Cotton

Cotton is a shrub from the family Malvaceae. It grows on every continent, as there are many varieties suited to a range of climates, and many ways of growing it. Cotton plants are cultivated for their fruits, known as “bolls”. Once ripe, the bolls split to reveal a small white ball: cotton. The raw product is know as seed cotton, which is ginned to separate the fibre from the seeds. The fibre is spun to make yarn and woven to make fabric. The seeds provide oil and presscake.

Cotton, white gold from the Americas

Cotton, Gossypium , is a shrub from the family Malvaceae, which includes hollyhocks, mallow, hibiscus, okra, and cocoa. The genus Gossypium includes 50 cotton species, four of which have been domesticated for the fibre produced by their seeds. Two species that originated in the Americas now account for most of the cotton produced worldwide: Gossypium hirsutum, from Mexico, which produces some 90%, and Gossypium barbadense, from Barbados, which produces the best fibre but only accounts for around 5% of the total. The other two species originated in the Old World: Gossypium herbaceum, from southern Africa, and Gossypium arboreum, from India, account for around 5% of global output. Their shorter, thicker fibres are generally used by local spinners and weavers.

Cotton plants flower as they grow, and are thus said to have a continuous growth cycle. This means that at any given time, a plant may bear buds, flowers and fruits, known as “bolls”, which contain the seeds. The bolls split to reveal a small white ball of fibre.

Fibre, a living cell

Cotton fibre is a living cell, with outer walls surrounding cytoplasm. The walls are made up of several layers of microscopic fibres: cellulose microfibrils.

Before the pod splits, the fibres dry out and die: the tube loses its contents and the walls become flattened and twisted. In its raw state, the fibre is an almost pure cellulose sheath with a fine coat of wax that makes it waterproof (or hydrophobic). Under a microscope, it looks like a long, thin corkscrew.

Cotton fibre is naturally either white or coloured. There are varieties that produce brown, khaki, yellow and greyish-green fibre. The American Indians commonly used coloured cottons, while in industrialized countries, the development of chemical dyes boosted demand for white fibre.

Cotton is grown on every continent

If cotton is grown on every continent, it is because there are many varieties suitable for a range of climates, and also many ways of growing it. With irrigation, cotton can grow in the desert, for instance in Arizona and Uzbekistan. Cotton plants need heat (25-35° for 150 days), a lot of sunshine, and water, particularly during flowering.

In many African countries, it is grown as a rainfed crop, ie without irrigation, and requires considerable labour. Cropping operations are done by hand or using draught animals, and the cotton is always picked by hand.

In the main producing countries, cotton is generally irrigated, and operations are entirely mechanized. The crop thus requires little in the way of labour and is given huge quantities of pesticides and fertilizer.

On a global scale, 55% of the area planted with cotton is irrigated, supplying three quarters of the world crop: 30% is irrigated in India, 43% in the United States and 75% in China.

From sowing to picking

Cotton is sown in rows. On non-mechanized farms, farmers dig small holes where they place the seeds. When a seed germinates, a seedling appears; this is known as emergence. The seedling then stops growing for a few days to allow the roots to become established and supply it with water. After a month, the seedling has four fully open leaves and is around 15 centimetres tall. At this point, the farmer pulls up all but the best two seedlings in each hole; this is known as “thinning”.

Around fifty days after germination, the first flowers become fruits, or bolls. The bolls ripen and then split, one by one, revealing the fibre inside. Each boll contains around thirty seeds, each of which is surrounded by very fine hairs: cotton fibre. This is why the picked product is called “seed cotton”. On average, it comprises 55% seeds, 40% fibre and 5% trash. From sowing to picking takes between 140 and 230 days, depending on the cotton variety.

Picking

Hand-picking is very labour-intensive. In West and Central Africa, for instance, whole families are involved, as the farms in the region are generally small and only grow cotton. Children, parents and grandparents pick the crop in successive rounds as the bolls split. A picker can harvest 50 to 80 kilos of seed cotton a day. Hand-picking results in clean fibre, with no undesirable crop waste.

In Brazil, on the other hand, where cotton is picked mechanically, farmers count the bolls in their fields. Once 80% of the bolls have split, they spray the field with a product known as ripener, which, as its name suggests, speeds up ripening. They then spray it with a defoliant that makes the leaves fall off so as to facilitate mechanical harvesting. Once 95% of the bolls have split, the crop is ready for picking, and the cotton pickers are brought out. In the United States and Europe, like in Brazil, mechanical pickers are used. A cotton picker can harvest 800 kilos of seed cotton an hour. Mechanical picking has the disadvantage of collecting bits of bolls, twigs and leaves at the same time as the seed cotton, and more subsequent cleaning is required than with hand-picked cotton.

Cotton has to cope with many enemies

Diseases caused by viruses, bacteria or fungi disrupt cotton plant growth or destroy the bolls. There are also insects that eat the leaves and bolls or attack the roots.

The damage caused is often substantial. It results in crop losses and deterioration of the fibre, which can no longer be used for spinning.

There are 1300 insect and animal species that feed on cotton, including almost 500 in Africa alone! The most common include:

the boll weevil (Anthonomus grandis), which caused a major cotton production crisis in the United States in the late 19th century. Thousands of dollars were spent to eradicate it in certain southern States.
the bollworm Diparopsis watersi, which as its name suggests, attacks the bolls. It is one of the most dangerous insects for cotton.
the aphid Aphis gossypii, which sucks the sap and weakens the plant. It also damages the fibre with its sugary excrement, known as honeydew. If honeydew contaminates the fibre in open bolls, it makes it sticky and thus difficult to spin. Sucking insects also carry viral diseases such “blue disease”.
 

