1. crop 301 weeds as pests

8/4/2019 1. CROP 301 Weeds as Pests

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CONCEPT OF PESTS:Pests are organisms that are nuisance ordamaging to i/ Crops ii/ Livestockiii/ Humans and/or iv/ Land fertilityPART III: WEEDS AS PESTSConcept of Weeds as pests: Weeds are plantsgrowing where they are not wanted. Theyinterfere with desirable associated crop plants.Weeds are not as spectacular as plant diseasesand insects in their manner of crop destruction,but they cost farmers as much as both insectsand diseases combined.


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i/ These three kinds of pests together costdamages to agriculture. They can operateseparately or dependentlyii/ Weeds are members of the plant kingdom.They are recognized worldwide as undesirableeconomic pests.

iii/ However, the value of any plant as weed isdetermined basically by the perceptions of itsvaluers. Plants considered weeds in someregions may not in others.


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v/ The combined effect of all negative plantinteractions is called negative interference.

vi/ Between weeds and crops the interference

includes competition, allelopathy and parasitism


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v/ Many weeds are also hosts for disease-causing organisms e.g. some of the fungaldiseases that infect food crops spent part oftheir life cycles on weeds that typically grownear the crops.

vi/ Many weeds can poison livestock if eatenduring grazing.


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Weeds are plants that originated under a naturalenvironment and, in response to imposed or naturalconditions, are now interfering associates of crops andhuman activity.

Interference To have undesirable effects either todelay, hinder, or obstruct the natural or desired causeof growing crop plants.

The interference can be inform of :a/ Competition, b/ Allelopathy,c/ Parasitism and d/Epiphytic behavior


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b/ Allelopathy: It is a type of negativeinterference that occurs when one plantproduces and releases chemicals into

the environments (air and soil) whichare deleterious to the growth anddevelopment of another plant.


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Direct losses include:i/ Reduction in yield due to competition byweeds with crop plants.Interference with crop growth by competing foressential resources (e.g. light, water, mineralnutrients, and space) for growth and

development. These result in yield losses.


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Weeds may delay drying of grains Slow down harvest operationsDelay in harvest can result in increased post-harvest losses as pests and diseaseinfestations may set in.

iv/ Losses of livestock due to poisonous weedsor chemicals used in weed control.


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the end

Classification of Weeds


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1. Germination requirements fulfilled in manyenvironments

Such requirements include:

a) Water

b) Oxygenc) Temperature

d) Light


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3. Dual modes of reproduction by seed andby vegetative reproduction.

4. Discontinuous germination (internallycontrolled) and great longevity of seed.

Dormant seeds:

Innate dormancy: Seed is dormant whenshed from the plant. Could be due togenetic control, immature seed embryo,not fully developed when shed from theplant.

Induced dormancy: Seed developsdormancy after exposure to environmentalstimulus eg. Temperature extremes,drought elevated.


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5. Continuous seed production for as long asgrowing conditions permit.

6. Self compatibility.

7. Weeds seed mimicry.

8. Cross-pollination by unspecialized visitors orwind.

9. Very high seed output in favourableenvironmental circumstances.


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10. Special adapted seed dispersal mechanism Adaptations for short and long distance

dispersal.Agents for dispersal:Wind, water, animals, man, explosive

mechanism

11. Production of some seed in wide range ofenvironmental conditions: tolerance andplasticity.

Tolerance: Ability to evolve differentstrategies for exploiting environmentalresources.


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Plasticity: It is the capacity for phenotypicadjustment in morphological characteristicsand physiological responses under charging

conditions.

12. Vigorous vegetative reproduction ofregeneration from fragments (perennial) eg.

stolons, rhizomes, tubers, bulbs, corms,cormels, stems, roots

13. Brittleness hence not easily drawn from soil.


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14. Parasitism Root/stem parasitic weeds

Root parasitic weeds are obligate parasites because

because require a host for the chemical stimulant

necessary for germination.

Stem parasitic weeds are hemiparasites.

They attach themselves to the stem of host plantsby means of haustoria and draw minerals and water

from the host plants.


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15. Adaptations that repel grazing such asspines or chemicals that impart bad odour toherbivores.

16. Difficult to control and found everywhere.

1. crop 301 weeds as pests

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