Herbicides – The Complete Guide

It doesn’t matter if you do agriculture at an industrial level, if you’re a beginner farmer or if you simply have a small garden and want to get rid of the weeds in your lawn. For all these, there is only one solution: herbicides.

Herbicides are chemicals that are used responsibly to kill or control the growth of weeds.

Without the help of herbicides, agriculture, landscaping, and lawns would suffer.

But how do these help the environment? In agriculture, they are used to increase the production of crops and lawns. Another example is their use in the forest to preserve the habitat and certain plants that are endangered.

Of course, like any other chemical substance, they also have a negative effect on the environment. But do these negative effects outweigh the positive ones? I will cover this topic below in the article, but overall, herbicides are tools with advantages over many fields.

This article is a guide in which I included everything about herbicides, from how they are used to their types, classifications, and application.

What are herbicides

Often used in agriculture, forestry, or landscaping, herbicides are chemical substances with a phytotoxic action.

They are used to kill and control weeds, but also to prevent the growth of other unwanted plants.

Herbicides - The Complete Guide
Herbicides – The Complete Guide

Types of herbicides

There are many types of herbicides. But one thing is quite clear: there is not only one way how herbicides to be classified.

The classification of herbicides can be done in a variety of ways. The official classifications like those from HRAC (Global Herbicide Resistance Action Committee) and WSSA (Weed Science Society of America), are divided into categories from A to Z or from 1 to 34.

But these types of classifications are quite detailed and cumbersome to be known by normal persons.

Therefore, another common classification that is often used is the one based on four criteria.

The criteria are the following:

How do herbicides work?

We’ve already mentioned that herbicides are nothing but chemicals. But in order to have a better understanding and to know how to use them well, it is good to know in more detail how herbicides kill weeds and unwanted plants.

What exactly is happening when you spray herbicide? How does the plant absorb these and how do herbicides affect photosynthesis?

In the following lines, I will try to detail how herbicides work in a way that everyone can understand.

Table 1. Summary of herbicide characteristics.
Herbicide groupExamples*Uptake siteTranslocationMechanism of actionSelectivitySymptomology
Cellulose inhibitorsCasoronSoilXylemInhibits new root/shoot growth and seed germinationSelected grasses and broadleaves controlledAffected weeds often don’t emerge from soil or re-grow in spring
Inhibition of VLCFAsDevrinolSoilXylemInhibits synthesis of fatty acids and cell divisionSelected grasses and broadleaves controlledAffected weeds often don’t emerge from soil or re-grow in spring
Carotenoid biosynthesis inhibitorsEvitalSoilXylemCarotenoid pigment synthesis blocked; cell membranes leakSelected grasses and broadleaves controlledBleaching (whitening) of above-ground vegetation
Synthetic auxins2,4-D, Stinger, Weedar 64Primarily postPhloem (with sugars)Excess synthetic hormones disrupt cell division and growthBroadleaves controlledTwisting of stems and leaves, leaf cupping
Lipid synthesis inhibitors (ACCase inhibitors)Poast,Select Max, FusiladePostPhloem (with sugars)Blocked enzyme inhibits lipid productionGrasses controlledGrowing point turns brown, new leaves pull easily
Amino acid synthesis inhibitors (EPSPS inhibitors)Glyphosate (Roundup and others)PostPhloem (with sugars)Blocked enzyme inhibits amino acid and protein productionNon-selectiveRelatively uniform yellowing, followed by necrosis (dead tissue)

* Herbicide labels change frequently. Always read and follow the label prior to any herbicide use.

Herbicide absorption

Plants absorb the herbicide through the root, leaf, or stem.

When the herbicide is applied to the soil, the roots of plants, especially broad-leaved plants, begin to absorb the chemical substance.

If the weed is still seeding, it will not absorb the herbicide at all. Of course, there are some plants that have the so-called Casparian strip. The Casparian strip is a thick layer of wax found in the root of plants that minimizes herbicide absorption.

In the case of the herbicide applied to the leaves, it must pass several barriers to be absorbed.

Plant leaves have waxy cuticles, walls, and cell membranes that must be crushed to absorb the herbicide. In general, the herbicide must be applied to a larger surface of the leaves for a successful operation.

These herbicides will inhibit the photosynthesis process, and the plant will no longer be able to convert light energy into chemical energy.

