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Weed Killer & Weed Control

Weeds sure are a pain for homeowners, commercial property managers, and farmers around the world. Seeds blow in from your neighbors’ yards or nearby fields, and boom, before you know it, your property is being taken over by weeds. Some of them may not look all that insidious, but they can be. They have the inborn ability to spread like a virus and wreak havoc on your beautiful landscape or crops that you spent your hard-earned money on. It’s OK, though. You don’t have to let those pesky weeds take over your yard when there are so many weed killer options out there. The “space race” to create effective weed killers has evolved so that farmers and homeowners alike can win the battle with weeds if they know what they are doing, and they implement a thorough year-round plan. In this article, we will break down the history of weed killers and where the weed control industry is heading.

History of Weed Killers

Before we dig into all of the different types of weed killers, here’s a bit of history in that area. Way before widespread herbicide use became the norm worldwide, several cultural controls were being used, including the alteration of the fertility levels, PH, and salinity of the soil. Besides mechanical control, which included tillage, was used for controlling weeds. Using fire to burn out sections of forest or land was also an early form of weed control that had meager results.

Early Hazardous Chemical Weed Killers

During the late 1800s, selective broadleaf herbicides were first discovered in France and included sea salt, as well as certain industrial oils and by-products. They were used in cereal crop fields, and the practice didn’t take long to start spreading all over Europe. They also used nitrates of iron and copper but soon discovered that sulfuric acid was more effective. All too soon, sodium arsenite became the popular alternative and was being widely used to sterilize soil and was sprayed on thousands of miles of rubber and sugarcane plantations, as well as railroad rights-of-way. Soon, it was being used in massive quantities, which often caused the poisoning of animals and sometimes even humans.

Additional Resources

Broadleaf Weeds  –  Grassy Weeds  –  Weed Control Link Guide

Wartime Advancements

In 1896, Sinox became the first known significant organic chemical herbicide developed in French research laboratories. The late 1940s became widely known as “the era of miracle weed killers” when the development of a wealth of new herbicides came from intensive World War II research involving the use of herbicides in herbicidal warfare. They used substances that were mainly designed for destroying plant-based ecosystems in specific areas to disrupt agricultural food production. The practice also theoretically allowed the military to kill plants that might be providing concealment or cover for the enemy. Although the practice was never meant to poison or asphyxiate human beings, herbicidal warfare was banned by the 1978 Environmental Modification Convention.

The First Selective Herbicides

In 1946, the first of the modern herbicides, which was called 2,4-dichlorophenoxyacetic acid (2,4-D), was released commercially, triggering a worldwide agricultural output revolution and making it the first among what would soon become a string of successful selective herbicides. Its release made it possible to significantly enhance the control of weeds in cereal, corn, rice, and wheat crops because it kills broadleaf plants without killing most grasses. The relatively low cost of 2,4-D led to the continued use of the substance even today. It remains among the most commonly used herbicides worldwide. Much like other known acid herbicides, all of the current formulations utilize either an amine salt (i.e., trimethylamine) or an ester of the parent compound, which are much easier than the acid to handle.

Groundwater Contamination from Triazine Herbicides

During the 1950s, the triazine herbicide family (including atrazine) was introduced and soon became the herbicides of greatest concern because of groundwater contamination. Atrazine causes “carryover,” which is an undesirable property in a herbicide because it doesn’t break down readily when applied to soils with an above neutral pH. When the soil is in an alkaline condition, atrazine can be transported by water passing through the soil carrying it to groundwater. The development of effective herbicides has continued, and some, like glyphosate, are widely used worldwide, despite ongoing controversy regarding safety and the threat of creating superweeds that are immune to all herbicides.

Important Early Weed Killer Innovations

The Early 1940s into the 1950s brought about the airplane sprayer and the ground-based mist blower. Low volume/low-pressure nozzles were developed for applying new phenoxy herbicides. Band-applicators were created for pre-planting and pre-emergent application of liquid and granular herbicides on row crops. Boom-jet nozzles were also invented for replacing or supplementing conventional ground rig spray booms for broadcast spraying.

