Cotton

Cotton is an important vegetable fibre grown in the tropics and is principally used in the production of textiles. 

Current estimates for world production are about 25 million tons annually, accounting for 2.5% of the world’s arable land. China is the world’s largest producer of cotton, but most of this is used domestically. The United States has been the largest exporter for many years.  Developing world countries that grow cotton include Sudan, Egypt, Nigeria, South Africa and Ghana. 

Cotton is frequently grown and purchased under marketing boards.  This enables the growers to purchase seed and fertilizer on credit, but means a fixed price for the end crop. It also gives a lot of power to the marketing boards and they can (as in the case in Ghana) ruin a sector of the market economy by refusing to pay a decent price for the end crops.

Cotton is processed in a ginnery where the lint is separated from the seed and then compressed into bales for sale. Seeds are pressed to extract oil which is used in the manufacture of margarine, salad oil and soap.  The residue from the oil extraction is used as cattle food.

Varieties of commercial cotton 

Meko cotton – produces short staple and thrives in the wetter areas

Ishan cotton – produces longer, silkier and better quality lint than Meko variety. Needs relatively less rainfall.

Allen’s Longstaple  -Produces superior fibre and is grown for export .It withstands the dry season well and is resistant to several bacterial diseases and insect pests.

Sea Island Cotton – This provides most of the quality cotton sold on the world markets.

Place in crop rotation

Cotton is a cleaning crop as a lot of weeding has to be done which brings the land into good condition before the following year’s crop. This is why it is usually the first crop in a rotation.  The second crop should be a legume to fix the nitrogen which reduces the amount of artificial fertilizer needed in subsequent cotton years.

A typical rotation might be like this:

   Year Crop
A 1 Cotton
  2 Cowpeas or soya bean
  3 Maize, interplanted with beans
      
B 1 Cotton
  2 Sorghum and maize
  3 Cowpeas
      
C 1 Cotton
  2 Millet and cowpeas
  3 Groundnuts
  4 Sweet Potatoes

 

In addition to crop rotations, it is not uncommon to interplant with maize, millet or in later years, yams. This may reduce yields from the yams.

 

Cultivation and plant spacing

The land should be deeply cultivated before sowing as it helps the tap roots penetrate the soil more easily.  Cotton is generally grown on ridges spaced 75cm, 90cm or 120cm apart. In low rainfall areas, the crop can be grown on raised beds or on the flat if required.

Seeds should be de-linted by treating with concentrated sulphuric acid and then dressed with copper sulphate. In agriculture, a seed treatment or seed dressing is a chemical, typically antimicrobial or fungidal, with which seeds are treated (or “dressed”) prior to planting.

Sowing should commence at the beginning of the rainy season as soon as the soil is moist. Three to four seeds should be sown 2.5cm deep with 45cm between plants.  The seed rate is approximately 44kg/ha.

 

Weeding

Frequent weeding should begin as soon as the seedlings are established.  Plants should be thinned to 2 plants per stand at the first weeding.  Once the flowers have appeared,  weeding needs to be done with care to prevent damage to the plant roots.

 

Fertilizer

Cotton does not require fertiliser if it is growing on good soil.  An application of farmyard manure and super phosphate at 50kg/ha should be used with the seed when the soils are poorer.  Nitrogen fertilizer is not recommended as this encourages excess leaf growth.

 

Pests and diseases      

Cotton is beset by a large number of pests and diseases that prevent the plants from producing clean open bolls

 Bollworm

Bollworms damage the plant by eating the inside of the bolls and by getting in and out of the bolls by the tiny bore holes they bore. Bollworm damage may kill the boll so it goes hard and brown and many buds and bolls fall to the ground before they are developed. Many bolls fail to open and the cotton becomes stained and worthless.

Historically, in North America, one of the most economically destructive pests in cotton production has been the boll weevil. Due to the US Department of Agriculture’s highly successful Boll Weevil Eradication Program (BWEP), this pest has been eliminated from cotton in most of the United States. This program, along with the introduction of genetically engineered Bt cotton (which contains a bacterial gene that codes for a plant-produced protein that is toxic to a number of pests such as cotton bollworm and pink bollworm), has allowed a reduction in the use of synthetic insecticides.

In other parts of the world, control is by means of a season close.  After harvest, all the debris is gathered up and burned.  This enforced by legislation in some countries that there must be no cotton left in the ground or unburnt between the end of harvest and the start of planting time. This is the close season.  Other methods of control are by hand picking the caterpillars and destruction of severely infected plants.   Routine insecticide spraying is also used.

Other pests

Other significant global pests of cotton include the pink bollworm, the chili thrips,; the cotton seed bug,  the tarnish plant bug,; and the fall armyworm.

Bacterial Blight

This disease causes blackarm – dark green or black lesions appear on the leaves, peticoles (where the leaf meets the stem) and even the bolls.  The bolls often rot on the inside. Control is by planting resistant varieties, apply strict rotation and dressing seed with copper.

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Written by Fiona Johnson

February 2012