About Herbicides

Herbicides are chemicals designed to control weeds. The use of these materials must be exact for satisfactory results. Proper rate selection, timing of application, activation, and observance of all precautions on the label must be followed to obtain optimum performance. Each herbicide controls certain weeds or families of weeds. Therefore, knowledge of the type of weed species present in the field is essential for good weed control. Once the weed problem is known, select the proper herbicide.

Herbicides can be systemic, meaning that they are absorbed and moved throughout the plant, while others are contact herbicides, meaning that they only affect the plant tissue they come into contact with. 

Preemergence Herbicides (PRE): These herbicides are applied prior to the emergence of weeds.  In general, they work by preventing weeds that are germinating from seeds from growing, but some can also act on perennial plants that spread by rhizomes, tubers, and stolons.  Application is timed to coincide with when the target weeds are about to begin germinating.  These herbicides prevent new weeds from establishing, but most will not impact weeds that are already emerged. 

Preplant: These herbicides are applied before the crop is planted.  They can be preemergence products that are incorporated into the soil or applied to control existing vegetation. 

Postemergence Herbicides (POST): These herbicides work on actively growing weeds.  They can be absorbed by the leaves or roots of the plants, depending on the chemical. 

Adjuvants: These products are added to a pesticide mixture to improve its effectiveness.  They include surfactants, stickers, penetrants, compatibility agents, etc.  Pesticide labels may list specific types of adjuvants that will maximize effectiveness of the pesticide.  Be sure to use the proper category of adjuvant if the manufacturer makes a specific recommendation.

When adjuvants are recommended, it is because research has shown that their addition increases efficacy of the herbicide.  If the label advocates the usage of an adjuvant, do not omit the adjuvant solely to save money.  The most common adjuvants used with herbicides are nonionic surfactants (NIS) and crop oil concentrates (COC) which can consist of petroleum, vegetable, or methylated vegetable or seed oils. They increase penetration of the herbicide through the leaf cuticle.

Herbicides and Crop Rotation Restrictions

Some herbicides can have long-lasting activity.  Many herbicides have crop rotation restrictions where they have been applied.  For example, Devrinol has a 60-day plantback interval for leafy greens. Check the label of each product for details.

Toxicity of Herbicides

The toxicity of pesticides varies by the active ingredient, concentration of active ingredient, and the formulation of the product (e.g. liquid, powder, etc.).  The toxicity of a pesticide is expressed in terms of oral (administered internally) and dermal (applied to the skin) LD50. LD50 is the dosage of poison that kills 50% of test animals (usually rats) with a single application of the pesticide product and is expressed as mg/kg of body weight. The lower the LD50 value, the more toxic the material.

The acute (short-term) toxicity of the formulated product is conveyed on label by a “signal word” stated on the front page. 

DANGER - pesticide product is highly toxic by at least one route of exposure. It may be corrosive and cause irreversible damage to the skin or eyes.  If the product is highly toxic if eaten, absorbed through the skin, or inhaled, then the word “POISON” must also be included in red letters.

WARNING - pesticide product is moderately toxic if eaten, absorbed through the skin, inhaled, or it causes moderate eye or skin irritation.

CAUTION - pesticide product is slightly toxic if eaten, absorbed through the skin, inhaled, or it causes slight eye or skin irritation.

General Principles for Safe Use

  • Know the herbicide. Read the label.
  • Check the output of sprayer frequently.
  • Replace worn nozzles. It may be necessary to replace them several times a season if the sprayer is used constantly.
  • Rinse spray equipment immediately after use. Use one sprayer for herbicides and another for insecticides and fungicides.
  • For restrictions on rates, timing, and crops for which the herbicide is approved, see Table 27.
  • For degree of susceptibility of each weed to a specific herbicide, see Table 28.
  • For limitations and special requirements of the herbicide, refer to the product label (or supplemental label).

Rate Selection

Always check the label to determine the proper rate to apply. For most soil-applied herbicides, knowledge of the type of soil and the percentage organic matter usually determines the rate. Generally, the more clay and/or organic matter present in the soil, the higher the herbicide rate necessary for good weed control. For postemergence herbicides, the type of weed, as well as its size, will usually determine the rate.

Incorporation of Herbicides

Some herbicides must be incorporated into the soil to be effective. Herbicides are incorporated because they are volatile and evaporate into the air if left on the soil surface or they will decompose when exposed to sunlight. Herbicides differ in their incorporation requirements; check the product label for the manufacturer's requirements.