Usually, they interfere with the production of chlorophyll and other pigments of photosynthesis.

Translocation of the herbicide

Its translocation is related to its type.

If the herbicide is systemic, then the plant will absorb it and conduct it throughout the body.

If the herbicide is a contact herbicide, then it will not be translocated everywhere in the plant’s body but will be absorbed only in the place where it was sprayed.

In general, contact herbicides have a faster effect, while the systemic one, or the one that is translocated, has a slower effect.

Translocated herbicides move through the xylem or phloem.

Xylem is responsible for transporting water from the roots to new plant growth, while the phloem is a general system for transporting sugar and other plant materials to all living plant tissue.

Herbicide metabolism inside the plant

Plants have a metabolism.

Through this metabolic process, plants can degrade herbicides. That’s why selective herbicides are sometimes used, that is, they can hurt uncontrolled weeds, but they do not affect the crop plant at all because they can metabolize that herbicide in a form in which it is no longer toxic to the plant.

When should herbicides be applied?

The timing of herbicide application has to do with several factors. One of the most essential is the type of crop and the type of weeds you want to kill.

You have to recognize your enemy and how it works before you act.

Another way to answer the question of when to spray herbicide is to consider the growth stage of the weeds and their sensitivity.

It is advisable to apply the herbicide during the period when the crop plants have maximum tolerance to the substances contained in the herbicide, and the weeds have maximum sensitivity to them.

That is why herbicides are applied before sowing, and others at the stage when the weeds are very small with leaves.

Why are herbicides important?

Herbicides play an important role in weed control and successful agricultural productivity. Unwanted weeds can cause significant damage to agriculture.

There are approximately 30,000 types of weeds that compete with the desired crops for space, water, nutrients, and sunlight.

A farmer who does not use herbicide can lose up to 22% of his production. Therefore, the damage caused by weeds is quantitative, through the partial or total compromise of the culture, but also qualitative by decreasing the quality of the harvested crops.

With the help of herbicides and the killing of unwanted plants, productivity can increase, and with it the profitability of the farmers.

Another example where herbicides play an important role is in maintaining or preserving certain habitats. As we’ve already mentioned, weeds will take nutrients and water needed by native plants, and in the end, they can even outcompete them.

Thus, biodiversity and other natural habitats would suffer.

Herbicide Storage

The correct storage of the herbicide is important in order not to injure the animals or the environment.

Sometimes the product label reports certain information about the storage needs of the specific herbicide. It is advisable to consult the label and follow the instructions there if there are any.

If the label does not specify the necessary information, then you must be guided by your own knowledge of the surroundings. It is vital that the place where the herbicides are stored is protected in case of any fire, flooding, or any possible massive water leakage. All this can produce a disaster and contaminate both the soil and the water in the area. Moreover, the place of storage must provide the herbicide with good ventilation.

A good storage place would be a closed building with concrete floors. Concrete minimizes any risk in case of pesticide leakage. The building must be marked with appropriate signs specifying that strong chemicals are stored inside.

After storage, it is good to check the containers from time to time to make sure that everything is in order.

Herbicides and Environmental Impact

Herbicides were created with the aim of protecting crops against weeds and improving yields.

However, their level of toxicity also has some negative effects on the environment. Over time, several studies have been done on this topic. Some resulted in the fact that herbicides can pollute the soil, and water, and also affect certain insects.

For example, researchers believe that herbicides can contaminate water sources by the fact that when they are sprayed they can be carried by the wind or washed by precipitation and pushed into the ground.

To minimize these disadvantages to the environment, researchers from Cornell University tried to develop a system to measure the toxicity of several herbicides. Their system is called the environmental impact quotient (EIQ) and includes the effect of toxicity on various animals and insects, as well as their persistence in the soil. The EIQs values can be calculated by taking into account the percentage of active ingredients in the product, and the application rate and frequency.

Considering such formulas created by researchers, farmers or those who want to use herbicides can make more informed and better choices with a friendlier impact on the environment.

This table shows some EIQs values for herbicides and their classification by group. It is considered that group 2 herbicides are some of the most environmentally friendly due to the fact that they contain a lower dose.

