NC State Extension Publications

Introduction

Trade Names: Roundup (many forms), Glyfos, Glystar, Touchdown, many others Common Name: glyphosate Formulations: 4L*, 5.5L, 94 WD, many others

Uses

Nonselective, foliar-applied, systemic, postemergence control of most weeds – broadleaves, grasses, sedges and many woody species. Uses include:

  • Site preparation – general weed control before planting most crops or landscape plants.
  • Postemergence directed or spot spray for general weed control in established woody crops, including Christmas trees, orchards, nurseries and landscape plantings.
  • Broad spectrum weed control in glyphosate-resistant (Roundup-Ready®) crops
  • Weed control under benches and in walkways of greenhouses when no plants are in the house.
  • General weed control in non-crop areas including residential and commercial lawns, golf courses, sod farms, etc.
  • Certain formulations are labeled for use in aquatic sites and irrigation canals for non-selective weed control.
Amount of Active Ingredient Amount by Formulation
4L 94 WD
Per Acre 1 to 5 lb 1 to 5 qt 0.9 to 4.4 lb
Per 1,000 sq. ft. 0.3 to 1.8 oz 0.3 to 1.6 oz
Spot Treatment 0.67 to 2.67 oz/gal spray

Major Weeds Controlled

Glyphosate is a nonselective herbicide. Most plants are controlled or severely injured. Woody perennial weeds often require higher doses than seedling broadleaf weeds.

Major Weeds Not Controlled

Several species are not well controlled by glyphosate including dayflower, doveweed, cutleaf eveningprimrose, established white clover, morningglory and several pine species. Weeds under poor growing conditions such as water stress or disease and insect damage may show erratic or reduced control. Many perennial weeds, particularly woody species, are not well controlled when treated at non-optimal times. Field horsetail (Equisetum arvensis) and legumes are not well controlled.

RESISTANT WEEDS: Several weed species have developed resistance to glyphosate including horseweed, palmer amaranth, ragweed, and annual ryegrass.

For Best Results

Apply to actively growing plants. Do not apply if rainfall or overhead irrigation will occur within 1.5 to 6 hours, depending upon the formulation used. Check label for specific guidelines. Treat before mowing or after regrowth to specified size as described on the label. Coverage should be uniform and complete, but do not apply to the point of runoff. Season of application is very important for controlling many perennial weed species. The addition of an adjuvant may be necessary for some formulations; consult label for details. May be applied using a wiper, or as a cut-stem / cut-stump treatment.

Cautions and Precautions

Avoid contact with foliage, green stems, or fruit of desirable plants since severe injury or death may result. Injury to plants receiving small amounts of drift may be expressed one to two years after the occurrence. Do not use more than 25 gallons of spray solution per acre. Keep people and pets off treated areas until dry to prevent transfer to desirable foliage. Limited amounts of drift to leaves will damage many plants.

Residual Activity (In Soil)

While glyphosate may be moderately persistent in soil, it is rapidly bound by clay particles and is not bioavailable resulting in no soil residual activity; however, in soilless substrates or pure sands, crop injury from root uptake has been observed.

Volatility and Leaching Potential

The vapor pressure is very low rendering it nonvolatile. Off-target drift may cause injury to non-target plants. Although glyphosate is very water soluble, it binds rapidly and tightly to soil colloids so that leaching does not occur. Further, glyphosate has a low tendency to runoff unless it is sorbed to eroding soil colloids.

Fate in Water

Glyphosate is highly soluble in water. Further, hydrolytic or photolytic processes do not readily degrade glyphosate and major reductions in water are due to sediment adsorption and microbial degradation.

Symptoms, Mode of Action, and Degradation

Absorbed through foliage and green stems and translocated throughout the plant. Growth inhibition occurs within days, but symptoms may take 7 to 10 days to develop. Symptoms include general chlorosis followed by senescence. Woody plants that are not killed may show injury symptoms on the new growth for two or more years. The symptoms may include chlorotic, strap-shaped leaves, shortened internodes, and witches-brooming. Mechanism of action involves inhibition of aromatic amino acid synthesis (a process unique to the plant kingdom) and the primary route of degradation is microbial. Additional information on glyphosate injury symptoms is available in the herbicide injury factsheet Glyphosate.

Additional Information

Manufacturer: Monsanto, Syngenta, others

EPA Reg. # 524-44

Updated Toxicology Information – The Debate Continues

September 2018 update

Recently, concerns have been expressed about the safety of glyphosate. This has also been the subject of litigation. Although hazard analysis by some non-governmental organizations have raised concerns, In 2016 the US EPA released a report from a review of glyphosate carcinogenicity and concluded that glyphosate is "not likely to be carcinogenic to humans." This conclusion is consistent with conclusions of other regulatory agencies. The full EPA report is available.

More recently (2017) EPA released a draft review of glyphosate human health and ecological risk assessments.

This draft human health risk assessment concludes that glyphosate is not likely to be carcinogenic to humans. The Agency’s assessment found no other meaningful risks to human health when the product is used according to the pesticide label. The Agency’s scientific findings are consistent with the conclusions of science reviews by a number of other countries as well as the 2017 National Institute of Health Agricultural Health Survey. EPA’s human health review evaluated dietary, residential/non-occupational, aggregate, and occupational exposures. Additionally, the Agency performed an in-depth review of the glyphosate cancer database, including data from epidemiological, animal carcinogenicity, and genotoxicity studies. The ecological risk assessment indicates that there is potential for effects on birds, mammals, and terrestrial and aquatic plants.

Does this mean glyphosate is "safe"? It is my opinion that we must limit our exposure to all pesticides, including glyphosate. When using any pesticide: wear appropriate protective clothing, use it carefully to avoid off-target deposition, store the pesticide in a safe and secure site, and follow all label directions.

Authors

Professor of Weed Science, Extension Specialist & Department Extension Leader
Horticultural Science
Assistant Professor
Crop & Soil Sciences

Publication date: Feb. 29, 2016

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