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 weed species or families. Therefore, knowledge of the weed species present in the field is essential for good weed control. Once the weeds are identified, select the proper herbicide.

Herbicides can be classified several different ways and understanding what the classifications mean can help you better select the most appropriate herbicide to use for different situations.

Herbicides can be pre-emergent or post-emergent.

• Preemergence herbicides (PRE) are applied prior to the emergence of weeds. Some need to be incorporated into the soil with tillage (pre-plant incorporated) and others are applied to the soil surface and are sometimes incorporated with irrigation or rainfall (pre-soil-applied). In general, they prevent new weeds from emerging and will not control emerged weeds, however each product is different. Some pre-emergent herbicides will control small weeds that have recently emerged and some have some post-emergence activity on certain species of weeds. Often, application rates of pre-emergent herbicides are dependent on soil texture and organic matter content, with higher rates required for finer soil with higher organic matter content. These products will provide residual control, meaning they will act to kill emerging weeds in the soil for a length of time after application. They also usually require water to activate the herbicide and get it to the germinating weed seeds.
• Postemergence herbicides (POST) work on actively growing weeds. They are usually absorbed by the leaves of the weeds and therefore they need to remain on the leaf long enough to act (so don't apply them in the rain). They need to be applied at certain growth states of the weeds, so pay attention to any height requirements mentioned in the label.

Herbicides can be systemic or contact.

• Systemic herbicides are absorbed by the plant and move through the plant to the site of action, where the herbicide kills the plant.
• Contact herbicides only affect the plant tissue they come into contact with. It is especially important to achieve good coverage of weeds when using these herbicides.

Herbicides can be selective or non-selective.

• Selective herbicides only work to kill certain species or types of plants. Selectivity is usually due to the physiology of the plant and the rate at which an herbicide is applied.
• Non-selective herbicides kill or injure any plant they come into contact with (as long as the weed has not developed resistance to the herbicide). These types of herbicides are used as burn-down products in situations where it is desirable for all living plant material to be killed.

These different classifications are usually well-defined, but not necessarily always exclusive (e.g., a pre-emergent herbicide can also have some post-emergent activity). The different types of classifications interact together. For example, 2,4-D is a postemergent, systemic, selective herbicide meaning that it is active against actively growing weeds, is absorbed by the plant and travels to meristems, causing death by uncontrolled growth, and only affects broadleaf plants (not grasses). Paraquat is another postemergent herbicide, but it is a contact and non-selective herbicide meaning that it is applied to actively growing weeds and kills all plant tissue the spray touches but does not harm tissue that is not sprayed.

Adjuvants are added to a pesticide mixture to improve its effectiveness. They include water conditioners, surfactants, stickers, penetrants, and compatibility agents. 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 use 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. Also be sure the pH of your application water is not too basic and add a water conditioner if need be; ammonium sulfate (AMS) is a common recommendation. These are especially important for systemic herbicides.

Herbicides and Crop Rotation Restrictions

Some herbicides can have long-lasting (residual) activity. Many herbicides have crop rotation restrictions where they have been applied, which can be a year or more, which can complicate crop rotation planning. Check the label of each product for details.

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

Pre-emergent herbicides must usually 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 degrade when exposed to sunlight. 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 psi 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. See Pesticide Equipment Calibration.

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

There are many techniques that can help prevent or 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 uncontrolled weed species.
• 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 prevents 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. 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 HRAC number have the same mode of action. Most labels now come with this group number assigned to them (some have not been updated to include the HRAC number, but will in the future).

When selecting herbicides to rotate between 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 31.

NOTE: The group number is specific for each type of pesticide (insecticides, herbicides, and fungicides). For example, there is no problem with using material from the herbicide Group 1 with 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. Check with your state for restrictions on their use in sensitive areas. For example, Massachusetts has Zone II designations near groundwater wells and regulates the use of these products in those areas:

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

Herbicides and Plastic Mulches

With the increased use of plasticulture for the production of various vegetable crops, it is very important to keep in mind a few points for proper weed management. 

If soil is fumigated (e.g. with Vapam) under plastic beds, there is no need for an herbicide under the plastic. Herbicides can be used under black plastic but are often unnecessary 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 at least 1/2" of rainfall or overhead irrigation; 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 beds 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 beds after planting, use a registered preemergence herbicide for that crop. Do not use nonregistered herbicides between beds 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, damaging or killing the crop. 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 (for example, 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, as some may volatilize and cause crop injury. Some herbicides are prohibited from use under row covers. 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.
• Vapam is not recommended for use in the spring if the plastic is to remain in place. Under cool, spring soil conditions, the soil will not aerate properly and the crop will be damaged. In plasticulture, Vapam is best used if the plastic is applied the fall before planting. With all fumigants, be sure 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 plasticulture 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').