Herbicides

 How much training does it take to effectively teach someone the proper use of a hoe or how to use a pair of gloves, recognizing that there is a right and a left hand and there may be some confusion? If you make a mistake using a hoe or pair of gloves, the effects may be small, since few on us can pull up a tree or hedge or even hoe one out.
      Such is not the case when considering the use of an herbicide. If the applicator is not properly trained, the product will not be properly applied, the wrong application equipment may be used, the material may be applied at the wrong time or the control of the wrong weeds and when herbicide injury occurs, it is not temporary, and unlike you and your hoe, an herbicide can kill a mature tree. In dealing with the use of herbicides, discussions will be centered on: herbicide labels, classification of herbicides, and herbicides and the environment - plant and soils interactions.

     When considering the use of an herbicide, the most critical component is the ability to be able to read, understand, and follow the label. When looking at an herbicide label the homeowner should be able to identify the following pieces of information: Trade Name - name the manufacturer gives the product. Ingredients Statement: Active Ingredient - the name of the compound that is managing the weeds also referred to as the "common name" of the herbicide. Inert Ingredient - non-herbicidal compounds in the formulation.

      How much active ingredient is in a particular container in terms of lbs../gal or % of the products overall weight. Why is it important to know, it helps understand that all herbicides are not created equal, even when they have the same active ingredients because the amount of active ingredient found in the container will directly influence the amount of product to be added to the spray tank in order to achieve the desired active ingredient level in the sprayer. EPA Registration Number, Company who manufactured the product. Signal Toxicity Word - which will be one of the following:

Toxicity Word -- EPA Category -- Meaning

• "Danger" -- I -- Highly Toxic
• "Warning" -- II -- Moderate Toxicity
• "Caution" -- Ill -- Slightly Toxic
• "Caution" -- IV -- Very Low Toxicity

     Precautionary Statements which include: Statement of Practical Treatment - what to do if the herbicide gets in your eyes, is swallowed, is inhaled, or gets on your skin. Note to the physician - telling him what to do. Container Storage and Disposal - how to store the herbicide and what to do once the container is empty. Listing of weeds that will be controlled with the particular herbicide. Timing and Rates of application - this is critical, since some herbicides have to be applied before the weeds emerge while others can be applied following emergence. It is also against the law to apply an herbicide in any fashion that is not in harmony with the labeled directions. Turf and Ornamental Sensitivity - some herbicides are safe for certain turf and ornamental species while others may injure the turf or ornamental species in question.

     Associated with the storage and disposal statement is the question of how to dispose of unused herbicides. The situation has occurred where you mixed up a gallon of solution and only used one quart of the mixture. What are you going to do? The best way to deal with this situation is to not let it occur in the first place. When mixing herbicides, and any other pesticide, mix up only that which you will use. It will require more effort on your part as the actual area to be treated will have to be determined and from there the amount of herbicide to be used can be determined, mixed and applied with no unused product left. Remember - the best way to use unused herbicides is to not create unused herbicide mixtures, and should it happen, dispose of it according to the label.

When dealing with the classification of herbicides, the following terms are used to explain when and how the herbicide is to be used:

     The following table is prepared to show how some of the more common herbicides fit into the different classifications. It is also intended to point out that the different classifications are just that, different classifications and it is not possible to make generalities between the different groupings. For example, just because an herbicide is a "preemergence" it will not necessarily control both broadleaf and grassy weeds. Also, not all residual herbicides are preemergence, because some have both pre and post activity.

* The values here represent:

• "Yes - B&G" means yes selective, controls broadleaves and grasses "
• "Yes - B" means yes selective, control only broadleaves
• "Yes - G" means yes selective, controls only grasses

     In order for an herbicide to do it’s thing in the plant there are a couple of requirements which have to be met: The herbicide has to come in contact with the plant. This is not as critical with postemergence herbicides as with the preemergence herbicides. Preemergence herbicides are applied with the understanding that they will be moved into the soil prior to the germination of the weed species. Often times the material is applied and left alone on the top of the soil. Though the label may indicate that it will remain in it’s herbicidally active form for a couple of weeks, the weeds may not wait and may germinate before the active ingredient is moved into the zone of herbicidal activity.

