Contributed by Dr. Jimmie Yeiser, T.L.L. Temple Chair,
Arthur Temple College of Forestry, Stephen F. Austin State University
Adapted from a two-part series originally published in The Monitor, Fall 2000 and Spring 2001.

Herbicide use continues to be of major concern, primarily due to a lack of understanding about such chemicals. Many non-industrial forest landowners lose thousands of dollars annually because they do not regenerate their pine stands after harvest. As a result, stands produce only a portion of their potential because of poor stocking. With good site preparation, unwanted, small, and poor-quality trees are removed providing access for tree planting, pine seedling establishment and growth, and well-stocked stands. Site preparation with herbicides has demonstrated its usefulness to this work. Use is growing in popularity because herbicides, (1) reduce competitors above (crown and stem) and below the ground (roots), (2) are well-suited for use on small or large tracts, (3) cause minimal soil disturbance, and (4) offer selectivity for specific landowner objectives and competitors.

• Herbicide application
• Hardwood response to herbicide treatment
• Choosing an effective herbicide
• Herbicide performance
• Common competitors for plantation trees
• Types of herbicide treatments
• Common herbicides and their characteristics
• A sample herbicide "prescription"
• Herbicide retention
• Herbicides degradation
• Herbicide safety and toxicity
• Loblolly pine response to herbicide treatment
• In summary

Herbicide application

Four commonly used methods of herbicide application are tree injection, foliar spraying, soil application and basal bark application.

Tree injection is used to control unwanted hardwoods and pines at least 0.5" in dbh. A hatchet and squirt bottle are examples of low-cost tools for injection. The hatchet is used to make incisions in the bark equally spaced around the tree. The squirt bottle is used to apply herbicide into the incisions. This method is labor intensive and more often used on small tracts, mixed stands where individual stem selectivity is desired and in sensitive areas requiring precise delivery of herbicide inside a tree.

Foliar spraying is accomplished with a helicopter or ground applicator. This method is fast, relatively inexpensive and well suited for large tracts. Caution must be exercised to avoid drift.

Soil applications with aerial or ground applicators can achieve broadcast weed control; individual tree control can be achieved with a spotgun. Following application, soil-active herbicides move into the soil by rain and are absorbed by plant roots.

Low volume basal bark application is usually made to the lower 14 inches of tree stems 3-inches in dbh and smaller. An herbicide mixture is applied completely around the stem until bark is saturated but not to the point of runoff. The penetrant in the mixture carries the herbicide through the bark and to vascular tissues for translocation throughout the tree. Often herbicides are used in combination with fire and mechanical methods of site preparation, depending upon landowner objectives.

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Hardwood response to treatment with herbicides

Hardwood response to treatment varies with soil texture, site quality, season of the year, soil moisture at the time of treatment, and the chemical and treatment method used. Most hardwood species are more susceptible to herbicides in the spring, when vigor is high, but also can be treated at other times of the year. Species susceptibility to herbicide varies greatly which significantly influences the effectiveness and cost of the treatment.

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Choosing an effective herbicide

When selecting an herbicide consider: the crop species, planting method and date, management style (even-aged, all-aged, etc.), soil type, target species and size, terrain, available application equipment and personnel, site features and neighboring restrictions, herbicide effectiveness and cost, and environmental and regulatory restrictions.

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Herbicide performance

Always use a product according to the label. In most cases, the failure of an herbicide treatment has more to do with improper use than the failure of the product.

Increasing use rate may or may not improve herbicide performance. Competitors may be grouped as susceptible, marginally susceptible and resistant. Generally, susceptible species are commonly controlled by labeled rates. Increasing the rate further may slightly improve control of marginally susceptible species. Resistant species will not be controlled at any practical rate. Therefore, tank mixing and site-specific prescriptions often are recommended for control of diverse plant communities.

Formulation may influence herbicide performance. For example, triclopyr is formulated as Garlon 3A and Garlon 4. The ester formulation of Garlon 4 provides better penetration of waxy leaf surfaces than the amine formulation of Garlon 3A. At equal rates as foliar sprays, Garlon 4 commonly provides better control of waxy leaf species than Garlon 3A.

A nonionic surfactant is a common spray additive in forestry. Surfactants break surface tension on spray droplets to improve leaf coverage and adhere spray to leaves. Surfactants are most beneficial when herbicides are applied under less than ideal conditions such as drought. However, a surfactant does not substitute for proper application timing or plant vigor. Common surfactant rates are 0.25% to 0.5% by volume. Adding too much surfactant may be detrimental to competitor control. Always follow the herbicide label when using spray surfactants or other additives.

