By: Paul A. Baumann, Peter A. Dotray and Eric P. Prostko
Introduction
Whether you are producing agricultural crops or tending a lawn or home garden, weed control will be important to your success. Weeds can be controlled mechanically, culturally, biologically, and chemically, and all these methods may be important in an integrated weed control program that is economical and friendly to the environment.
Chemical control with herbicides has been an important tool for managing weeds in crops and home landscapes for many years. Many of today’s herbicides are more effective and selective. These traits make them less harmful to the environment when they are used properly. Although herbicides are widely used, few people understand how they work to control undesirable plants.
Herbicide application
Generally speaking, herbicides are applied either preemergence or postemergence. That means they are applied either before or after weeds emerge from the soil and begin to grow. Preemergence herbicides kill weeds shortly after they germinate or emerge through the soil surface. Postemergence herbicides control weeds that are already growing and easily visible.
Some herbicides are applied to the soil and are taken up by seedling plant roots or shoots. They are said to have soil activity. Herbicides that are applied to plant foliage have foliar activity. Some herbicides have both. Herbicides with a high degree of soil activity usually are applied preemergence.
Selectivity
Selectivity is the process by which a herbicide controls or kills certain plants but leaves others unharmed. Selectivity may be as simple as controlling broadleaf plants but not grass plants. Many new herbicides have more sophisticated selectivity that differentiates between several broadleaf and/or several grass plants.
Herbicides with no selectivity, such as Roundup Ultra®, are called nonselective. These products kill all types of plants. Selectivity usually depends on the time or placement of the herbicide applied. Most herbicides can be harmful, even to normally tolerant plants, if the dose is too high.
Translocation
Some herbicides move (translocate) within the plant. Systemic herbicides translocate once they are taken up by the leaves, stems or roots. Herbicides that do not move after they enter the plant are called contact herbicides. Some products can be either contact or systemic herbicides, depending on the way they are applied.
Mode of Action
Mode of action refers to the effect a herbicide has on a plant. Herbicides work in many different ways. If we understand a herbicide’s mode of action, we will know what symptoms it produces at lethal or sublethal doses.
Other problems such as disease, nutrient deficiency, and insect damage may mimic the effects of herbicides. These other possibilities must be ruled out before herbicide injury is diagnosed. The following mode of action categories cover most of the herbicides used in Texas.
Growth Regulator Herbicides
These herbicides are widely used to control broadleaf weeds in grass crops such as wheat, corn, sorghum, forages, and turf grasses. One member of this group, 2, 4-D, was one of the first selective herbicides developed. Growth regulator herbicides upset the normal hormonal balance that regulates processes such as cell division, cell enlargement, protein synthesis, and respiration. That is why this group of herbicides is sometimes called the “hormone herbicides.” These herbicides are very versatile for weed control. They usually are applied to the foliage, but are also effective in the soil. Any herbicide that falls on the soil instead of the foliage can be percolated into the soil with rain or irrigation and will be taken up by weed roots.
Herbicides in this category
Phenoxy growth regulator herbicides have the least plant activity and soil residual activity; the carboxylic acids generally have the most. Broadleaf crops and turf grasses should not be planted into soils recently treated with these herbicides because they severely inhibit seedling emergence.
Symptoms
The most common symptoms for these herbicides are leaf and stem malformations. In broadleaf plants stems curl, twist, and droop, while leaves are cupped, crinkled, or have a “drawstring” appearance caused by irregular growth at the leaf edges. In cotton plants, points develop on leaf edges. Overapplication or application at the wrong stage of development in corn can cause leaf rolling and crinkling, brace root malformation, sterile flowers, and missing grain (blasting). Blasting and malformed seed heads are common symptoms of ill-timed applications in wheat.
Special considerations
Vapor from these products can easily drift to desirable plants, so they must be applied carefully. Equipment should be cleaned according to label instructions before it is used to treat susceptible crops with other herbicides.
Photosynthesis Inhibitors
Herbicides in this category inhibit photosynthesis, the process by which all green plants convert light energy from the sun into sugars (food). Photosynthesis inhibitors are broadleaf herbicides, but also control annual grasses to some extent.
Herbicides such as Buctril®, Basagran® and Tough® are used as foliar, selective, postemergence products. Others such as atrazine, Bladex®, Caparol®, or Cotoran® are generally used as preemergence herbicides, but are sometimes used postemergence. Their selectivity when applied postemergence depends on the crop and application method.
Herbicides in this category
Several herbicides in this category are critical to cotton, corn, and rice production in Texas.
Symptoms
Symptoms depend on the product’s mobility within the plant. Herbicides in the triazine, triazone, uracil, and urea families move within the plant and exhibit these symptoms in older leaves first: yellowing between the leaf veins or in the veins, and yellowing of the leaf margins that eventually turn brown and die. Herbicides in the benzothiadiazole, nitrile, phenyl-pyridazine, and amide families are not mobile within the plant and affect only treated leaves. Symptoms include speckling, spotting, and yellowing or bronzing that may kill affected tissue.
