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.