Effective timing varies with the herbicide and the target species. This is especially true for perennial plants and translocated herbicides, a scenario common to forestry. It is most economical to apply at the correct time, however, increasing the rate, adding an adjuvant or using a different application method can help compensate for less than ideal timing.

Soil type influences the performance of soil active herbicides. Clay content, organic matter and pH are the most important characteristics to monitor. Clay content and organic matter bind soil active herbicides to the colloid making it unavailable to the plant. Extremes in soil pH may alter the performance and use rate also.

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Common competitors for plantation trees

Herbaceous grasses and forbs - Grasses are serious competitors during the first, three-years of plantation development. Their aggressive, fibrous roots occupy the same zone as young seedlings and compete very effectively for water and nutrients. Broadleaf weeds are generally less competitive than grasses but can form a canopy over seedlings and compete for light in addition to water and nutrients.

Woody trees, shrubs and vines - Low density, hardwoods seedlings will compete with planted pines from age four onward. High density, hardwood seedlings will compete with planted pine seedlings from plantation establishment onward and both and will cause significant pine growth loss. Dense stands of shrubs like sumac (Rhus spp.) compare with grasses in their competitiveness with planted pine seedlings for site resources. Perennial vines may grow rapidly and disrupt pine seedling establishment and development and even inhibit thinning operations. Honeysuckle and kudzu are two examples requiring special treatment programs that last more than two years.

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Types of herbicide treatments

Site preparation treatments are applied in the absence of crop trees and provide favorable conditions for crop tree establishment (Table 1).

Release treatments free the established crop tree from surrounding competitors.

Higher rates and more herbicide options exist for site preparation and correspondingly product performance is generally more consistent than for the same product during release. Crop trees must tolerate release herbicide rates, which are commonly lower than those used during site preparation and generally less effective on hard-to-control competitors.

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Common herbicides and their characteristics

Each herbicide has unique characteristics. See Table 1 (common herbicides, use, plant uptake, mode of action and signal word), Table 2 (herbicide behavior in soil) and Table 3 (toxicology).

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A sample herbicide "prescription"

Prescriptions match herbicide characteristics, landowner objectives and site conditions for efficacious competitor control and for nontarget organisms safety.

Assume you wish to prepare your forested site with herbicides before planting loblolly pine seedlings.

1. Divide your property into two parts. One receives an aerial application of herbicide by helicopter or ground application by skidder to the foliage of unwanted trees or brush. On the second, a ground crew injects herbicide into unwanted trees and then burns the brush.
2. Following treatment, a neighbor alleges your treatment damaged off site, nontarget trees on his property.
   
3. Your dog drinks from a water puddle on the treated tract.

Consider point number one. Your objective-prepare the land for planting with loblolly pine seedlings-is the same for both areas. Site characteristics warrant different approaches to meeting your objective. Options in herbicide brand, rate and application method exist to meet your needs for both tracts. [back to list]

Next, consider point number two and the alleged drift. Each active ingredient has a mode of action inside the plant that creates unique plant symptoms. Therefore, plant damage can be matched with the symptoms of the active ingredient and identified. Furthermore, commercial applicators are required by law to maintain a log documenting dates, times, locations, brands and rates of application. An assessment of symptoms and applicator records may reveal the origin of the active ingredient. [back to list]

For point number three, your dog is safe. Drinking from the puddle is an example of acute exposure. To reach the LD50, a 75 lb dog would need to drink over 5 gallons of spray (assumes a labeled rate of Arsenal AC at 24 oz in 10 GPA total volume). More importantly, the group of herbicides called amino acid inhibitors (herbicides containing glyphosate, imazapyr, metsulfuron and sulfometuron - see Table 1) act through unique biochemical pathways that differ for specific organism groups to stop plant production of amino acids. Plants or animals lacking the path are not injured. In this example, Arsenal AC inhibits amino acids leucine, isoleucine and valine. If the dog in this example did drink over 5 gallons of spray containing Arsenal AC, it has a 50% chance of surviving (Table 3). The dog in this example is unharmed, because 5 gallons of spray is too much to drink and because the dog lacks the physiological path needed for imazapyr activity. Therefore, the ingested imazapyr was excreted through waste (amino acid inhibitors are not stored in animal tissues).

In summary, these three points help illustrate selectivity, symptomology, and biochemical specificity, three reasons why forest herbicides are thought to be very safe. Specific use recommendations such as application method, mixing, rate, timing, soil, crop tolerance, weed control, etc. will promote safety and are provided on product labels. Labels may be acquired from a distributor, company representative, consultant or the web at Crop Data Management Systems, Inc. (Acrobat Reader needed).