The 1960s saw continued progress with herbicide research. Many improvements were made to weed killers from a molecular level, including research on many new promising compounds. Advances in molecular biology in other scientific fields created a ripe setup for advancements in weed control technologies. The 1960s also saw many improvements in the equipment used and protocols around the use of weed killer.

Mechanisms of Action (MOA)

Herbicides are often classified based on their mechanism of action. This is because, in general, herbicides within the same site and action class are responsible for producing comparable symptoms on susceptible plants. A classification that is site-based is useful because of its general efficacy. However, mode of action (MOA) classification is often preferable because it can indicate the first biochemical step, enzyme, or protein in the plant that is affected by the application. When scientists study weed control and potential herbicides, it reminds us of how cancer scientists target cancer cells. They both target biochemistry cycles within the organism with the goal of interrupting an essential chemical process. With cancer, the goal is to leave surrounding healthy tissue undisturbed. With weed control, the goal is to kill weeds without harming surrounding grass.

List of MOAs in Modern Herbicides

Below is a breakdown of the most common mechanisms of action in modem day weed killers. Every day PhD level scientists are hard at work trying to identify new molecules and new sites within weed biology to target.

Auxins (synthetic) brought about the organic herbicide era. Their discovery goes back to the 1940s following long studies of auxin, a plant growth regulator. Synthetic auxins mimic plant hormone in many ways and have proven effective in controlling dicot plants, whose seeds have two cotyledons (embryonic leaves) and include broadleaf weeds. Popular synthetic auxin herbicides include 2,4-D and 2,4,5-T.

ACCase inhibitors (ACCase) work by affecting the production of cell membranes in grass’ meristems. Dicot plants’ ACCases aren’t at all sensitive to this type of herbicide, however, those in grasses are.

ALS inhibitors (ALS aka acetohydroxyacid synthase, or AHAS), include a variety of sulfonylureas (SUs) like flazasulfuron, imidazolinones (IMIs), Metasulfuron-methyl, pyrimidinyl oxybenzoates (POBs), sulfonyl amino carbonyl triazolinones (SCTs), and triazolopytimidines (TPs). Because the biological pathway of ALS only exists in plants and not in animals, ALS-inhibitors are some of the safest herbicides available.

EPSPS inhibitors (EPSPS) are used for synthesizing amino acids like phenylalanine, tryptophan, and tyrosine and can affect both dicots and grasses. Glyphosate, the controversial ingredient in the popular weed killer Roundup, is a systemic EPSPS inhibitor, which is inactivated by contact with the soil.

HPPD Inhibitors, like Sulcotrione and Mesotrione, cause the breakdown of tyrosine in the weeds. Without tyrosine, they suffer chlorophyll loss, turn white, and die.

Photosystem I inhibitors, such as the Bipyridinium herbicides Paraquat and Diquat work by stealing electrons from the weeds’ normal pathway via FeS to Fdx to NADP+, which leads to plant death.

Photosystem II inhibitors, like the triazine herbicide Atrazine and the urea derivative Diuron, work via the reduction of electron flow from water during the photochemical step of photosynthesis. They prevent quinone from binding to this site, causing accumulation of electrons on the weed’s chlorophyll molecules, resulting in oxidation that is beyond what can be tolerated and the plant dies.

Intended Outcomes

  • Weed control occurs with the destruction of unwanted weeds. It also includes damage to weeds to a certain point where they can no longer compete with crops, lawns, or landscape plantings.
  • Weed suppression is control that is incomplete but still provides some benefits, including a reduction in competition with crops, lawns, or landscape plantings.
  • Crop protection is the respective lack of any stress or damage to a crop via the use of selective herbicides. The majority of selective herbicides do end up causing a certain amount of visible stress to plants.
  • Defoliants are somewhat similar to herbicides, however, they are designed for removing foliage instead of killing the entire plant.

Pre-planting Herbicides

Pre-plant herbicides (non-selective) are applied to the prepared soil before crop or lawn planting. Some can be incorporated into the soil mechanically, but they are usually sprayed into the ground and often tilled. There are numerous crops currently being grown in soil that has been treated with pre-plant herbicides, including (but not limited to) corn, soybeans, strawberries, and tomatoes. Soil fumigants, such as dazomet and metam-sodium, are often used as pre-plant herbicides.

