Weed control and management

Weed control and management

               Weed is defined as a plant that is undesirable, unattractive or troublesome especially one that grows where it is not wanted and often grows or spreads fast or take the place of desired plants.

                  The classification of weeds is important for applying weed management for a particular group of weed instead of individual weed species. Weeds are classified according to their life cycle, morphology, structure and form. According to the life cycle, weeds are classified into three groups. Annual weed take one year or one season to complete their life cycle. Biennial weed take two years or season to complete their life cycle. Perennial weed take more than two years or several years to complete their life cycle.

                  According to the habitat weeds are classified into two groups such as terrestrial and aquatic. According to the morphology, weeds are classified into two groups such as narrow leaved weeds and broad leaved weeds. Grasses (family Poaceae) and sedge (family cyperaceae) are the examples for narrow leaved weeds. Grasses have hollow stem and lignite alternative or opposite leaves with leaf sheath. Sedges are angular and have solid stem. Broad leaved weeds are dicot plants. The difference between broad leaved weeds and narrow leaved weed helps to use of selective control methods without injuring broad lead weeds.

                  Correct identification, impacts and life cycle of weed are useful to idenify proper weed management. Several methods are available to control weeds. Preventive methods are used to stop the spread of weeds. Preventing the introduction of weeds and spreading is usually easier than controlling them after establishment.

              Cultural weed control methods make the crop to be the best completer with the weeds. Planting the best crop varieties and dense crop populations and applying lime and fertilizers help to achieve the aim of the cultural weed control. It is more practicable and efficient method for controlling weeds where other methods are impossible such as within rows of rice fields. But it is difficult to distinguish weed seedlings and rice seedlings at early stages so uprooting of rice seedlings cannot be avoided. It is not practical to apply for large scale rice fields.

                        Biological control involves the use of natural enemies to reduce the reproductive potential of weeds.  They can be weed feeders like insect feeders or disease cause organisms like fungi, bacteria and viruses to kill the invasive plants. It is environmentally friendly methods. It has ability to self-propagative so there is no need of again and again introducing them for field. Weeds are unable to develop resistance against biological methods. The damage to the untargeted pant species is minimized due to its selectivity.  As biological control agents are also dependent on the environmental conditions, it causes fluctuations on the weed populations.

                        Chemical control involves the use of herbicides to kill a particular group of weed. It is an efficient method for both small scale and large scale. It supports to kill the deep rooted, vegetative propagative plants. After application, it is not allowed weed to regenerate for a considerable time period. Unlike other methods, herbicides are able to reach all places and kill the weeds.  But excess use of herbicides makes the weed to develop resistance against them. The people who apply the herbicide must have proper technical knowledge in order to apply the appropriate dose with correct dilution.  In addition to suppression of growth weed, it treats to the growth of crop plants as well as animal species. Excess use of herbicide pollutes the soil, water and the atmosphere.

                        Mechanical or physical methods destroy or make the environment less favorable for seed germination and survival.  Mechanical methods are tillage, uprooting, slashing, moving, ring banking and dark stripping. It requires less time than the manual control. It is only suitable for small scale weed management. It cannot be used to control weeds growing much closer to the crop plants.  Improper tillage can damage the crop plants too. It cannot be used for soil with too dry or too wet conditions. It causes serve disturbance to the soil which leads to soil erosion.

               Integrated weed management (IWM) involves employment of different methods of weed prevention and control. IWM is a combination of physical control, chemical control, biological control and cultural control method. It reduces the chance to the plants to adapt to the control techniques. It reduces the weed stock and extent in the soil. It is the most sustainable method of weed control and management.  But it takes relatively more time for the longer process.

                        Risk of plant to be invasive is assessed by using a protocol including 32 questions.  At least 28 answers for flora are essential to evaluate the potential of risk. Each question must be marked with either yes or no. Using final percentages value for answers with yes, the plant is placed in one of following categories.

                        >80%- very high risk

                        60-80%-high risk

                        40-60%- moderate risk

                        <40%- low risk