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Herbicide retention

When an herbicide is applied, several factors retain it on site. Plant uptake keeps it, at least temporarily, from being transported from the site. Some herbicide coheres to the leaf while some reaches the soil. Soil behavior is determined by the properties of the herbicide in addition to environmental factors-temperature and rainfall levels as well as soil properties-organic matter content, soil type, moisture and pH. In general, the greater the organic matter content of the soil and the smaller the particle size of the soil, the greater its potential to retain herbicide.

Herbicide volatility from soil particles and soil water into the air is negligible (Table 2). Thus, once herbicide reaches the soil, it remains in the water or binds to colloids. Some herbicides strongly cohere to soil (Table 2). For example, glyphosate is highly soluble and would be expected to readily move off site in water. It seldom does. Instead, it adsorbs to soil. With heavy rainfall, surface runoff, leaching of water through the soil profile and erosion can carry herbicides off site. Also, herbicides can wash off bare soil, foliage and stems.

Site characteristics--hydrology, soil type, vegetation, and topography, plus product solubility are among the factors determining how readily this movement occurs. Effective use of buffers can reduce the likelihood of movement off site. Some herbicides like Rodeo™ (glyphosate), require no buffer and can be applied over wetlands. Generally, highly water-soluble products move in greater quantities if they do not bind tightly to soil. For example, hexazinone is highly soluble but with low adsorption to soil (Table 2). The risk of hexazinone movement off site in water is higher than for glyphosate. But the mobility can be an advantage. For example, the mobility of hexazinone makes pre- and post-emergence soil applications viable options. On the other hand, the immobility of glyphosate renders pre-emergence and soil applications meaningless.

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Herbicides degradation

Herbicide degradation begins immediately upon its introduction into the environment. Naturally occurring agents immediate act to degrade it in three ways: (Table 2)

• Biological decomposition is degradation by a living organism, either plant or microorganism.
• Chemical decomposition is breakdown by chemical process in the absence of a living organism-oxidation (loss of electrons), reduction (addition of electrons), or hydrolysis (addition of water).
• Photodecomposition is breakdown by sunlight.

Speed of degradation varies with the product and the environment in which it exists and influences the chance of off-site movement. Speed is influenced by soil moisture, pH and temperature. Generally, persistence is shorter under warm-wet conditions than for cold-dry one. Though herbicide persistence offers increased control of target species, it can be a disadvantage. The longer a product is in the environment, the greater the chances of movement from the site (Table 2). Products commonly used today, all have a short (< 1 month) to medium (1-6 months) half-life and soil persistence (Table 2). These products normally decompose into nontoxic substances that further decompose into elements naturally occurring in nature.

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Herbicide safety and toxicity

Unfortunately, I hear herbicides commonly referred to as "poisons." The implication is that they pose as an extreme hazard to the environment, much in the way of long-lasting pesticides. General toxicity to fish and wildlife is expressed as LC50 or LD50.

LC50 is the lethal concentration of a chemical in air or water that kills 50% of the test organisms in specific conditions.

LD50 is the dose of a chemical that kills 50% of the organisms in specific test conditions. It is expressed in weight of the chemical per unit of body weight and the toxicant may be fed (oral LD50), applied to the skin (dermal LD50), or administered in the form of vapors (inhalation LD50).

LC50 levels have been established for fish and wildlife and examples are presented in Table 3. Values there, serve as a base for ecotoxicity. Acute oral LD50 values presented in Table 3 are commonly >5000 mg/kg. In comparison, the acute oral LD50 for aspirin is 1,240 mg/kg and for table salt is 3,320 mg/kg. This means that under test conditions, acute doses of aspirin or table salt are more lethal than acute doses of the forest herbicides called amino acid inhibitors.

Chronic ingestation of commonly used herbicides seems to pose limited health risk for malformations in the embryo or fetus, as reproductive toxins, or inducers of tumors (Table 3). These data do not justify unsafe use practices. Instead, they provide a minimum justification for continued product use in conjunction with accepted safety standards.

Parameters such as acute and chronic toxicity, combined with product solubility, persistence (Half-Life) and agents of degradation can be used to indicate potential hazards, and when combined with common sense, help users apply herbicides safely. Users and workers need a simple, prominently displayed assessment of overall herbicide safety.