Pre-emergent Herbicides

Pre-emergent herbicides are applied to prevent seedlings from sprouting in the first place. Their mechanism of action may differ, but they all prevent weeds from taking root in the first place. It’s important to note that weeds that have already taken root and grown will not be susceptible to pre-emergent weed control treatments.

Post-emergent Herbicides

Post-emergent herbicides are generally applied after the weed seedlings have completed their emergence through the surface of the soil. These herbicides could be root absorbed or foliar, non-selective or selective, systemic, or contact applied. The use of these herbicides should be avoided altogether when it’s raining since they could be washed off, making it ineffective. One popular post-emergent product, 2,4-D, is a foliar absorbed, selective, systemic herbicide.

Foliar Based Applications

Foliar based herbicides are applied to weeds that are above ground and are absorbed by plant tissues that are exposed. They are usually post-emergent herbicides that are either “contact herbicides” or “translocated systemic herbicides.” Dicamba, glyphosate, and 2,4-D are foliar-applied herbicides. Some plants and weeds, however, can often have external barriers, such as cell walls, cuticles, waxes, and thick waxy leaves that can affect herbicide action and absorption.

Soil Based Applications

Herbicides that are applied to the soil, like trifluralin and EPTC, and are generally taken up by the shoots or roots of emerging seedlings, are usually used as soil-based pre-emergents. There are numerous factors responsible for influencing soil-applied herbicides and their overall effectiveness. Weeds can absorb herbicides via both active and passive mechanisms. Herbicides that are absorbed into organic matter or soil colloids often see a reduction in the available weed control absorption amounts. That’s why it’s so important that herbicides be positioned in the correct soil layer. This can be achieved either via rainfall or mechanical application. Soil applied herbicides are often subjected to several processes that cause a reduction in their availability. Both photolysis and volatility are common processes responsible for reducing herbicide availability. Most soil based weed control applications are absorbed via plant shoots while they’re underground, which leads to injury and/or death of the weed.

Weed Killer Terminology

Weed killers are generally classified or grouped in several ways, including by their specific activity, application timing, chemical family, or mechanism of action. This has resulted in different terminologies to describe them and their respective use. This includes:

Selective Weed Killers

Selective weed killers suppress or control specific plants without any effect on the growth of other plant species. Selectivity could be due to differential absorption, physical (morphological), physiological differences between the weed species, or simply translocation. For example, 2,4-D, dicamba, and mecoprop are capable of controlling numerous broadleaf weeds while remaining completely ineffective against grassy weeds.

Non-Selective Weed Killers

Non-selective weed killers, like glufosinate, paraquat, and glyphosate, are not targeted in their action against specific plant species. Instead, they can kill a wide variety of plants they end up coming in contact with. They’re often used for clearing industrial sites, railways and railway embankments, and waste ground.

Weed Killer Sub-Categories

  • Pre-emergents: Applied before weed seedlings have emerged through the surface of the soil.
  • Post-emergents: Applied after weed seedlings have emerged through the surface of the soil.
  • Contact Herbicides: These include diquat, paraquat, and sulfuric acid, and are effective at killing only those plant organs that they come in contact with.
  • Translocated Herbicides: Include amitrole, 2,4-D, and picloram that are highly effective against weed roots that are transported from treated soil surfaces that are above ground.

Herbicide Resistance

Herbicide resistance is a weed biotype’s natural ability to survive a herbicide application that usually would kill it. Hundreds of herbicide resistance cases have been documented in 100+ weed species worldwide, presenting an on-going challenge to both the agriculture industry and growers, requiring the development of weed management strategies that are not only effective but also economical, practical, and reliable. Diverse techniques in crop management can help growers with mitigating the spread of herbicide-resistant weeds.

Herbicide Rotation

In the proper planning of any weed control program, herbicides need to be selected from groups using different sites of action for controlling the same weed as well as being utilized in mixtures or successive applications. In general, weed populations have a broad range of genetic fluctuation. Small numbers of weeds could include certain herbicide-resistant genetic traits. When herbicides are applied, the majority of the susceptible weeds will be controlled, however, the resistant ones will continue to grow. If they’re allowed to set seed, they could easily continue to grow, setting seed again the following year if the same weed killer is used again. And, the continued application of that same herbicide would result in the property becoming dominated by the resistant plants.

