According to the Weed science society of America," Weed is any plant that is objectionable or interferes with the activity or welfare of human". In simple words, weed is any crop or plant that grows in an unwanted place, particularly with agriculture crops. For instance, a chickpea plant in a wheat field may also be classified as a weed. Weeds compete with light, space and nutrients with the main crop and thereby reduce the yield of the main crop. Hence, they need to be removed from the field. For this purpose, various intercultural operations are carried out from time to time by farmers.
Detrimental effects of weeds-
● Degrade ecosystem- Exotic species spread in the new environment vigorously. They replace the existing natural plants growing in that area and the population of insects and animals associated with those plants is also affected. Thus, they disturb the natural ecosystem of a particular area.
● Foul water bodies- aquatic weed species such as water hyacinth grow in water bodies and reduce the oxygen level present in the water. This causes the death of the aquatic species living in the water bodies and destruction of the lake or pond.
● Reduces the value of land- weeds grow anywhere on fallow land and use the nutrients of the soil. the weeds are known to complete the life-cycle within a short period of life and hence, extract more nutrients from the soil in a short period of time. Thus, the soil fertility is reduced which reduces the value of the land.
● Reduces crop yields- weeds compete for the light space air and nutrients with the crop. They grow vigorously then crop plants and hence, the reduced yields of the crop is observed.
● Harbour pest and disease- weeds also harbour various pests and diseases of the crops. The dormant stages of spores of various fungi and bacteria, as well as eggs and cocoons of pests, are harboured by weed plants. the act as a secondary host for diseases and pests.
Different methods of weed control-
Conventional methods use various chemicals known as herbicides to control the weed. These herbicides may be used as pre-emergence or post-emergence depending upon the emergence of weed in the field. Since weeds grow in a very large amount, the amount herbicide used is directly proportional to it. Thus, the environment has a lot of chemicals which degrade at a very slow pace and pollute the environment. Nowadays there are various other methods which are incorporated to destroy weeds that do not pollute or harm the environment. These modern practices of weed control are as follows-
1.Agronomic measures of cultural control-
Such methods are also referred to as precautionary measures to prevent weed growth. They involve cultivating the field in such a way that it creates favorable conditions for the growth of crops rather than weeds. Agronomic measures include practices such as as-
● Clean field cultivation- in this method the field is irrigated prior to sowing of crops. The moisture in the soil allows germination of the weed seeds in the field. When the seedlings emerge, the field is cultivated by a plough or disc Harrow to destroy these seedlings. Thus, the field is free from any dormant seeds of weeds.
● Growing crop varieties with high vigor- as we know that weeds grow more vigorously than crop plants, we must grow crops that have high vigor so as to compete with the weeds.
● Increasing a diverse microbial population- by increasing the natural microbial population in the soil we create conditions that are unfavorable for the growth of weeds.
● Crop rotation- some weeds are associated with particular crops. In order to reduce the growth of such weeds, we must incorporate crop rotation with crops of other families.
● Changing sowing time- some weeds emerge at the particular time of crop growth. Hence by altering the sowing time during the rainy season we can easily reduce weed growth.
● Harnessing allelopathic action of plants- allelopathy is the process by which some plants secrete chemicals that reduce the growth of other plants. This natural mechanism can be harnessed to use against the wheat crops by growing allelopathic crops.
2.Mechanical weed control-
Mechanical weed control involves the usage of some implements or hand weeding to remove the weeds that have already grown in the field. It is also a post remedial measure.
● Tillage- tillage or manipulation of soil with the use of arrows for disc plough can be done to destroy the weed population in the field. Weeds are uprooted by tillage.
● Removal by hands- hand weeding can also be applied to remove the plants which are not large in number.
● Removal by implements during inter cultivation- various inter cultivation implements are available that can be used to remove weeds from the field. Conventional inter cultivation implements are known as Guntakal whereas nowadays harrows are available.
3.Biological methods of Weed Control.
It involves the usage of various natural enemies of weed plants to control their population. The natural mechanism of predation is hardness in biological control.
● Use of Mexican beetle Zygogramma bicolorata to control Parthenium- The Mexican beetle was introduced to control a perennial weed Parthenium which caused a lot of problems to human beings. The beetle feeds on the leaves of parthenium and slowly the plant is killed.
● Alligat or weed flea beetle- used to control floating aquatic weeds. This beetle also feeds on the leaves of the weed plant.
● Use of cutworms
● Use of bindweed gall mite( Aceria malherbae) to control bindweed.
● Bioherbicide- use of bacteria and fungi as biocontrol agents to reduce weed growth. Example use of rhizobacteria to reduce weed growth in wheat fields.
The various advantages of modern practices to control the weed population are as follows-
● Weeds do not pollute the environment as they do not leave any residue as in the case of herbicides.
● Agronomic measures and mechanical measures are much cheaper than chemical herbicides.
● These measures help in sustainable agriculture.
● The quality of products is also not toxic.