Table 2 *. Environmental Impact Quotients (EIQs) and field use ratings for several herbicides.
HerbicideHerbicide GroupEIQActive ingredientApplication rateKg of product / acField use rating (EIQ x kg product/ac)
Metsulfuron-methyl (Ally)Group 216.6760%3 g/ac0.00180.03
Clethodim (Select)Group 117.0240 g/l0.08 L/ac0.01920.326
Quizalofop-P-ethyl (Assure)Group 151.796 g/L0.15 L/ac0.01440.744
Glyphosate (Roundup Original)Group 915.3356 g/L0.3 L/ac0.1071.64
Sethoxydim (Poast Ultra)Group 127.5450 g/L0.45 L/ac0.20255.57

* Tabel credit.

Recommendations: When possible, choose herbicides such as Group 2’s, which have a low dose and low environmental impact.

Herbicides Facts

Can herbicides be mixed?

Herbicides can be mixed, but this should be done with caution. Before mixing the herbicides, it is recommended to read the product label. The label of the herbicide indicates which substances can be mixed. Plus, it provides instructions on how to mix chemicals or obtain the so-called tank mixing.

The instructions must be respected and followed step by step.

Some herbicide mixtures are not compatible with each other. If you combine such unsuitable substances, you risk getting a herbicide that can create unwanted chemical reactions and that can kill even the plants you want to keep. Therefore, always consult the product label for more details.

Can herbicides freeze?

Herbicides sometimes come in liquid or dry formulations and granules. These forms of herbicides behave differently when it comes to frost.

If I talk about liquid herbicides or the liquid left after spraying, then they are affected by the lower temperatures in a more or less different way. In general, a liquid herbicide does not necessarily freeze, but it will separate the active ingredient from solvents and emulsifiers.

Of course, there is also the risk that the liquid expands when it freezes, which will lead to the cracking of the container. It is best to try not to create a large excess of herbicide that needs to be stored in the winter. If it is really necessary to store the liquid herbicide over the winter, then consider putting it somewhere protected from freezing temperatures.

In contrast, herbicides with dry formulations or granules are not affected by cold or near-freezing temperatures.

Can herbicides kill trees?

If the herbicide is applied correctly, then it should not affect trees or any other plant that you want to keep.

However, if the herbicide reaches the roots of the trees, especially in a large dose, then the trees can be killed by the herbicide. The herbicide works by penetrating the bark and entering the tree’s vascular system. Once distributed everywhere, the tree will begin to die.

If we are only talking about a small dose, then it is possible for the tree to recover from herbicide damage.

Can herbicides harm humans?

If the herbicides are not used according to the instructions on the label and are misused, then they can be harmful to humans. Herbicides must be used with care. Plus, those who spray with herbicides must wear certain protective equipment.

Inhalation of a large quantity of herbicides can be manifested through the following symptoms such as headaches, nausea, vomiting, and eye or skin irritation.

Herbicide equipment

Application equipment for herbicides

For the correct application of herbicides, you need equipment specially created for this process. Herbicide application equipment generally depends on several factors. The type of herbicide and the size of the land or area must be first evaluated. Let’s take some of these scenarios in turn:

  • For individual use and a small portion of land, you can use hand-held sprayers. These are mostly made of a tank for herbicide, a pump, and a nozzle for spraying the herbicide. Another good option that can spray for a longer time, are backpack sprayers. They are even easier to use because you can carry them on your back and your hands will be free to spray more easily.
  • For large areas of agricultural land, for example, you can buy boom sprayers. Boom sprayers are mounted on a tractor, and with multiple nozzles, you can cover large areas in a short time.
  • For areas such as forests, mist blowers are suitable.

Protective gear for spraying herbicides

When you spray herbicide, you must be prepared for any gust of wind that will blow the herbicide towards your skin, but also for any other leaks. Eyes and skin must not be exposed to chemical substances.

Moreover, the herbicide should not be inhaled. Protective gear must include a long-sleeved shirt and pants, chemical-resistant gloves, chemical-resistant boots or shoes, goggles or face shield, a respirator mask, a disposable jumpsuit, and a rubber apron.

All these must be worn properly to be effective. If any of the protective items are contaminated or quite worn out, then it is important to replace them.

About the author: Dani Martelli
Dani FarmerDB

My name is Dani and I am a farmer with 10 years of experience. I will share with you everything about farming from processes, tips, machines, and more. Farming for me is not just a job but a way of life. Keep reading ...

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