Weeds and weed control methods in agriculture

Weeds are plants that compete with crops for nutrients, water, sunlight and space.

Importance of weeds

Positive importance /advantages of weeds

• provide humus and nutrients to the plants after decomposition
• Can be used for mulching
• Some of used as vegetables and source of vitamins
• Can act as soil cover to prevent soil erosion and excessive evaporation
• Are used as animal feeds
• Some fix nitrogen to the soil
• Some are used for medicinal purposes
• Take up and store nutrients thereby preventing leaching.
• Weeds, such as Chrysanthemum cinerariifolium, provide insecticide pyrethrum.
• Weeds tap nutrients from deeper soil layers and return them to the surface through litter leading to nutrient recycling
• Habitats for wildlife

Negative importance/disadvantages of weeds

• Compete with plants for water, light, space, and nutrients and therefore reducing crop yields
• Are host for vectors and pests
• Some weeds are poisonous to livestock and man
• Weeding requires money
• Block irrigation and drainage channels
• Water weeds suffocate fish, impede fishing and water transport
• The presence of weeds limits farm size.

Qualities/characteristic/factors than make weeds successful competitors

• Rapid rate of growth and maturation
• Drought resistant
• Resistant to pest and disease
• Some are parasitic
• Produce large volume of seed to increase their chance of survival
• Vegetative reproduction allows weeds to develop from any part of the plant
• Weeds employ various mechanisms of dispersal using wind, water, animals and self-dispersal mechanisms.
• Produce poisonous substances to crops
• Can obtain nitrogen from carnivorous behavior
• Are protected by structures such thorns and itching hairs that protect them herbivores.
• Some are resistant to herbicides

Methods of weed control

(a) Tillage to remove weeds with hand hoe or machines

Benefits/Advantages of weed control by tillage

• Improve soil aeration
• Loosen soil particles
• Nonpoisonous to the crop
• Requires little skill
• Burying of weeds promotes their decomposition
• Fast when machines are used

Disadvantages of weed control by tillage

• Destroy soil structure
• Increase water loss through evaporation
• Damage roots of the crops
• Burry and encourage germination of weed seed

(b) Mulching

Advantages of mulching

• Protects the soil from erosion
• Retains moisture in the soil and reduces evaporation
• Reduces the growth of weeds
• Improves the fertility and nutrient content of the soil
• Insulates the soil from extreme temperatures

(c) Lethal wilting with high heat; Heat is reflected from hard surfaces to cook weeds to death

(d) Burning of weeds

Advantages of burning as a method of control of weeds

• Quick and effective method of destroying weeds and their seeds
• Add nutrients to the soil
• Kill pests and disease causing organisms
• Non-toxic

Disadvantages of burning weeds as a method of weed control

• Risk of fire to unwanted parts of the farm
• Destroy soil structure
• Releases carbon dioxide that leads to global warming
• Promote leaching of the released nutrients.

(e) Controlled grazing/biological control or use of an animal to feed on weeds such as rabbits for example use rabbits to control McDonald’s eye in banana plantation, beetles to control water hyacinth, control of lantana camara by bugs and goats.

Advantages of biological control of weeds

• Reducing the pest population by using a natural parasite, predator or pathogen of the pest.
• Does not pollute the environment
• Does not affect soil structure
• Enhancing crop yield by managing pests that damage crops, leading to more productive farming.
• Preventing economic damage to agricultural products.
• cheap

Disadvantages of biological control of weeds

• biological control agents are not selective for weed
• biological control agents do not control all weed
• cannot eliminate weeds with dormant seed
• Use of herbicides

(f) Use of herbicide to control weeds

An herbicide is a substance that is toxic to plants, used to destroy unwanted vegetation or weeds

Examples of herbicides

• Phenoxyalkanoic acids, such as 2,4-D, which are selective herbicides that kill broadleaf weeds but not grasses.
• Substituted alkanoic acids, such as dicamba, which are also selective herbicides that control broadleaf weeds in crops like corn and soybeans.
• Chlorinated aliphatic acids, such as dalapon, which are non-selective herbicides that kill grasses and sedges.
• Substituted triazines, such as atrazine, which are selective herbicides that inhibit photosynthesis in weeds.
• Substituted triazoles, such as amitrole, which are non-selective herbicides that interfere with the synthesis of chlorophyll and other pigments in plants.

Advantages of using herbicides

• Requires less labour
• 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.
• Herbicides can get rid of deep-rooted weeds
• Herbicides prevent weeds from growing for a long time
• Herbicides can get rid of structurally similar weeds

Disadvantages of herbicides

• Poisonous to human
• Non-selective herbicide cause injury to crops.
• Lead to herbicide resistant weeds
• Expose soil to soil erosion
• Require high skill in measurement
• Pollute the environment

Precautions  when using herbicides

• Read the instructions carefully before using the chemicals.
• Wear protective gears such as gloves, rubber boots, and overalls.
• Wash your hands clean after using the chemicals and before eating
• Keep the chemicals out of reach of children
• Dispose of empty containers safely from the reach of children and animals.
• Spray crops in the direction of wind

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Dr. Bbosa Science