WEEDS AND WEED CONTROL

Introduction

  • Weeds cause heavy crop losses if not controlled.
  • Their control is important so as to maintain high quality and quantity produce.

Definition:

  • A weed is any plant growing where it is not required.
  • And whose economic disadvantages outweigh the advantages.

Harmful Effects of Weeds

  • Weeds compete with crops for nutrients, space, light and soil moisture.
  • Some weeds, for example, Striga spp are parasitic to cultivated crops such as maize.
  • Some weeds lower the quality of agricultural produce for example:
  • Mexican marigold gives undesirable flavour to milk if dairy cows feed on it.
  • Devils horsewhip, black jack, bristly fox-tail and others get attached to sheep wool thus lowering its quality.
  • Some weeds are poisonous to human beings and livestock for example:
  • Thorn apple (Datura stramonium)
  • Sodom apple (Solanum incanum)
  • Some weeds have allelopathic effects to cultivated crops.
  • Water weeds block irrigation channels.
  • Aquatic weeds such as Salvinia in Lake Naivasha and water hyacinth in Lake Victoria affect fishing.
  • Some weeds are alternate hosts for insects, pests and disease causing organisms for example:
  • Wild oat (avena fatua) is an alternate host for rusts.
  • 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 thus reducing the efficiency in which they are controlled for example:
  • Double thorn (Oxygonum sinuatum),
  • Stinging nettle (Urtica massaica) ,
  • Devil’s horse whip (Achyranthes aspera).

Factors Contributing to the Competitive Ability of Weeds

  • They produce large quantities of seeds for example pigweed and black jack.
  • Their seeds remain viable in the soil for a long time awaiting conducive germination conditions.
  • They have effective seed dispersal mechanisms.
  • Some weeds propagate by means of elaborate underground storage structures.
  • They are efficient in utilizing little moisture, nutrients and sunlight.
  • Some have short life cycles.
  • They have elaborate root systems for supporting the plant and absorbing nutrients and water.

Weed Classification

 It is based on:

  • Life cycles for example:
  • Annuals – complete their life cycle in only one season.
  • Biennuals complete their life cycles in two seasons o
  • Perennials – complete their life cycle in more than two seasons.
  •  Morphology – leaf formation such as size, shape and venation.
  • Broad leaved weeds for example black jack, lantana, pig weed, oxalis and others.
  • Narrow leaved weeds for example couch grass, setaria, nut-grass, manyatta grass and others.
  • Habitat some weeds are terrestrial (grow on land) while others are aquatic (grow on aquatic/marine conditions).

Weed Identification

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

 

Common Name                               Botanical Name

  • Black Jack  ………………………Bidens pilosa
  • Mexican marigold ………………Tagetes minuta
  • 0xalislsorrel ……………………..Oxalis spp.
  • Double thorn ……………………Oxygonum sinuatum
  • Thorn apple ……………………..Datura stramonium
  • Couch grass ……………………..Digitaria scalarum
  • Nut grass ………………………..Cyperus rotundus
  • Wandering Jew …………………Commelina bengalensis
  • Sow thistle ………………………Sonchus ole race us
  • Devil’s horsewhip ……………….Achyranthes aspera
  • Macdonald’s eye/ Gallant soldier. Gallinsoga parviflora
  • Sodom apple …………………….Solanum incanum
  • Black night shade ………………..Solanum nigrum
  • Chinese lantern…………………. Nicandra physalodes
  • Bracken fern ……………………..Pteridium aquillium
  • Love grass/ Bristly foxtail ……….Setaria verticillata
  • Cleavers …………………………Gallium spurium
  • Stinging Nettle …………………..Urtica massaica
  • Fat hen/Goose foot ………………Chenopodium spp.
  • Rape weed ………………………Brassica nap us
  • Wild oats …………………………Avena fatua
  • Lantana/Tick berry ………………Lantana camara
  • Water hyacinth ………………….Eichhornia crassipes
  • Striga/Witch weed ………………Striga hermontheca
  • Creeping indigo ………………….Indigofera spicata

Weed Control Methods

     The methods of weed control determined by:

  • The weed being controlled.
  • Weather conditions.
  • Capital availability.
  • Effect on environment.

