Agricultural Science


Weed is any plant that grows in a place where it is not expected to grow or a plant which grows out of place. Weed can also be defined as a plant that grows where it is not desired in such a way that it constitutes nuisance either to human, livestock or crops

All farmers have to work hard to prevent their crops from being invaded by weed. Losses in crop yield due to weeds may be greater than those due to plant pest and diseases. If a farmer fails to control weed growth, the chances of successful crop production are seriously reduced.


The harm done by wed is due mainly to their competition with crops. The effects or economic importance of weeds include

  1. Weeds compete with crops for space: when a young plant is developing, weeds rapidly crowd and smother the crop and the space needed for healthy is reduced.
  2. Weeds compete crops for sunlight: weeds are capable of producing many large leaves which shade crops seedlings and therefore reduce the amount of sunlight which is available for photosynthesis.
  3. Weeds compete with crops for Nutrients: the root of weeds rapidly develop and remove considerable proportion of the available plant nutrients thereby reducing the healthy growth of the plants.
  4. Weeds harbor Crop Pest and Diseases:  some weeds may have habour crop pest and diseases by acting as a host, i.e. providing the insect food when crops are not available.
  5. Weeds compete with crops for soil moisture: their root is capable of developing fast and removing a large portion of the soil moisture required for the normal growth of the plant.
  6. Losses in crop yield : the combined competition of weed and crops eventually will result in low yield.
  7. Weeds compete with crops for soil Oxygen: weeds are capable of developing long root which remove considerable amount of oxygen required for crop root respiration .
  8. Losses in quality of crops : the quality of harvested crops will reduce and this will affect market values of such crops .
  9. Losses in income of the farmer : the low yield coupled with the reduction in the quality of produce , and increase in the cost of controlling the weeds consequently leads to losses in the income of the farmer .
  10. Reduction in the palatability of some grasses : weeds also invade pastures ,reducing the palatability of useful grasses.
  11. Some weeds are toxic to farm animals : the leaves and stem of some weeds contain toxins which may be dangerous to farm animals .

However some weeds are still useful to the farmer in the following ways :

  1. Some weeds can acts as cover crops
  2. Some are used as forage grasses and legumes for feeding farm animals
  3. Some are used to prepare compost mad green manure
  4. Some weeds can help to control soil erosion

Characteristics and growth habit of selected weeds

Weeds often possess characteristics which increase their growth habit and mode of dispersal. Factors which aid the growth habits of weeds include :

  1. Production of large quality of seeds
  2. Some weeds reproduce by seeds
  3. Some weeds reproduce by vegetation propagation
  4. Growth of large leaves within a short time
  5. Weeds growth is favoured during the rainy season
  6. Most weeds are wind-pollinated
  7. Production of large quantities of pollen grains

Dispersal of weeds seeds and fruits

Most seeds and fruits of weeds are generally dispersed by wind and animals.

Mode of Dispersal of weeds includes :

  1. Dispersal of seeds by winds
  2. Dispersal of seeds by animals
  3. Dispersal of seeds by water
  4. Dispersal of seeds by explosive mechanism

Common /types of weeds found on the farmland

Common NamesBotanical Names
Guinea grassPanicum maximum
Bahama grassCynodon dactylon
Carpet grassAxonopus compressus
Sensitive plantMimosa pundica
 Spear grassImperiata cylindrical
Wire grassAspilia africana
CentroCentrosema pubescens
Blue featherCommenlina nudiflora
Stubborn grassEleucine indica
Stubborn weedSida acuta
Goat weedAgeratum conizoids
Elephant grassPenisetum purpureum
Giant star grassCynodon plactostachynum
Pig weedBoerhavia diffusa
Bush greenAmaranthus spinosus
Water leafTalinum triangulare
Sedge plantCyperus rotandus
TridaxTridax procumbens
CalopoCalopogonium mucunoides
Northern gambaAndropogon gayanus
Sun hempCrotalaria juncea
MucunaMucuna utilis
StyloStylosanthes gracilis
Southern gambaAndropogon tectorum
Kudzu or pueroPueraria phaseoloides

General control of weeds, grasses and legumes

Weeds can be controlled through a number of methods

  • Mechanical /physical method : the mechanical methods of controlling weeds are :
    1. Hand pulling /hand picking: this involves uprooting of weeds from the soil by hand.
    2. Hoeing : this involves uprooting weeds from the soil by using hoe
  • Slashing : through this method, the Arial part of the weeds are removed with cutlass leaving behind the root in the soil
  1. Rotary cultivation: this involves using hand drawn implements to undertake the ploughing and burying of weeds.
  • Biological control : this involves the introduction of some insects and herbivores animals to farmland to eat up the leaves of these weeds
  • Cultural control: this involves the practice of crop rotation and the growth of cover crops to suppress the growth of weeds on the farmland.

The cultural methods of weed control are

  1. Flooding
  2. Burning
  • Mulching
  1. Cover cropping
  2. Crop rotation
  • Chemical control: this involves the use of chemical solutions called herbicides to control the growth of weeds.

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