Cropping or Cultural Methods of Weed Control
Use of cropping methods alone is not effective for control of weeds. However, these methods are useful for minimizing the weed population and weed and weed crop competition to some extent. These methods need suitable combination of mechanical and chemical methods for effective control of weeds.
i) Crop Rotation:
Many weed problems exist with us because of practicing monoculture i.e growing of same crop year after year in the same field. Monoculture leads to increase the infestation of certain crop associated weeds.
Phalaris minor and chenopodium album
Maize , Pearl millet and Sugarcane
The proper crop rotations or inclusion of pulse crops in rotation is helpful for control of crop associated and parasitic weeds. E .g Striga on Jawar or Orobanche crops like groundnut, soybean, cotton etc in the crop rotation.
A quick growing dense crop can successfully complete with weeds. Past growing and fast shading crop is called smother crop. Sweet potatoes, Lucerne, soybean, sun hemp, etc are good smother crops. These crops cover the field within short period due to higher plant density, fast growth and canopy coverage suppress weeds by fast shading effect and also make the under ground parts of weed feeble. The smother crops are sown at close spacing with high plant population to suppress weeds.
In this method the soil temperature is further increased by 5 to 10 0 c by covering a pre-soaking fallow field with transparent plastic sheet. In some regions the technique raised the soil temperature to as high as 56 0 c which was sufficient to kill vegetative propagation. This techniques followed for four weeks or longer period which reduced weed seed viability by 80 % or more. Israel has so far made maximum use of this technique as a part of integrated weed management. However, this is costly technique.
iv) Use of Fertilizers or Selective Crop Stimulation:
Many crops get fillip from the fertilizers applied before sowings or as top dressing. The band application ( near to crop roots) of nitrogen for cereals, sugarcane and Sugarbeet etc is said to result in their vigorous growth that carries them beyond weed competition. Some fertilizers like calcium Cyanamid and ammoniums sulphate directly destroy the delicate weeds.
v) Summer Tillage or Summer Fallow:
Deep ploughing after harvest of the rabi crop and exposing underground part of weeds to strong sunlight during summer months is helpful for destroying many annual and perennial weeds.
vi) Proper Crop Stand and Narrow Spacing:
Weed crop competition is more in gapy and lower plant population. Therefore, practice like use of good quality seed, proper method of sowing, optimum seed rate, spacing and time of sowing and plant protection measures against insect pests and diseases are very important to obtain proper and uniform stand of crop, capable of offering initial competition to young weeds. Higher plant density and narrow spacing is useful to minimize weed crop competition than the lower plant density and wider spacing.
vii) Lowering Area under Bunds:
Reduction in area under bund and water channels of crop fields is essential to minimize weed infestation as weeds on bunds and channels are often ignored by the farmers since they do not complete directly with crops.
viii) State Seed Bed:
Stale seed bed is one where initial 1 to 2 flushes of weeds are destroyed by harrowing before planting or sowing of the crop. This is achieved by soaking a well prepared field with either irrigation or after receiving rain and allowing the weeds to germinate. These newly emerged weeds are destroyed by harrow with spike tooth or blade harrow. This should be followed immediately by sowing the crop. Non selective herbicides like Glyphospahate or paraquat can be used to destroy weeds instead of harrowing or paraquat can be used to destroy weeds instead of harrowing or light tillage.
Many a time intercropping with crops like soybean , cowpea, red gram, green gram , groundnut etc. helps in suppression of secondary growth of weeds e.g Intercropping of soybean or cow pea in sorghum or pearl millet is helpful to suppress secondary growth of weeds as the inter row space is covered by intercrop at latter stage of crop growth. In this method at initial stage of crop growth it is necessary to control the weeds by mechanical or chemical methods.