 

METHODS OF WEED CONTROL INCLUDE:

Chemical Weed Control

  • The use of chemicals known as herbicides to control weed

Classification of Herbicides

Based on:

Formulation – the physical form of the herbicides for example:

  • Liquids
  • Wettable powders
  • Emulsion
  • Dust

Time of Application

  • Pre-emergence – applied before the planted crop germinates.
  • Post emergence – applied after the planted crop germinates.

Selectivity

  • Selectiv
  • Non selective.

Mode of Action

  • Contact – herbicides that kill only the parts of the plant which it comes into contact.
  • Translocated systemic herbicides that will kill the whole plant even if it comes into contact with only a small part of it.

Methods of Herbicide Application

  • Spraying – application of solutions.
  • Dusting – application of dusts.
  • Fumigation – application of fumigants into the soil.

Safety Measures in the Use of Chemicals

  • Read manufacturer’s instructions and follow them.
  • Wear protective clothing such as overalls, breathing mask, gloves and boots.
  • Avoid inhaling the herbicides.
  • Wash thoroughly after handling chemicals.
  • Do not blow or such blocked nozzles.
  • Avoid herbicide drift to unintended crops and other plants.
  • Avoid herbicide drift to livestock feed and water.
  • Avoid spilling herbicides in pastures and fodder crops.
  • Dispose off the empty containers properly for example burying them in the soil.
  • Do not wash spraying equipment in water sources used by animals and human beings.
  • Store chemicals in a safe place.
  • Wash the spraying equipment thoroughly.
  • Sink left over chemicals into the soil after the day’s work.

 

Advantages of Chemical Weed Control

  •  It is less laborious.
  • Effective in the control of difficult weeds such as couch grass and sedges.
  • It does not disturb crop roots and other underground structures.
  • It makes the control of weeds in certain crop easier.
  • It is efficient in both wet and dry conditions.
  • It does not destroy soil structure.
  • Cheaper in large scale production than the use of manual or mechanical cultivation.

Disadvantages of Chemical Weed Control

  • It requires skilled labour in mixing and application.
  • Cause environmental pollution.
  • Herbicides have long residual effects which may interfere with future crops.
  • It is very expensive.

 

Mechanical Weed Control

  • It involves the following operations:

Tillage/Cultivation

  • This is the opening and loosening up of the soil.
  • It can be done by hand tools or tractor drawn implements.

Advantages

  • Cheap in small scale production.
  • Increases water and air infiltration into the soil.
  • Incorporates crops residues into the soil.
  • The earthing-up done during tillage encourages root growth.

Disadvantages

  • If done repeatedly it destroys soil structure.
  • It is laborious and expensive in large scale production.
  • It may not effectively control weeds.
  • It may lead to soil erosion and loss of soil moisture.
  • Damage crop roots.

Slashing/Mowing

  • Mechanical removal of shoots from weeds.
  • It is effective in the control of annual weeds.

Uprooting

  • It is done when the crops are too close
  • To allow mechanical cultivation or where weeds are scattered.

Cultural Weed Control

It involves the following practices:

  • Mulching.
  • Cover cropping.
  • Crop rotation.
  • Timely planting.
  • Use of clean seed/planting materials.
  • Proper spacing.
  • Proper seedbed preparation.
  • Flooding.

Biological Weed Control

  • The use of living organisms to control weeds.

     Examples are:

  • Use of livestock to graze and control growth of weeds especially in plantations.
  • Use of weed eating fish to control aquatic weeds.
  • Use of moths to control cactus.

Limitations: the method is not reliable.

 

Legislative Weed Control/ Quarantine

  • It involves government laws and regulations which prevent the introduction and spread of foreign weeds in a country or an area. Done by KEPHIS.

Limitations: Only samples are checked while the bulk of the materials may have some weed seeds.

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