Weeds and Weed Control - Agriculture Form 3 Notes

Introduction

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

Weeds

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
Oxalislsorrel ……………………..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 weeds.

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
- Selective
- 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.