Weeds or unwanted plants negatively affect the growth of crops or other commercially important plants. Herbicides are chemical agents that are used for removing these unwanted plants. Even though herbicides are mostly nontoxic to human beings and animals, they can cause harm to non-targeted plants and insects whose survival depends on these plants.
Herbicides. Image Credit: Kritchai7752/Shutterstock.com
Herbicides play an essential role in integrated weed management programs. However, the efficiency and sustainability of this chemical agent for its utilization as a weed management tactic depend on the knowledge of its mechanisms.
History of Herbicides
Researchers have traced back the first use of herbicides to France in the early 1800s and soon after it gained immense popularity throughout Europe. Mostly, chemicals such as copper sulfates, copper nitrates, iron nitrate, and iron sulfate were used extensively during that period. However, sodium arsenite soon gained popularity because it could be applied by spraying.
In 1896, the first organic chemical herbicide, namely, Sinox, was developed in France. Research on herbicides grew rapidly and within the next two decades over one hundred new chemical herbicides were developed and utilized. Some of the examples of popularly used herbicides are 2,4-D (2,4-dichlorophenoxyacetic acid), 2,4,5-T (2,4,5-trichlorophenoxyacetic acid), IPC (isopropyl-N-phenylcarbamate), and glyphosate. In the middle of 1980, genetically engineered herbicide-resistant crops were developed. These crops showed resistance and thrived on the land treated with specific herbicides.
Types of Herbicides
Herbicides are predominantly categorized into two groups, i.e., selective and non-selective herbicides. These are further classified into various other groups with regards to their mechanism and mode of application. The following section describes different kinds of herbicides and their mode of application.
These herbicides only kill targeted weeds, thereby, other desirable crops are not affected by their application. These chemical agents are applied with precise dosages and application rates. Selective herbicides are mostly useful for treating weeds in gardens and lawns.
These herbicides are also known as knockdown herbicides. As the name suggests, the application of this kind of herbicide in sufficient quantities may damage or kill most of the plants. These herbicides are highly efficient, improve timelines for the sowing of seeds, reduce the risk of erosion, and are cost-effective.
However, the overuse of knockdown herbicides promotes herbicide resistance. Some of the common examples of non-selective herbicides are glyphosate and paraquat.
These herbicides travel to the site of action via a plant's transport system i.e., the xylem and phloem. In the plants’ system, the main function of the xylem is to transport water and nutrients from the soil to the growth sites. Whereas, the phloem transports the products of photosynthesis, for example, sugars, from the leaves to the other parts of the plant for its growth and storage.
In the case of translocated herbicides, it may take up to two weeks for the development of symptoms on the targeted weeds. The timeline generally depends on the environmental conditions, the herbicide rate, and the target species. An example of translocated herbicide is glyphosate.
These herbicides have limited movement within the plant. Hence, these herbicides are to be applied such that all the target weeds are completely covered with them. Mostly, these herbicides show symptoms rapidly, i.e., within 24 hours of their application. Some of the examples of contact herbicides are oxyfluorfen, bromoxynil, paraquat, and diquat.
These herbicides remain active in the soil for a longer period depending upon seasonal conditions. They can be functional to successive weed germinations. For example, the effect of triazine residues is much less in the rainy season than in dry conditions. Sulfonylurea residues are soluble and are therefore less effective in the rainy season.
These herbicides have minimal soil activity and are rapidly deactivated in the soil. Sometimes, they get attached to soil particles and hence become less available to be absorbed by the plant root system. Some of the examples of non-selective herbicides are glyphosate and paraquat.
Pre-emergent and post-emergent herbicides
These are types of selective herbicides. The pre-emergent herbicides control the weed growth between the growth of a radical from the seed and seedling through the soil. Whereas, the post-emergent herbicides include the foliar application of herbicide, i.e., after the weeds grow out from the soil.
Environmental conditions, the nature of the soil, and the time of the application play a crucial role in the persistence and availability of the pre-emergent herbicides.
Herbicide mixtures and sequential applications
The application of more than one herbicide is carried out to manage the problem of herbicide resistance. It also improves the spectrum of weed species controlled. Sometimes, different herbicides are mixed and applied on the field, while at other times when herbicides are antagonistic and cannot be mixed, they are applied sequentially.
Degradation of Herbicides in the Soil
When the herbicides reach the soil system, it begins to degrade by various biological and chemical processes. However, the majority of degradation is carried out by soil microbes. The process of degradation is heavily dependent on the type of soil, moisture level, temperature, etc.
Herbicides are applied on millions of hectares of land, particularly agricultural lands, forests, pastures. Many other sophisticated pieces of equipment, sprayers, spreaders for pelleted herbicides, etc., are conveniently utilized for easy and effective herbicide treatments. Sometimes, airplanes are also used to spray herbicides over a large area.
Several precautionary measures must be followed while handling herbicides, for example, avoiding the application of herbicide on a windy day. One must also not apply herbicide near water bodies. Both animals and humans must not inhale high amounts of herbicides, and the use of protective masks, long sleeves, and gloves are highly essential.
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