Special Considerations
Soil pH higher than 7.2 can make injury from the triazine and triazone families more severe when used preemergence.
Pigment Inhibitors
Herbicides classified as pigment inhibitors destroy the green pigment (chlorophyll) in leaf tissue. Chlorophyll is necessary for photosynthesis; without it, plants die. These herbicides are often described as “bleaching herbicides” because they cause new leaves to appear yellow or white.
These herbicides are absorbed by roots and translocate to the shoot tissue where they inhibit the production of carotenoids—substances that protect the chlorophyll molecules that make plants green. Without carotenoids, chlorophyll is destroyed. These herbicides do not destroy carotenoids already formed, but prevent the formation of new ones.
Herbicides in this category
There are three families of herbicides that bleach plant tissue.
Symptoms
Injured leaves turn yellow or white, then often translucent. New growth is yellow to white with sometimes a hint of purple or pink. These symptoms can be found on cotyledons to the newest leaves of susceptible plants. Zorial® initially causes bleaching within veins; Command® initially causes bleaching between veins.
Special Considerations
In order to use Command® in cotton, an organophosphate insecticide (Thimet® or DiSyston®) must be used in-furrow first. If the insecticide is placed incorrectly or applied at the wrong rate, cotton may be injured. Some formulations of Command® are volatile and should be used with care. Consult the label for further precautions.
Seedling Growth Inhibitors
Some herbicides act on seedling weeds shortly after they germinate and before they emerge. These herbicides work beneath the soil so their effects are seldom seen. If overapplied, however, they may inhibit growth of weed or crop seedlings that do emerge through the soil surface. These herbicides can be divided into two groups—root inhibitors and shoot inhibitors.
Root inhibitors. These herbicides interrupt cell division, which stops root growth in seedling weeds. Plants die because they cannot take up enough water and nutrients to sustain growth. The root inhibitors are most effective on small-seeded broadleaf and grass weeds. Large-seeded weeds and crops generally survive normal dosages because their roots and shoots grow through the herbicide treated zone in the soil.
Shoot inhibitors. The seedling shoot growth inhibitors also act on newly germinated weed seeds. They are absorbed by the seedling shoots of grasses and roots of broadleaf plants, and they disrupt cell growth. They are most effective at controlling small-seeded grass and broadleaf weeds. Large-seeded crops and weeds are not usually affected. Once tolerant or susceptible plants emerge they can generally overcome the effects of the herbicide.
Symptoms
Root inhibitors. Injury to tolerant plants is caused by root damage. Grass crops may be stunted and have a purple discoloration because roots cannot take up enough phosphorus. Root systems appear stubby and thick, especially the lateral roots. Broadleaf plants may have swollen and cracked hypocotyls. If these herbicides are incorporated shallowly or applied preemergence, they sometimes cause callus tissue (tumors) to form on the plant stem at the soil surface. This weakens the stem and causes lodging. Dinitroaniline herbicides applied postemergence to broadleaf crops may cause stunting.
Shoot inhibitors. Symptoms caused by the shoot inhibitors are much different than those of the root inhibitors. Overapplication or extended periods of cool, wet weather shortly after planting may sometimes cause injury to tolerant crops such as corn or sorghum. Symptoms include leafing out underground, improper leaf unfurling (shepherd’s crooking), buggy whipping (tightly rolled leaves), and leaf crinkling. In broadleaf plants, the center vein (midrib) may draw in the leaf edge in a drawstring effect. Leaf puckering is also a symptom on broadleaf plants. There may also be stunting that persists until the soil warms enough to promote plant growth.
Herbicides in this category
These products are widely used in Texas row crops, turfgrass, and horticultural crops.
Special Considerations
The growth inhibitor herbicides have no postemergence activity; therefore, the timing of application is critical. Dinitroanilines have various requirements for incorporation into the soil. Consult the individual product label for specific recommendations. Grain sorghum treated with Concep® seed safener is tolerant to the acetamide herbicides.
Cell Membrane Disruptors and Organic Arsenicals
The cell membrane disruptor postemergence herbicides control both grasses and broadleaf weeds by destroying cell membranes and causing rapid desiccation of the plant. There are two types of cell membrane disruptor herbicides: the bipyridylium and the diphenylethers. The bipyridylium herbicides require thorough plant coverage to be effective, and they have no soil activity. The diphenylether herbicides act in a similar way but more slowly. Some of them are more selective between crops and weeds. The herbicides Goal and Reflex have significant soil activity.
The organic arsenical herbicides DSMA and MSMA are often called contact herbicides, as are the cell membrane disruptors. However, their true mode of action is unknown. They are used to selectively control wide-leaved grasses such as crabgrass or dallisgrass in narrow-leaved grasses such as bermudagrass lawns. They are also very effective on cocklebur and common ragweed. These herbicides bind tightly to soil clay and organic matter, so they have no residual, preemergence activity.
