Agriculture Form 3 – Weeds and Weed Control

Introduction

• Weeds cause heavy crop losses if not controlled effectively.
• Controlling weeds is important to maintain high quality and quantity of produce.

Definition:

• A weed is any plant growing where it is not required.
• It is a plant whose economic disadvantages outweigh its advantages.

Harmful Effects of Weeds

• Weeds compete with crops for nutrients, space, light, and soil moisture, reducing crop yields.
• Some weeds, such as Striga spp, are parasitic to cultivated crops like maize, causing severe damage.
• Some weeds lower the quality of agricultural produce, for example:
  • Mexican marigold imparts an undesirable flavor to milk if dairy cows feed on it.
  • Devil’s horsewhip, black jack, bristly foxtail, and others attach to sheep wool, lowering its quality.
• Some weeds are poisonous to humans and livestock, for example:
  • Thorn apple (Datura stramonium)
  • Sodom apple (Solanum incanum)
• Some weeds have allelopathic effects on cultivated crops, releasing chemicals that inhibit crop growth.
• Water weeds block irrigation channels, disrupting water flow.
• Aquatic weeds such as Salvinia in Lake Naivasha and water hyacinth in Lake Victoria negatively affect fishing activities.
• Some weeds serve as alternate hosts for insects, pests, and disease-causing organisms, for example:
  • Wild oat (Avena fatua) is an alternate host for rust diseases.
  • Mallow (Malva verticillata) is an alternate host for cotton stainers.
• Weeds lower the quality of pasture, for example:
  • Tickberry (Lantana camara)
  • Nut grass (Cyperus rotundus)
  • Manyatta grass (Eleusine jaegeri)
• Some weeds irritate workers, reducing the efficiency of weed control, for example:
  • Double thorn (Oxygonum sinuatum)
  • Stinging nettle (Urtica massaica)
  • Devil’s horsewhip (Achyranthes aspera)

Factors Contributing to the Competitive Ability of Weeds

• Weeds produce large quantities of seeds, for example pigweed and black jack, increasing their spread.
• Their seeds remain viable in the soil for long periods, waiting for favorable germination conditions.
• They have effective seed dispersal mechanisms, enabling wide distribution.
• Some weeds propagate through elaborate underground storage structures such as tubers and rhizomes.
• They efficiently utilize limited moisture, nutrients, and sunlight, outcompeting crops.
• Some weeds have short life cycles, allowing rapid reproduction.
• They possess elaborate root systems that support the plant and absorb nutrients and water effectively.

Weed Classification

It is based on:

• Life cycles, for example:
  • Annuals – complete their life cycle in only one season.
  • Biennials – complete their life cycles in two seasons only.
• Perennials – complete their life cycle in more than two seasons, often persisting for many years.
• Morphology – based on leaf formation such as size, shape, and venation.
  • Broad-leaved weeds, for example black jack, lantana, pigweed, oxalis, and others.
  • Narrow-leaved weeds, for example couch grass, setaria, nutgrass, manyatta grass, and others.
• Habitat – some weeds are terrestrial (grow on land) while others are aquatic (grow in water or marine conditions).

Weed Identification

• Weeds are identified by their common (individual) names and botanical names.
• They are named according to specific features or after the person who identified them.

Common Name