     Once inside the plant, the herbicide has to make it to it’s site of herbicidal activity within the plant. It is this principle that is involved with some of the selectivity that is observed. There are several factors which are going to influence whether the herbicide comes in contact with the plant or makes it to it’s site of herbicidal activity within the plant. Some of the more common ones include:

Age of the plant - it is well understood that younger more actively growing weeds are more susceptible to control than are the established more mature plants. Associated with this growth factor is the fact that older leaves exhibit a reduced translocation rate which will decrease the movement from the site of herbicidal uptake to the site of herbicidal activity.

Shape of the leaf - broader leaves will affectively retain more herbicide than narrow leaves.

Surface makeup of the leaf - here we are concerned with the amount of cuticular waxes deposited on the leaf surface. It is this layer of waxes that keeps water in the plant and keeps the herbicide in the water carrier on the outside of the leaf, just wanting to get in. Varying level of these waxes will then influence the movement of the herbicide molecule through the leaf, with the thicker layer posing a greater barrier than the thinner layer. Also associated with the surface of the leaf is the presence, or absence, of hairs or trichomes. It is these structures, often, that keep the herbicide spray droplet from coming into direct contact with the leaf surface. As a result, there is a reduction in the amount of actual active ingredient which makes it into the plant.

Environmental Condition - the influence of the environment is obvious and considerable. It is the principle of stress placed on the plant, as a result of its environment, that is of interest here. Stress due to lack of moisture, or heat, or even physical stress results in a reduction in translocation or movement in the plant. The formation of a thicker cuticle, which makes it more difficult for the herbicide to get from the outside in, is also the result of stress placed on the plant. By understanding some of the factor involved with the interactions of herbicides and plants helps one to appreciate what it says on the label, "do not apply to stressed plants".

As we look to the interaction between herbicides and the soil, we first ask ourselves what are the major players. It becomes quite obvious there are two, the herbicide and the soil. With the herbicide there are four major factors involved with it’s behavior in the soil, solubility, adsorption, degradation or breakdown, and residual. It is the first two which are the most critical in relationship to the movement of herbicides through the soil profile.

Solubility: the solubility of an herbicide is based upon the amount of that particular material that will dissolve in water or the liquid phase of the soil. These will range from being practically insolubility, such as is the case with trifluralin, pendimethalin, or oryzalin, to those which readily dissolve in water, glyphosate, dicamba, and 2,4-D amine salts.
     As water moves downward through soil, it carries with it water-soluble compounds. This movement is referred to as "leaching." It is understood that the greater the water solubility, the greater the potential for movement through the soil with the liquid phase. In looking at the solubility values of these two examples, it is easily to predict which of the compounds will dissolve in water more readily. Glyphosate. 1,000,000 mg/I sol., Oryzalin 2.5 mg/I sol.

Absorption also referred to as "magnetic charge" is the next factor to consider in relationship to the herbicide and it’s relationship to soil movement. Along with its characteristic solubility, each herbicide has a characteristic charge associated with it. This charge can range from little or no charge to strongly magnetic. Herbicides or compounds with an overall positive charge will be tightly bound to the soil fraction, while those with an overall negative charge will be repelled from the soil fraction to varying degrees. Many soil factors influence pesticide adsorption. Soil texture is one of the major factors. Soils high in organic matter or clay are going to be more adsorptive than coarse, sandy soils. Other factors include pH, temperature, and water content of the soil.
     In reporting this characteristic, the term "Koc" is used to measure the degree to which a particular herbicide is bound either through chemical or physical bonds. When looking at the following examples, note that "Koc" values above 1,000 indicate that the herbicide is held quite tightly to the soil fraction. If an herbicide has a strong "Koc" value, what will that tell you about it’s movement in the soil? In looking at one examples we now have two pieces of information to report: Glyphosate: Solubility 1,000,000 mg/liter, Koc Value 10,000
     Using these two pieces of information what can be concluded regarding this particular herbicide ability to leach. From the solubility value, one would conclude it should leach, yet on the other hand, the Koc value is in the range which indicates a strong binding to the soil phase. What we see here is opposing factors, and one has to over ride the other. In this case it is the adsorption or binding that over rides the solubility. This results in the fact that glyphosate will not leach through the soil, why, because it is held to the soil very tightly, so tightly that it can not be moved through the soil with the soil water. This is understood, because when this herbicide is used, we find there is not soil activity. This is a postemergence systemic herbicide which offers no residual activity.
     As important as solubility potential of a particular herbicide may be, it is found that the magnetic charge, as it relates to the soil binding of the herbicide, is a more important factor to consider when discussing the movement of herbicides in the soil.

