Control of weeds, pests and disease

Control of weeds, pests and disease

Weeds, pests and diseases that negatively affect crops have to be controlled by the farmer. Control will ensure that yield is not reduced. Two main methods of control are:

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  cultural control
  
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  chemical control
  

Cultural control

This has evolved from traditional methods of cultivation. Cultural control is preventative but requires long-term planning. Examples of cultural means include:

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  Ploughing – this buries perennial weeds so they die and decompose.
  
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  Time of sowing – sow when annual weeds are in dormant seeds and perennial weeds have been ploughed.
  
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  Critical time for removal of weeds – weeds removed early in life of crop allows the crop to become stronger.
  
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  Removal of alternative hosts – alternative hosts of pests can be weeds at the edge of fields, removing these controls pests. However this can reduce wildlife.
  
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  Destruction of crop residue – some pathogen spores survive on straw or stubble left over after harvest.
  
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  Cover crop – grown on fallow land, these crops compete successfully with weeds.
  
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  Crop rotation – a series of different crops are grown on the same piece of land over time. The current crop will not have the same pests as the next crop (see image below).
  

Chemical control

Farmers will also use chemical methods to improve crop yield as traditional methods alone are not enough.

Farmers can use pesticides that include:

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  Herbicides - to kill weeds
  
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  Fungicides - to kill fungal parasites
  
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  Molluscicides - to kill mollusc pests
  
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  Nematicides - to kill nematode pests
  

These chemicals act in selective or systemic ways.

Types of herbicide	How the herbicide acts
Selective herbicide	Have a greater effect on certain plant species for example broad leaved weeds. They mimic plant growth hormones. Therefore they increase the growth and metabolism of weeds. This happens to such an extent that the weeds exhaust their food supply and die. Crop plants absorb little of the chemical. Selective herbicides are biodegradable and do not cause harm to the soil.
Systemic herbicide	Systemic herbicides enter the weeds vascular system. The chemical is transported to all parts of the weed, which has a lethal effect. Systemic herbicides reach and kill underground organs and roots preventing regrowth.

Systemic insecticides, molluscicides and nematicides spread through the vascular system of the plants and kill pests that are feeding on the plants.