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What Are Herbicides
Herbicides

What are Herbicides

Understanding herbicides requires understanding the importance of weed control. Weed control has been a necessary part of agriculture for a long time, even prior to the development of herbicides. When you grow crops, you might also find some unwanted plants called weeds growing alongside. The presence of these unwanted plants can negatively impact the growth and yield of your desired crops, since they consume the same resources like water, nutrients, sunlight and harbour insects and fungi.

Before herbicides gained widespread popularity, some of the methods used to control weeds included hand weeding, cover crops, modifying the soil pH, fertility levels, or salinity. Sea salt, industrial by-products, and oils were used for chemical weed control. However, it was not until breakthrough research was conducted during World War II that herbicides for modern application were developed that could kill a wide range of weeds without affecting the crops. Today, it can be said that herbicides form the foundation of modern weed management and control for commercial applications in different industries, including agriculture. For agriculture, herbicides have been a boon and their widespread use was quickly adopted by farmers as herbicides proved to be a much more efficient form of weed control.

Use of Herbicides

Herbicides get rid of weeds that would otherwise compete for light, moisture, and nutrients with the crops, affecting the quality and quantity of produce. They can also interfere with and damage harvesting equipment, harbour pests and diseases, contaminate aquatic resources, and can even have toxic properties that cause health issues.

Herbicides are primarily used in agriculture but the use of herbicides also extends to other industries where unnecessary plants have to be cleared. In agriculture, herbicides are useful because they can control weeds in almost any stage of growth, providing flexibility to the crop growers. Every crop is impacted by different kinds of weeds, and the herbicides that need to be used will be different. Some herbicides can be applied before the seeds are planted to ensure that there is no trace of herbicides beforehand. In some cases where weeds might already be present alongside growing crops, a selective herbicide might be used that can control the weeds without damaging the crops. The use of herbicides will continue to remain important as they can significantly impact the yield and prevent harmful weeds from growing and being processed alongside crops.

How Herbicides Work

The chemicals in herbicides can prevent or inhibit the growth of weeds. Herbicides can be incredibly effective even in small doses of concentration. There are many different types of herbicides and each of them work differently but overall, their method of action can be classified into two: systemic or translocated herbicides and non-systemic or contact herbicides.

Systemic or translocated herbicides are those which are absorbed by the plant from the absorption site and moves within the plant’s vascular system to sites of action where the chemicals then work to control the growth of the plant. Non-systemic herbicides, or contact herbicides, kill the parts of the plant that it comes in contact with. While these are more fast-acting than contact herbicides, they require repeated application, especially to control undergrowth growth and are not as effective for perennial weeds.

Different Types of Herbicides

Herbicides can be classified in various ways. This can include classification based on the chemicals in herbicides, method of application, site of action, or timing of application. Some of these classifications of types of herbicides are listed below:

Types of Herbicides Based on Specificity

  • Selective Herbicides

    - Selective herbicides can prevent the growth of some specific species of plants without having an impact on crops. These are the types of herbicides that are most commonly used in agriculture as they can be used without affecting crops.

  • Non-Selective Herbicides

    - These herbicides are broad-spectrum and will affect any plant it is applied to. Non-selective herbicides are mostly used in pre and post-sowing burn down, chemical follows, plantations and also for industrial applications that require clearing a wide area of land of all plants.

Types of Herbicides Based on Method of Application

  • Soil Applied Herbicides

    - These herbicides are applied to the soil and absorbed by the roots or the shoots of the emerging seedling of the weed. For these herbicides to achieve their proper efficiency, they have to be applied to the proper layer of the soil and adequate soil moisture.

  • Foliar Applied Herbicides

    - Foliar herbicides are applied to the foliar parts of the weeds above the soil and then absorbed by the exposed tissues of the weeds.

Types of Herbicides Based on Timing of Application

  • Pre-plant Herbicides

    - These herbicides are applied to the soil before planting so the weeds are controlled before the crops are sown. These are typically selective as well as non-selective herbicides. The herbicide-treated land can then be used for agricultural purposes to grow crops.

  • Pre-emergence Herbicides

    - This type refers to the herbicides that can be applied before the weed seedlings emerge from the soil. When applied, these herbicides control the weeds as they grow out of the soil but they do not affect the weeds that are already grown.

  • Post-emergence Herbicides

    -These herbicides are applied after weeds are already grown. They can be selective or non-selective depending on the needs and can have soil or foliar application. They typically require multiple applications for effective control and are avoided for use during rains as they can be washed off which reduces their effectiveness.

Advantages of Herbicides

Herbicides come with many advantages over other methods of weed control because there are so many different ways that they can be used. Here are some advantages that highlight the importance of herbicides:

  • Herbicides can be applied before the weeds grow

    Unlike other methods of weed control, it is possible to apply herbicides in the pre-plant and pre-emergence stages. This lets the crops grow in a weed-free environment without competing for nutrients even in the early stages of development.

  • Herbicides can be effectively used in different crop arrangements

    While mechanical forms of weed control can be effective in some cases, when it comes to broadcast sown and narrow-spaced crops, they cannot reach all the weeds present in the field which herbicides can easily do. Herbicides can also effectively reach intra-row weeds.

  • Herbicides can get rid of deep-rooted weeds

    Weeds that have roots deep into the soil cannot always be effectively removed with mechanical methods as while the top part may be removed, the weeds can grow back from the roots. For these, a combination of mechanical and chemical herbicides might be necessary for effective weed control.

  • Herbicides prevent weeds from growing for a long time

    When herbicides are applied, they tend to be effective for long periods of time and prevent the weeds from growing back soon.

  • Herbicides can get rid of structurally similar weeds

    Weeds that appear similar to the crops can be missed out when they are removed through mechanical methods as it is hard to distinguish between the two. However, herbicides can be applied to kill these weeds that may look the same but be biologically different without causing any damage to the crops.

Herbicide Resistance

Since herbicides were first used, some plants inherited resistance to it that would otherwise kill the plant species - this is herbicide resistance. This is a natural evolutionary process. When herbicides are applied, it kills off the susceptible weeds but some remaining resistant weeds survive and reproduce. Resistant weeds can start off as individual plants or limited patches in a field but can spread across the whole field and prove to be a challenge to remove. One of the early signs of herbicide resistance is the presence of a high number of weeds that survive after the application of herbicide.

Hypersensitivity of a particular herbicide can also lead to resistance, as the most susceptible weeds are eliminated in a single application and the few that remain can rapidly evolve into a resistant species. There are also some other factors related to herbicide usage that can lead to resistance. This includes a lack of rotation of herbicides which contributes to the thriving of resistant weeds, usage of herbicides with long residue periods that provides resistant weeds longer time to grow unchallenged, and using herbicides with a single site of action which makes it easier for weeds to develop resistance.

These herbicide-resistant weeds are a growing concern, especially in agriculture. There are more than 450 herbicide-resistant weeds across the world which impacts 92 crops in 70 countries. UPL is constantly working towards innovating to improve our herbicide products and create new ones that will combat the growing herbicide resistance.

References

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