Degradation or Breakdown is the process whereby reactions take place to render the active ingredient inactive. The following ways are some of the different ways herbicides can be degraded:

Microbial degradation of herbicides is the breakdown by fungi, bacteria, and other microorganisms which use the herbicide as a food source. Soil conditions such as, moisture, pH, aeration, temperature, and the amount of organic matter affect the rate of breakdown under the microbial method.

Chemical decomposition is the breakdown of the herbicide by non-living soil processes. This includes hydrolysis, dealkylation, substitution, and others. Temperature, moisture, pH, chemical and physical properties of the herbicide, and adsorption are all involved in the chemical degradation process.
     With these two breakdown systems there is a tight relation to the soil conditions. Those conditions which favor microbial decomposition, also favor chemical degradation. These include: High organic matter; Warm soil temperatures; Adequate moisture.
     Under conditions of low organic matter, hot and dry soils, which is the situation for a lot of our state, what does this tell us about the breakdown of herbicides under those conditions -is it slower.

Photodecomposition certain herbicides can be broken down by sunlight. This is done in a way in which the bonds between certain elements are excited to the point of breaking or shifting. When this occurs the molecule changes shape and is rendered inactive as an herbicide.

Persistence is the ability of an herbicide to resist breakdown. The rate of breakdown, which is persistence, is measured in terms of half-life of the active ingredient. The chemical half-life is the time required for half of the original herbicide application to be broken down into its metabolites.
     Some of the factors influencing persistence include: Herbicide Solubility; Herbicide Adsorptivity; Mechanism of Breakdown; Environmental Conditions; Soil Type; Organic Matter Content. In dealing with the soil aspects of this interaction between herbicides and the soil, we readily identify a couple of critical factors.

Soil Texture with this we are concerned with the different soil particles, sand, silt, and clay. The soil texture affects the movement of herbicides in several ways: Movement of water through the soil, with coarser soils having a more rapid downward movement. Binding or absorption coarser soils have fewer binding sites and can be saturated with the herbicide at a lower application rate than can the heavier soils. Degradation with the heavier soils having more organic matter, moisture, and ideal temperature range for the degradation to occur microbially and chemically. Organic matter content is the amount of organic matter in the soil. This value will greatly influence how well the soil can retain the herbicide. Increasing the soil’s organic matter content, through practices such as application of manure or plowing under cover crops, increases the soil’s ability to hold both water and dissolved herbicides in the zone of herbicidal update and degradation.
     We have covered the two major players involved in the process of pesticide movement. We have identified the different factors associated with the different processes. The status of herbicides in the soil is a complex situation. There are three major fates associated with the reaction of an herbicide and the soil, these include, adsorption, movement, and degradation. These are all interrelated and difficult to separate on their own, but it is necessary to have a general understanding of them in order to have a better feeling for what is going on in the soil once an herbicide comes in contact with the soil. By taking the time to discuss these different principles and factors influencing the activity of herbicides in the soil, it becomes apparent there are no simple statements which can be made regarding the status of herbicides in the soil without a good foundational background.