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. 

Herbicides can also be pre-emergent or post-emergent.

Preemergence Herbicides (PRE): These herbicides are applied prior to the emergence of weeds. Some need to be incorporated into the soil (pre-plant incorporated) and others need to remain on the soil surface to be effective (pre-soil-applied). 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. These herbicides prevent new weeds from establishing, but most will not impact weeds that are already emerged. 

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 (most dangerous) - 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 (least dangerous) - 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 an overview of herbicide use restrictions, safety concerns, and other information see Tables 28 and 29.
• For an overview of crops labeled for each herbicide, see Table 30.
• For an overview of weeds listed as controlled on each label, see Table 31.
• 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

Each herbicide has a mode of action – how and where the chemicals in an herbicide work to kill a target weed. However, through natural random genetic mutations, weeds can develop  resistance to a mode of action. Resistance in a weed makes it less sensitive to an herbicide. Since mutations that lead to resistance are genetic, they are inheritable. When an herbicide is used, weeds that are susceptible to an herbicide will be killed, but any with the genetic mutation that makes them less sensitive will be left behind. These then reproduce and pass on the genes that let them survive the herbicide exposure to their offspring. If the same herbicide is used repeatedly in the same field, over time the predominant genetic form of that weed in that field will be resistant to that herbicide. A higher rate of the same or a similar chemical from the same group usually will not control the weed and these weeds can become extremely difficult to manage.

There are many techniques that can help delay the onset of resistance.

• Integrate cultural, mechanical, and physical weed control techniques to reduce reliance on chemical tools.
• Scout fields so that you are aware of what is not controlled.
• Take steps to control escaped weeds, especially preventing seed set.
• 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, 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.

One of the most important ways to slow resistance development and extend the useful life of an effective product is to rotate the use of products with different modes of action. This can be done in several ways:

• Rotate the type of herbicide used on fields where the same crop is grown year after year.
• Rotate the crops grown on a field so that different herbicides are used on that field.

Even if herbicides have different active ingredients, they can share the same mode of action. It is very difficult to know which herbicides share a mode of action. To help growers effectively rotate herbicides, international groups have assigned group numbers to herbicides based on the mode of action. 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 herbicide categories. Herbicides with the same number have the same mode of action. Most labels now come with this group number assigned to them (some have not been updated to the HRAC/WSSA number, but will in the future).

When selecting herbicides to rotate for resistance management, use the group number as your guide and NOT the product name or active ingredient. These resistance groupings are listed in Table 28.

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.

The most common weed resistance issue that we have in New England is common lambsquarters that is resistant to atrazine.

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)

Herbicides and Plastics

With the increased use of plastic culture for the production of various vegetable crops, it is very important to keep in mind a few points for proper weed management. Here are some tips:

If soil is fumigated under plastic strips (e.g. Vapam), there is no need for an herbicide under the plastic. Herbicides can be used under the plastic but are often unnecessary under black plastic unless there is nutsedge or excessive weed pressure. If you use clear or white plastic, without soil fumigation, then you need an herbicide under the plastic mulch. To get the best results, take the following steps for herbicide application:

1. Prepare a pressed bed.
2. Apply the herbicide to the bed surface.
3. Activate the herbicide with rainfall or overhead irrigation (at least 1/2"); if the herbicide needs to be mechanically incorporated, do so and repress the bed.
4. Apply the plastic mulch.
5. If weeds are present between the plastic strips before planting, use a banded application of Gramoxone plus surfactant, or Roundup (if registered for stale bed use) to kill all existing vegetation. Depending on the level of weed vigor, a second application of Gramoxone plus surfactant may be needed before planting. Do not broadcast apply these herbicides to the surface of the mulch.
6. For weed control between the plastic strips, after planting, use a registered preemergence herbicide for that crop. Do not use nonregistered herbicides between strips as the crop roots will grow in that zone and, in many cases, the crop will be damaged or killed. Do not spray the surface of the plastic with any preemergence herbicides! During a rainfall the excess herbicide will wash into the holes where the crop was planted and will greatly concentrate the herbicide. Thus, the crop will be damaged or killed. When banded applications of herbicides are used, remember to adjust the rate of the herbicide downward to conform to the actual ground area being sprayed (with 30" beds and 30" areas between strips, only half the normal amount of herbicide would be required per acre since only half the amount of soil area per acre is actually being sprayed).

Notes:

• Herbicide use under row covers can be dangerous, some may volatilize and cause crop injury when used under plastic mulches or row covers and some are prohibited from use. Be sure to observe any cautions on the product label.
• Generally, ventilated covers are safer than solid covers or hot caps, from an herbicide injury standpoint. This is especially true with an herbicide that is moderately or highly volatile.
• Wherever possible, try it on a small scale before you apply it on a large scale.
• Use caution when spraying Gramoxone with a backpack or hand-held sprayer. Wear rubber boots, gloves and a mask or respirator. Avoid skin contact with the spray. If contact occurs, immediately rinse the area with water or rub soil on your skin. Soil will help soak up and bind the chemical, preventing its absorption into the skin. Spray when the wind is calm to avoid spray drift. Also, always use a surfactant and plenty of water with Gramoxone for good control.
• Vapam is not recommended for use in the spring if the plastic is to remain in place. Under the cool soil conditions at this time of year, the soil will not aerate properly and the crop will be damaged. In plastic culture, these materials are best used if the plastic is applied during the fall before planting. Be sure with all fumigants to space the injection shanks correctly. The label recommends a 6" spacing with Vapam.
• Backpack sprayers or modified boom sprayers are often used for herbicide applications in plastic culture systems. Be sure to calibrate them properly and maintain a constant pressure using a pressure gauge, especially on hand-pumped models. Maintaining a constant speed or walking pace is also necessary. Three miles per hour is a good speed (23 seconds per 100').