Herbicides in this category
Symptoms
Plants rapidly turn yellow or pale and may look water soaked; then they dry up. The effects of the bipyridylium herbicides are rapid. Even small droplets that drift to nontarget vegetation cause specks of burned tissue. Roots of perennial weeds are seldom killed because these herbicides do not usually translocate to the roots.
The organic arsenicals accumulate in root and leaf tips and symptoms are first seen on leaf tips. They rapidly kill leaf and stem tissue. MSMA and DSMA are more effective on grass weeds than on broadleaf weeds, except for common ragweed and cocklebur. When applied over cotton to control grasses or cocklebur, they sometimes cause speckled leaf burn and red stems on the cotton plants; however, this has little effect on overall growth.
Special Considerations
Be careful to prevent drift during application so that non target plants are not harmed. Applying systemic herbicides shortly after cell membrane disruptors or organic arsenicals is not advised. Paraquat and diquat are generally considered to be nonselective and harmful to both grass and broadleaf vegetation. In peanuts, however, some selectivity can be achieved by using paraquat at the cracking stage. Another bipyridylium herbicide called Avenge® is used in wheat and barley for selective postemergence control of wild oat.
Lipid Synthesis Inhibitors
Lipid synthesis inhibitors are unique because they act only on annual and perennial grasses, not on broadleaf plants. With the exception of diclofop, these herbicides are applied postemergence and have little or no soil activity. Crop oil concentrate or some other type of adjuvant must be used to increase herbicide uptake into the leaf. To be most effective, these herbicides should be applied to actively growing grass weeds. If grass weeds are stressed and slow growing, these herbicides will be less effective.
These herbicides disrupt lipid biosynthesis in grass plants. All plants contain lipids, which are fatty acids essential for plants to function normally. Plant cells contain lipid membranes. Membranes help the plant cell regulate what moves in, what moves out, and what remains out. Because these herbicides prevent the plant from producing fatty acids, membranes cannot form. Leaves absorb these herbicides quickly and within an hour they can not be removed by rain.
Herbicides in this category
There are two families of herbicides that disrupt lipid biosynthesis.
Symptoms
Symptoms develop slowly on grass plants and may not appear for 7 to 14 days. Initial injury is seen where the newest leaves are developing. These regions usually turn pale or yellow and then die. The area at the base of new leaves quickly becomes mushy, has a rotted appearance, and new leaves in the affected area can be pulled easily from the rest of the plant. Reddish blue pigmentation may also be observed on the stem sheath, leaf margins, and/or leaf blade.
Amino Acid Synthesis Inhibitors
This new category of herbicides can be used at extremely low rates, controls both grasses and broadleaf plants, has soil and foliar activity, and is essentially non toxic to mammals and most non vegetative life forms.
Amino acid synthesis inhibitors bind to a specific enzyme and prevent the development of amino acids essential to plant life. The enzyme to which they bind is abbreviated ALS or AHAS, so these herbicides are often called the AHAS/ALS herbicides.
Herbicides in this category
Symptoms
When these herbicides are applied preemergence, symptoms do not usually appear until the plants have emerged from the soil. Symptoms for grasses include stunting, purple coloration, and root systems that develop a “bottlebrush” appearance. On broadleaf plants, symptoms include red or purple leaf veins, yellowing of new leaf tissue, and sometimes blackened terminals.
Special Considerations
Herbicides in this category are very crop specific. The spray tank must be cleaned thoroughly before the sprayer is used on a potentially susceptible crop. It is very important that the susceptibility of future rotational crops be considered before herbicides in this group are applied. High soil pH increases the soil activity of sulfonylurea herbicides and the potential for rotational crop damage.
Other Herbicides That Inhibit Amino Acid Synthesis
The herbicides in this category also affect amino acid synthesis but in a different way than the previous group. These herbicides are nonselective and control a broad range of annual and perennial grasses, broadleaves and sedges. Roundup Ultra®, one of the most commonly used herbicides on the farm and around the home, is in this category.
Herbicides in this category
Herbicides in this category have not yet been classified by family. Instead, they are grouped by the active ingredient or common name.
Symptoms
Plants treated with glyphosate or sulfosate turn yellow in 5 to 7 days, then turn brown and die in 10 to 14 days. Glufosinate acts more quickly, in 3 to 5 days. An individual plant may have dead tissue, yellow tissue, and green tissue at the same time. Extremely low dosages of Roundup® cause leaf puckering.
Special Considerations
Because these herbicides are nonselective, it is very important to protect desirable plants from spray drift. These herbicides bind tightly to soil clay and organic matter and have no soil activity. For that reason they may be less effective when plants are dusty or when application water is dirty.
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