Herbicides can be defined as chemical agents used for killing and inhibiting the growth of weeds on a farm. They are also known as weed killers. Weeds are plants which grow in unwanted areas i.e. mostly found growing in between cultivated plants thereby disturbing the proper growth of the plants. Weeds compete with plants for air, nutrients, water and space hence, it is important to always eliminate weeds from one’s farm.
They offer a great advantage over mechanical weed control as is requires little manual effort and it kills more weeds within a short period of time. They are applied with the aid of sprayers, some large farms even employ the services of planes and helicopters to spray their farms. They play an essential role in weed management for most farmers as they are relatively cheap, effective and efficient weed killers.
Types of herbicides.
Herbicides are majorly categorized into two groups namely selective and non selective herbicides, but however, they can also be further classified into other types based on factors like timing, application method, mechanism of action, persistence etc.
There are two major types. These are;
Selective herbicides: These are herbicides that attack and kill targeted weeds leaving other plants unharmed in the process. They are the most commonly used herbicides for effective weed control in most gardens, lawns and farms. Examples are glyphosate, paraquat, NCPA, imazapic etc.
Non Selective herbicides: They are known as knockdown or wipeout herbicides as kill almost all types of plants. These herbicides kill most plants when applied in sufficient quantities. They are highly effective and should be used when the farmer intends to clear a farm site. Examples are; sodium chlorate, linuron, mazapyr etc.
Classification of herbicides based on method of application.
There are two classes of herbicides based on their method of application. These are listed below.
Soil applied type: They are applied directly to the soil to check weed growth. These herbicides are absorbed and taken up by the roots and shoots of weeds as they grow and in the process kills them. Several factors affect the effectiveness of soil applied herbicides.
These are; the level of organic matter in the soil as it reduces the amount of herbicidal chemicals present in the soil, application of the herbicide in the correct soil layer to prevent washing away by erosion etc. Examples of soil applied herbicides are: EPTC, Trifluralin etc.
Foliage applied type: They are applied to the exposed parts of the weed above the ground level i.e. the shoots, stems or leaves of the weeds. The weeds upon absorbing the chemicals translocate it throughout its body system thereby killing it. External protective plant structures such as cuticles, waxes, cell wall, barks affect the absorption of the herbicidal chemicals by weeds. E.g. triclopyr.
Classification based on timing.
Based on their time of application, there are of two types namely;
Pre emergent type: These are herbicides that are applied on the farm before any planting is done. This is done to prevent or suppress the growth of weeds. They do not prevent weeds from germinating, but they kill the weeds as soon as they grow or emerge through the herbicide treated zone in the soil.
It should be noted that weeds that have grown before its application are not killed by pre emergence herbicides. E.g. pendimethalin, metolachlor etc.
Post emergent type: They are applied on the farm after planting to prevent and kill weeds. They are applied on the weeds directly to stop its further growth, this is done after the weeds are spotted on the farm. E.g. imazamox, fluazifop, dicamba etc.
Classification based on mechanism of action.
This classification deals on how to classify herbicides based on how they affect the weeds. They are of two types which are;
Contact type: These are herbicides that kill almost all plants that they come In contact with. They have high toxicity as it is dangerous to most plants. The herbicidal chemicals have limited movement within the plant hence, it should be ensured that the herbicide is applied in a way that the target weeds are covered with it. These herbicides are rapid in action as the weeds die off after 24 hours. Examples are; oxyfluorfen, diquat, bromoxynil, glufosinate etc.
Translocated type: Upon application and absorption by weeds, they travel through the plants transport system and kills the plant by poisoning it. The plants transport system is made up of the phloem and xylem. The xylem tissue transports water and the phloem transports manufactured food and sugars. E.g. glyphosate.
Classification based on persistence or residual action.
This classifies them into types based on its ability to remain in the soil for a long time. The types are;
Residual herbicides: They are residual chemical remain active in the soil for a long period of time. These residues remain potent in the soil as they can still help prevent weed growth. E.g lopyralid, aminopyralid.
Non residual herbicides: They are characterized by its minimal soil activity and its rapid deactivation in the soil. Most of them are mostly soluble and easily dissolve into the soil where it is absorbed and can increase its level of organic matter. E.g., paraquat, glyphosate etc.
Precautions to be taken when applying herbicides.
Avoid the indiscriminate use of herbicides near water bodies.
The use of protective masks, long sleeves, gloves by farmers should be enforced.
Farm animals should not present on the farm during the spraying of the herbicides.
They should be stored in a clean, dry place out of the reach of children, pets or farm animals.
Cultivated crops should be protected when herbicides are to be applied on the farm.
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