Herbicide Decomposition

A wide range of modern herbicides have been formulated to decompose in a short time following application. This is a highly desirable trait since it can allow plants and crops to be planted later in the same soil, which might otherwise be adversely affected by another type of herbicide. However, those decomposing herbicides that have low residual activity (i.e., they decompose quickly) often don’t provide weed control for extended periods.

Weed Killer Group Labeling

The global Herbicide Resistance Action Committee (HRAC) is an organization formed for actively supporting accurate and informative labeling on weed control products. This helps to ensure that farmers, weed control professionals, and homeowners have the resources necessary for making the most responsible decisions about the use of herbicides on their property. The global HRAC has been developing both a herbicide classification system that is based upon herbicide sites of action and integrated weed management, as well as helpful guidelines for herbicide use.

Guidelines for Sustainable Use

  • Use sequential treatments or mixtures of herbicides that have varying sites of action. Each one should be targeting the same species of weeds.
  • Take into consideration any/all chemical control options before planting as well as in-crop and after harvest.
  • Avoid continually using the same herbicides (or even those with the same site of action) in the same field, however, that is unless you integrate them with other practices of weed control.
  • Limit the number of applications using the same herbicide, or even those with the same site of action, in the same growing season.
  • Understand that herbicide rotations and herbicide mixtures alone aren’t enough for preventing resistance. You must also use a diversified plan that incorporates biological, cultural, and mechanical practices.
  • Always ensure that you’re following label use instructions, including application rates, equipment recommendations, and timing.
  • Know all of the weeds in your yard or field as well as nearby non-crop areas, being sure that you’re tailoring your weed control program to both economic thresholds and weed density.
    Continuously monitor your herbicide results, always being aware of any changes in weed populations.
  • Maintain a detailed record for confirmation of your weed killer use and efficacy history.

Integrated Weed Management

Integrated Weed Management (IWM) is a common term referring to the use of biological, chemical, cultural, and mechanical methods of weed control. IWM should be utilized in an integrated manner for successfully controlling weeds. It doesn’t rely mainly on any single method. When IWM is used as part of an integrated approach, these tools can help with reducing the survival and selection pressures of any resistant weeds.

Biological – This is the introduction of pathogens and insects to control target weed species. This includes the purposeful post-harvest grazing on weeds that are already growing in an area.

Chemical – This is the application of any type of weed killer to fields or yards.

Cultural – This includes harvest timing for disrupting the weed cycle, altering planting dates, and row spacing. It can also involve buying certified (weed-free) seeds, planting plants that are capable of competing with the weeds, and the use of diverse crop rotation.

Mechanical – This requires the use of measures like cultivation, plowing, or hand-weeding for controlling the already-emerged plants and burying any non-germinated seeds. It can also include harvesting any weed seed destruction (like stubble) and then cutting or burning for the prevention of weeds setting seed.

Monitoring & Mitigation

Have you ever wondered exactly how weed scientists go about monitoring herbicide resistance and developing systems for mitigating the resistant weeds’ development and spreading? That’s a good question because monitoring and mitigation are both critical aspects of herbicide resistance management in weeds. The detection of weed resistance, gaining an understanding of the scope in any given area, and limiting its spread are all crucial steps in the overall management of the risk. If you are dealing with a resistant weed lookup who the authorities are in your state. Information is often provided by universities that offer weed management programs or other agricultural degrees.

Monitoring

Both private and public weed scientists are tasked with monitoring weed populations for early detection of herbicide-resistance plants and weed problems. Those monitoring methods can include:

  • Field surveys – The seeds are collected and then tested via the use of bioassays (analytical methods for determining potency and concentration of any substance by the effect it has on living cells);
  • Market research surveys – From both scientists, growers, and weed control professionals.
  • Tracking herbicide performance complaints – From growers and weed control professionals.

