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. Certain considerations should be made in this process.

Toxicity of Herbicides

All pesticides are poisonous. However, some are more toxic than others. The toxicity of the pesticide is usually stated in the precaution on the label. For example, a skull and crossbones figure and the signal word “Danger” are always found on the label of highly toxic (Toxicity Class I) materials. Those of medium toxicity (Toxicity Class II) carry the signal word “Warning.” The least toxic materials (Toxicity Class III) have the signal word “Caution.” The toxicity of a pesticide is expressed in terms of oral and dermal LD50. LD50 is the dosage of poison that kills 50% of test animals (usually rats or rabbits) with a single application of the pure pesticide for a given weight of the animal (mg/kg of body weight).

The lower the LD50 value, the more toxic the material. Oral LD50 is the measure of the toxicity of pure pesticide when administered internally to test animals. Dermal LD50 is the measure of the toxicity of pure pesticide applied to the skin of test animals. Generally, an oral application is more toxic than a dermal one.

• For restrictions on rates, timing and crops for which the herbicide is approved, see Table 18.
• For degree of susceptibility of each weed to a specific herbicide, see Table 19.
• For limitations and special requirements of the herbicide, see reference no. 16 for more information.

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.
• Avoid skips and overlapping.
• Rinse spray equipment immediately after use. Use one sprayer for herbicides and another for insecticides and fungicides.

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.

Weed Sprayer Systems

• Select a sprayer and pump that can deliver a volume of 20 to 50 gallons per acre. Most herbicides are applied at rates of 20 to 40 gallons of water per acre. Pressures of 20 to 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 80° or 73° 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

Weeds that survive the application of a herbicide are likely to pass the trait that enabled them to survive on to their offspring. Repeat applications with the same type of herbicide will eventually remove almost all the susceptible individuals from a pest population and leave only those with the resistant gene. 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.

To preserve the useful life of a pesticide, it is necessary to practice resistance management. 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 17.

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: Resistance is not interchangeable between 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.