The EPA assesses each product for safety and uses a one-word system to rate herbicide safety. The signal word - Caution, Warning and Danger - is the EPA's rating of overall product safety and it is clearly displayed on the product label. "Caution" is the lowest risk to health while "Danger" is the highest risk. Additional safety information is provided on product material safety data sheets (MSDS). Copies of MSDS may be acquired from a distributor, company representative, consultant or the web at Crop Data Management Systems, Inc. (Acrobat Reader needed). Additional information on toxicology and the fate of herbicides in the environment and in plants can be found in Ahrens (1994).

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Loblolly pine response to herbicide treatment

Herbaceous grasses and broadleaf weeds are major competitors of pine seedlings for water, light, nutrients and space. Prior to age four, levels of herbaceous competition are especially critical and may more negatively impact seedling performance than hardwood competitors. When competing herbs are controlled, especially for sites near the western border of the loblolly pine distribution, the survival and growth of newly planted seedlings is increased.

For example, in a summary of 28 studies in Arkansas and southeastern Oklahoma spanning 13 years and containing plots without and with herbaceous weed control at planting, 67% of the plantings had a significant increase in age-one seedling survival. Herbaceous weed control increased survival by an average of 15% or 96 more seedlings per acre. Planting failure due to drought occurred at 14% of the plantings and abundant rainfall at 18% of the studies resulted in no survival response to weed control. When failure occurred, herbaceous weed control reduced the needed replant by 110 seedlings per acre. In all 28 studies, herbaceous weed control increased growth in height (30%), ground line diameter (62%) and volume (236%). By rotation end, seedlings without herbaceous weed control need two additional years to reach the yield level of seedlings receiving herbaceous weed control at planting.

Herbicides commonly used at planting to control herbaceous competitors include Arsenal™+Oust™ and Velpar™+Oust™. One common use rate for Arsenal™+Oust™ is 4oz+2oz per treated acre and for Velpar L™+Oust™ is 1 qt+2oz per treated acre. Additional rate and product options exist. Users should refer to product labels, or contact consultants, company representatives and distributors for additional options.

Mid-rotation vegetation control involves removing unwanted woody and herbaceous competitors of established crop trees. The effectiveness of herbicide treatment is affected by season of year, vigor and size of tree, species and weather conditions.

Loblolly pine growth response increases (1) as the residual level of vegetation decreases, (2) with time, and (3) in magnitude as the initial level of vegetation increases. If hardwood basal area equals or exceeds 20% of the total basal area, weed control is recommended. A maximum response to mid-rotation weed control is probably 0.5 cords acre-1 year-1 (15%). This response has been shown to last at least eight years. Arsenal AC™+Accord™ (14oz+1.5qt) is an example of one of many herbicide options applied over-the-top for mid-rotation weed control. Herbicide selection should be prescribed for specific site conditions. Numerous rate, product and application options exist. Users should refer to product labels, or contact consultants, company representatives and distributors for additional options and information.

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In Summary

In summary, herbicides are used commonly for practical and economical vegetation management in loblolly pine plantations. Wise and safe herbicide use includes comprehension of local regulations and features of the site in addition to the use label for rates, application methods and timings. Once applied, adhesion to plants and soil keeps herbicide on site. Naturally occurring environmental factors promptly initiate product degradation into fundamental elements. To minimize risk from nontarget exposure, herbicide properties, such as mode of action in plants, persistence and behavior in soil and water, and toxicity for selected organisms in specific use areas, are established. For Texas, herbaceous weed control following planting enhances year-one, loblolly pine seedling survival (15%) and seedling volume (236%) and reduces rotation length by two years. If 20% or more of the mid-rotation basal area is hardwood, pine crop trees should be released. Mid-rotation weed control can increase growth up to 0.5 cord acre-1 year-1 and the response last for at least eight years.

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Tables

Table 1. Herbicides commonly used for woody plant control during
site preparation (SP) and release (R), or for herbaceous weed control (H).

Table 2. Herbicide¹ behavior in the soil.

Table 3. Toxicology¹(LC₅₀LD₅₀ of herbicides commonly used for woody plant control
during site preparation and release, or for herbaceous weed control.

¹Ahrens,W.H. (ed). 1994. Herbicide Handbook. Seventh Edition. Weed Science Society of America, Champaign, IL. 352 pp.
²For Daphnia, a 48-hour test; for Bobwhite quail, an 8-day dietary test; for Bluegill sunfish, a 96-hour test of technical grade.
³Oral toxicity tests were conducted with the technical grade.
⁴No effect level.
⁵Subchronic toxicity test (90-day dietary test). Does not cause malformations in the embryo or fetus (teratongenic).
⁶Subchronic toxicity test (90-day dietary test). Are not reproductive toxins.
⁷Chronic toxicity (24-month dietary test). Does not give rise to tumors (oncogenic).

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