There are two different types of monitoring programs, and they are reactive or proactive, with both having their challenges, goals, and levels of success. The early identification of herbicide resistance can be somewhat tricky. Throughout the initial years of each weed selection, the resistant ones end up only encompassing a minimal proportion of total weed populations. These ‘escapees’ could easily be mistaken for weed problems that are caused by application or weather issues. On the other hand, they could be the result of poor herbicide performance. And, the resistance question might not even come up until 30 or even 40 percent of the weed population has become resistant! In other words, this is a problem that is hard to tell initially!

Mitigating

When populations of resistant weeds are detected, some steps can be taken to reduce the overall impact they have on the ecosystem. Mitigation is the key, and it works by focusing on preventing the spread of the resistant weed. Successful mitigation is typically reliant on special training and education as well as the implementation of Best Management Practices (BMPs), which is one more valid reason for retaining a licensed lawn care company for handling your weed control problems.

Modern Lawn Treatment Plans

Commercial grade weed control products, applied by a licensed weed control company, is the key to successful weed management. It’s the most reliable way for getting a handle on your weed problems and keeping them under long-term control. So, if you have questions regarding weed killers, lawn care, and controlling lawn weeds, be sure to look up a local, trusted weed control service that has been around a while and looks financially stable.

Look for a Year-Round Plan

Modern-day lawn services offer year-round plans that focus on the deployment of both pre and post-emergent weed control products at different times of the year. They rotate the products they use, so they hit a very wide variety of weeds. What you want to look for is a reliable and trusted company that is locally owned and understands your area’s unique weed growth cycle. You also want the company you hire to be licensed and insured with well-trained lawn care technicians with several years of experience (like WinLAWN).

Professional Weed Control Services

What you want to look for in a professional weed control service should include:

  • Prevention of Pre-Emergent Weeds
  • Elimination and Control of Broadleaf Weeds
  • Elimination and Control of Grassy Weeds
  • Year-Round Weed Control
  • Customized Weed Control Plans Based on Your Type of Grass
  • Plans Designed for Lawns in Your Area
  • Treatments that Include Lawn Fertilization During the Growing Season
  • Selective Grassy Weed Applications (when necessary)
  • Licensed Providers
  • Professional Spray Equipment

Roundup Weed Killer (Glyphosate) Controversy

You can hardly turn on your TV these days without hearing about the Roundup class action lawsuits, and it’s main ingredient glyphosate. The product has been linked to cancer, and it has a long history of use. It’s been sold worldwide by Monsanto since 1974, plus the EPA allows it to be on the market. The company’s ability to sell it has been supported by EPA rulings in both 1993 and 2016. Those rulings stated that glyphosate not only isn’t carcinogenic but isn’t even likely to be. Now, those particular EPA rulings have become quite controversial based upon other decisions by other agencies, including:

2015:  The World Health Organization (WHO) International Agency for Research on Cancer (IARC) stated that their glyphosate studies showed that it is “probably carcinogenic in humans.”

2017:  The California Environmental Protection Agency labeled glyphosate as being “known to cause cancer.” Then, at the same time, the European Chemical Agency stated that there wasn’t any evidence implicating glyphosate as being a carcinogen, mutagen, or toxic to reproductive and/or other specific organs.

Today:  There are still numerous conflicting rulings from the World Health Organization and others, including the Australian Pesticides & Veterinary Medicine Authority, the Canadian Pest Management Regulatory Agency, the European Commission, the German Federal Institute for Risk Management, and the Joint FAO/WHO Meeting on Pesticide Residues (JMPR). Currently, some are still saying that it may be safe, while others are saying that it isn’t. Naturally, Monsanto is saying that it’s safe, and we can only hope that their scientists (who have been studying it the most) are being both honest and forthright with the public.

Amidst all of the many studies with mixed results, isn’t the EPA supposed to be the gold standard? Aren’t they still allowing Roundup to be sold nationwide? It looks like we’ll just have to wait and see how it all plays out on the world stage. For now, if you do choose to use it, protect yourself by covering your mouth, nose, and every inch of your skin. Then, make sure that you shower immediately after using the product. On second thought, it might be safer to just opt for an alternative weed killer!

Stress Less!

Now that you know a bit more about weed killers and weed control, you can stress less about the issue. Better yet, if you want to make the upkeep of your beautiful yard completely stress-free, consider a professional lawn care company. You’ll be glad you did!