in plants which disrupts cell membranes. Glufosinate is a contact herbicide with limited translocation within the plant. Control is best when weeds are actively growing and not under stress.

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

• treating small weed sizes (recommended weed size varies with species and application rate),
• utilizing spray nozzles and pressure that produce medium-size droplets (250-350 microns). Course droplets can result in reduced weed control,
• applying with a minimum of 15 gallons of water per acre.

Glufosinate Tolerant Crops

Glufosinate tolerant cultivars of corn, sweet corn, soybeans, and canola are available for use in Minnesota and marketed under the LibertyLink^® name. In addition, Enlist E3 soybean varieties are available which are tolerant to glufosinate as well as glyphosate and 2,4-D. Only 2,4-D choline products, Enlist One^® and Enlist Duo^®, can be applied on LibertyLink^® crops. Check the pesticide labels carefully prior to application.

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 to herbicides 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.³

Glufosinate resistant Italian ryegrass, Lolium perenne ssp. Multiflorum, has been reported in Oregon and California.⁴ Resistance in other weed species such as perennial ryegrass (Lolium perenne), ridged ryegrass (Lolium rigidum), and goosegrass (Eleusine indica) have been found in other parts of the world. Glufosinate resistant weed species have not been reported in Minnesota. To prevent the development of resistant weeds, utilize practices such as combing and rotating herbicides sites-of-action and mechanical weed control.³

Glufosinate Movement in the Environment

Glufosinate movement in soil depends on both soil properties and pesticides properties. The leaching potential of glufosinate is reduced with increasing soil clay and organic matter content. According to the National Pesticide Information Center’s Herbicide Properties Tool, glufosinate is likely to reach shallow groundwater in sandy soils (Koc = 10) but not in silty loam soils (Koc = 250).⁵ However, in soil column experiments, glufosinate and its degradants did not leach further than 6 inches in loam or clay soils, or further than 24 inches in sandy soils.⁶

Movement of glufosinate 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. Glufosinate may also move offsite via drift during application.

Detection in Minnesota Waters 

The MDA started monitoring for this pesticide in 2022. Water Quality Portal⁷ data indicated that glufosinate was detected in 0.7% of groundwater samples and 1.1% of surface water samples in the United States. The highest concentrations detected in groundwater and surface water on the Water Quality Portal were 4.5 and 3.2 ug/L, respectively. The Minnesota Department of Health has established a drinking water reference value for glufosinate of 5 µg/L. The lowest Environmental Protection Agency (EPA) OPP (Office of Pesticide Products) aquatic life benchmark for glufosinate is 72 ug/L for non-vascular plants.

Glufosinate and Non-target Organisms

Glufosinate is very toxic to non-target plants. Since it is a contact herbicide, drift of a small volume of spray can result in necrotic vegetation. However, thorough spray coverage would be needed to kill plants, especially if larger in size.

Glufosinate is moderately toxic to fish and aquatic invertebrates with a lethal concentration, LC₅₀, of >2400 ug/L for both rainbow trout and water flea.⁷ Glufosinate is slightly toxic to mammals and birds through ingestion with acute oral LD₅₀ values >2000 mg/kg. Glufosinate is classified as practically non-toxic to adult honeybees on an acute contact and oral exposure basis, LD₅₀ >100 μg ai/bee. However, the EPA states in their interim decision for glufosinate that "the Agency is unable to assess risks to pollinators at the present time" and may require additional pollinator studies related to honey bee chronic and larval toxicity testing before a final registration review decision is made for glufosinate. The toxicity of glufosinate degradates is similar to or lower than the parent compound.

Glufosinate and Human Health

Evaluation of human dietary exposure to glufosinate and its breakdown products by the EPA showed that the greatest risk from glufosinate was from contamination of drinking water. However, observed levels of exposure were found to be below levels of concern and were not considered a reasonable adverse health risk. Based on rodent studies, glufosinate is classified as not likely to be carcinogenic in humans.⁸

Glufosinate is a slight skin irritant and a severe eye irritant.⁹ EPA evaluation determined that short and intermediate term dermal exposure was not a concern if label personal protective equipment requirements, such as goggles and gloves, are utilized. The restricted entry interval for glufosinate treated areas varies among products, crops, and type of field activity.

References

¹Minnesota Department of Agriculture. 2017. Pesticide Sales Database. Accessed March 25, 2020.

²Minnesota Department of Agriculture, USDA: NASS Minnesota Field Office, 2016. 2013 Pesticide Usage on Four Major Crops in Minnesota. 

³Ohio State University Extension. 2019 Weed Control Guide for Ohio, Indiana and Illinois. Bulletin 789.

⁴ Heap, I.  The International Herbicide-Resistant Weed Database.  www.weedscience.org. Accessed March 30, 2020.

⁵National Pesticide Information Center. Herbicide Properties Tool. http://npic.orst.edu/HPT/. Accessed March 30, 2020.

⁶United States Environmental Protections Agency. 2014 Fate and Ecological Risk Assessment for the Registration Review of Glufosinate.

⁷National Water Quality Monitoring Council. Water Quality Portal. Accessed March 25, 2020.

⁸Donovan, W.H., D. Dotson, and K. Rury. June 19, 2012. USEPA Memorandum: Glufosinate ammonium. Updated of revised acute and chronic aggregate dietary assessment (food and drinking water) exposure assessment in support of the petition proposing tolerances for residues of glufosinate ammonium in citrus fruits, pome fruits, stone fruits, olives, and sweet corn. CAS No: 77182-82-2. Decision No. 402155. https://deq.mt.gov/Portals/112/Water/WQPB/Standards/Glufosinate.pdf?ver=2016-12-23-154821-317

⁹United States Environmental Protection Agency. Reregistration Review of Glufosinate Ammonium (PC Code 128850). Docket ID: EPA-HQ-OPP-2008-0190. . Accessed 4/12/20.