Herbicide Fact Sheet

Information Categories

The following information categories are presented for each chemical:

Activity

Herbicides are either foliar active, soil active, or both. This refers to how the chemical enters the plant. Foliar-active chemicals must usually have adequate leaf surface area in order to be absorbed by the plant, but in some cases foliar-active chemicals can be applied directly to the stem. Soil-active chemicals are pulled into the plant through the roots as they take up water and transpire.

Movement in Plants

Chemicals are translocated in the food transport system of the plant (the phloem) or in the water transport system (the xylem) or in both. Chemicals transported in the xylem are characteristically soil-active and move with the transpiration flow - from the base of the tree toward the tips of branches and leaves. Chemicals transported in the phloem move both up and down the plant.

Mode of Action

A brief description is given of how the chemical affects plant biochemistry.

Selectivity

General classes of plants which are resistant to the effects of the chemical are presented in this section.

Environmental Considerations

Information is provided on four key characteristics related to the environment:

• Volatility - Refers to a chemical's tendency to go into a gaseous state after application.
• Photo decomposition - Indicates whether a chemical is broken down or decomposed by sunlight.
• Mobility in Soil - Provides the potential for off-site movement through leaching.
• Half-life - Indicates the expected time after application that one-half of the chemical would naturally decompose in the environment. For example, suppose that an application is made of 2 pounds of an active ingredient with a half-life of 30 days. One month after application, 1 pound of the chemical would have decomposed and 1 pound would still be present.

Toxicity

Toxicity is provided in reference to mammals and to fish. LD50 is the lethal dose of a chemical required to kill 50 percent of a test-animal (rat) population. It is expressed in milligrams of chemical for each kilogram of test-animal weight. For reasons of comparison, the following toxicities are provided:

• Caffeine LD50 = 200 mg/kg (extremely toxic)
• Aspirin LD50 = 1,240 mg/kg (moderately toxic)
• Table salt LD50 = 3,000 mg/kg (moderately toxic)

LC50 is the lethal concentration in water at which a chemical becomes lethal to 50 percent of a test population of fish.

Product Formulation

Active ingredients are often sold under one or more formulations. This section provides the percentage of the active ingredient and indicates whether it is an amine (water soluble) or ester (oil soluble) liquid or a dry formulation. Trade names and manufacturers are also given.

Glyphosate

• Activity: Foliar
• Movement in plants: Translocated in food and water systems
• Mode of action: Amino acid inhibitor
• Selectivity: Non-selective

Environmental Considerations

• Volatility: Negligible
• Photodecomposition: Negligible
• Mobility in soil: Tightly bound to soil and organic matter
• Half-life: 61 days
• Toxicity: LD50 - 5,600 mg/kg (slightly toxic)
  LC50 - 86 ppm (slightly toxic)

Hexazinone

• Activity: Primarily soil-active; some foliar activity
• Movement in plants: Translocated in the water transport system
• Mode of action: Inhibits photosynthesis
• Selectivity: Broad spectrum control with some selectivity for conifers

Environmental Considerations

• Volatility: Minimal
• Photodecomposition: Slow; reported to be 10 percent per week
• Mobility in soil: Adsorbed by organic matter and clay; highly water-soluble with potential for leaching on sandy soils
• Half-life: 30 days
• Toxicity: LD50 - 1,690 mg/kg (moderately toxic)
  LC50 - 274 ppm (practically non-toxic)
  other - eye irritant

Imazapyr

• Activity: Foliar and soil
• Movement in plants: Translocated in food and water systems
• Mode of action: Inhibits the synthesis of specific amino acids
• Selectivity: Conifers generally resistant

Environmental Considerations

• Volatility: Negligible
• Photodecomposition: Can be significant
• Mobility in soil: Adsorbed by soil; leaching usually not a problem
• Half-life: 27 days
• Toxicity: LD50 - 5,000 mg/kg (slightly toxic)
  LC50 - 100 ppm (slightly toxic)

Metsulfuron

• Activity: Foliar and soil
• Movement in plants: Translocates in the food and water systems
• Mode of action: Inhibits cell division and amino acid synthesis
• Selectivity: Broad spectrum herbicide; some apparent conifer selectivity

Environmental Considerations

• Volatility: Negligible
• Photodecomposition: Negligible
• Mobility in soil: Low adsorption to clay but some adsorption to organic matter; solubility increases with increasing pH; some leaching potential but use rates very low
• Half-life: 42 days
• Toxicity: LD50 - 5,000 mg/kg (slightly toxic)
  LC50 - 150 ppm (practically non-toxic)

Sulfometuron

• Activity: Soil and foliar
• Movement in plants: Translocates in the food and water systems
• Mode of action: Stops cell division, particularly at growing tips
• Selectivity: Conifers and other woody perennials resistant

Environmental Considerations

• Volatility: Minimal
• Photodecomposition: Minimal
• Mobility in soil: Mobility increases with higher pH; adsorbed by soil organic matter
• Half-life: 10 days
• Toxicity: LD50 - 5,000 mg/kg (slightly toxic)
  LC50 - 12.5 ppm (slightly toxic)

Triclopyr

• Activity: Foliar
• Movement in plants: Translocated in the water and food systems
• Mode of action: Not known exactly, but similar to 2,4-D and picloram
• Selectivity: Most grasses tolerant

Environmental Considerations

• Volatility: Can be a problem with ester formulations
• Photodecomposition: Rapid
• Mobility in soil: Not readily leached
• Half-life: 46 days
• Toxicity: LD50 - 713 mg/kg (moderately toxic); LC50 - 117 ppm (practically non-toxic)

Last modified 2007-07-12