Herbicide Sprayer Systems

  • Select a sprayer and pump that can deliver a volume of 20-50 gallons per acre. Most herbicides are applied at rates of 20-40 gallons of water per acre. Pressures of 20-40 p.s.i. at the nozzle are recommended for most herbicides. Higher pressures result in finer droplets and increase the chance for more drift. Lower pressures sometimes cause uneven spray patterns.
  • Use 50-mesh screened filters for nozzles and suction lines.
  • Select 80º-73º flat fan nozzles. Because of wear, brass tips used exclusively for applying wettable powders should not be used on more than 30 acres before being replaced. Use stainless steel or hardened stainless steel tips for longer wear. Stainless steel nozzle tips are more than twice the cost of brass tips but last about 20 times longer. Hardened stainless steel tips are only slightly more expensive than stainless steel tips but last three times longer. Ceramic nozzles are the most durable.
  • Calibrate sprayers frequently and check for wear, especially when wettable powders have been used.

Resistance Management

Pesticide resistance is an inheritable (genetic) characteristic of a pest that makes it less sensitive to a pesticide.  Repeated use of the same pesticide (or pesticides with the same mode of action) over time kills pests that are susceptible to the pesticide and leaves behind individuals that are less sensitive. These then reproduce and pass on the genes that let them survive pesticide exposure to their offspring. With herbicides, this usually takes several years to accomplish. This is because of the seed bank in the soil that preserves individuals that are not resistant for many years.

International groups have been founded for a cooperative approach to resistance management. They have assigned group numbers to pesticides to help growers make decisions on how to rotate pesticides. They are based on mode of action –how and where the chemicals in the pesticide work on the target. The Herbicide Resistance Action Committee (HRAC) and The Weed Science Society of America (WSSA) have developed a harmonized classification system of herbicides using numbers to designate the categories. A key step in resistance management is to minimize the continuous use of herbicides with the same mode of action through rotations and combinations of products. One of the purposes of these classification systems is to make it easier for farmers and farm advisors to understand which herbicides share the same mode of action without having to actually know the biochemical basis.

Most labels now come with a group number assigned to them. Some active ingredients are available under several different product names, and sometimes different active ingredients have the same mode of action.

Resistance management may include alternating or sequencing products with different modes of action or limiting the total number of applications per season. When selecting herbicides for resistance management, use the group number as your guide and NOT the product name or active ingredient.

The most effective way to extend the useful life of an effective product is to rotate herbicides within fields if the same crop is grown or to rotate crops so that different herbicides might be used in following years. The most common weed resistance issue that we have in New England is common lambsquarters that is resistant to atrazine. To help select pesticides with a different mode of action, see chemical resistance groupings in Table 27.

Once a weed develops resistance to a group of pesticides with a particular mode of action, a higher rate of the same or a similar chemical from the same group usually will not control the weed.

There are many other techniques that can help delay the onset of resistance. Using other options and recognizing weeds that may be resistant is critical.

  • Integrate chemical control with effective cultural, mechanical, and physical options.
  • Scout fields so that you are aware of what is not controlled and can take steps to control the escaped weeds.
  • Good rate selection, spray coverage, and herbicide activation helps do the job right the first time and avoids unnecessary repeat applications: use the proper size nozzles and the correct angle or orientation and the right amount of water per acre.
  • Time postemergence applications so that the weeds are at the right stage of growth.

NOTE: The group number is specific for each type of pesticide (insecticides, herbicides, and fungicides). For example, there is no problem when using material from the herbicide Group 1 and an insecticide or fungicide from Group 1.

Groundwater Concerns

The following herbicides have the potential to impact groundwater due to their chemical characteristics and toxicological profile and have been discussed in the crops sections. Check with your state for restrictions on their use in sensitive areas.  For example, Massachusetts has Zone II designations and regulations pertaining to the use of these herbicides in those areas.

  • Atrazine (Aatrex)
  • Bentazon (Basagran)
  • Chlorthal-Dimethyl (Dacthal, DCPA)
  • Dimethanamid (Outlook)
  • Diuron (Karmex)
  • Fluthiacet-methyl (Cadet)
  • Metolachlor (Dual Magnum)
  • Metribuzin
  • Pronamide (Kerb)
  • Simazine (Princep)