From chemicals to integrated pest management

Chemicals were long seen as a universal solution to the problems posed by insects, but some insects have become resistant. Despite that, some countries still spray their cotton crops liberally, with up to 20 spraying rounds a year! Until recently, cotton growing accounted for 25% of global insecticide sales.

However, growers in many producing countries are now switching to "integrated" pest management, combining several techniques so as to cut pesticide use. This also accounts for the success of genetically modified cotton plants, which were developed for their resistance to bollworms. The best known and most widely grown GM cotton varieties produce a protein that kills bollworms, enabling farmers to cut their chemical insecticide use. In China, GM varieties have helped to revive the cotton sector, enabling a two-thirds reduction in insecticide use.

From raw cotton, through yarn, to fabric

After picking, the seed cotton is taken by truck to the ginning plant. It is cleaned by machines that remove any large impurities (leaves, twigs or bolls). It is then ginned, ie the fibre is separated from the seeds. The fibre is cleaned by machines that remove any smaller impurities. By the end of the line, the cotton fibre is clean. It is then compacted and compressed into 225-kilo bales. Modern ginning plants produce a bale a minute, or more than 1000 bales a day!

Spinning consists in making a linear textile from clumps of cotton fibre delivered as bales of various origins. The tangled mass of cotton fibre is spun to make a highly structured yarn. The fibre is first prepared, ie cleaned, untangled and separated, followed by several other, successive operations:

• carding: the fibres are separated from mineral or organic non-fibre elements and bunched together in long slivers.
• drawing out: the fibres in each sliver are combed, and several slivers are combined to make a rope, which is still relatively fragile.
• spinning proper: yarn is obtained by drawing out the rope and twisting its multitude of fibres. The fact of twisting the fibres into a spiral and the wax they contain give the yarn its cohesion and strength. How can such long, strong yarn be made from such short fibres? There are between 100 and 250 cotton fibres from 1 to 3 centimetres long in the thickness of a cotton thread. It takes just 20 grams of fibre to make a kilometre of fine yarn.
• weaving and knitting: weaving produces stronger fabrics. Knitting gives stretchier, softer and breathable fabrics (t-shirts, socks). Industrial looms can produce 500 metres of fabric a day.
• Special treatments can make cotton look or feel different (satiny), or render it stain-repellent, antibacterial, anti-UV, creaseproof, waterproof, fire resistant, etc. This can also be done by blending cotton with synthetic fibres with the appropriate properties.

Fabric and knits may also be specially treated, depending on their end use:

singeing: the fabric is superficially burnt to remove any small fibres that may be sticking out and stop it from pilling.
napping: conversely, it can be scraped for a “peach skin” effect.

boiling off and bleaching: the fabric is placed in a bath of water and caustic soda, which removes the natural wax, swells the fibres and makes the fabric absorbent. A second bath of hydrogen peroxide bleachnes the fabric so it will “take” dye better.

mercerizing: yarn or fabric can be soaked in an alkaline bath. This makes the fibres swell and gives them a sheen. It also increases their dye retention capacity. Mercerized fabrics are more compact and stronger than non-mercerized ones.

dyeing (soaking the fabric in a dye bath) or printing (stamping a monochrome or multicoloured pattern on the fabric), to make fabric more attractive and decorative. Yarn may be dyed directly, before weaving or knitting, to produce special effects (stripes, checks, etc).

The main uses of cotton yarn are ready-made clothing (60%), furnishings (35%), and work garments (5%). Cotton is also used in medical and hygiene products, notably cotton wool, compresses, gauze bandages, sanitary tampons, cotton buds, etc. It is the variety Gossypium herbaceum that is most used to make cotton wool, as it has short, thick fibres.

Seeds with many uses

Every part of a cottonseed has either an industrial or a food use.

The fuzz around the seeds (known as linter) comprises short cellulose fibres that are used to make felt, stuffing for mattresses, furniture and car seats, compresses, gauze, cotton wool, wicks, yarn for rug-making, etc. Food derivatives can also be extracted (dietary fibre, thickeners, excipients, etc). There is an international market for linter, by virtue of its many uses.

The hull of the seeds can be burnt to generate energy for oil mills. It can also be used as an animal feed or to make synthetic derivatives for the chemicals industry.

The seed kernel is very rich in oil and protein, but it contains a toxic pigment, gossypol, which is removed by traditional and industrial processes. Pressing the kernels produces an excellent edible oil, which can also be used to make soap. There are now naturally glandless cotton varieties (without the glands that produce gossypol), which are grown on a large scale in some countries. Cottonseed oil is the sixth most widely consumed oil worldwide. It is high quality, rich in polyunsaturated fatty acids (including vitamin E), and cholesterol-free. However, it is not as good for cooking as the other main oils. In certain cotton-growing countries (Mali, Chad, Burkina Faso, Togo, etc), it accounts for a major share of edible oil consumption.

The paste left after oil extraction is made into cake that is fed to ruminants (cows and sheep, the only animals that can detoxify gossypol during digestion). Cottonseed presscake has a high protein content (up to 49% over dry matter), three to six times higher that that of cereals and up to 20 times higher than some fodder crops. It is also the plant-based feed with the most phosphorus. Presscake can also be used for other agricultural purposes, for instance as a fertilizer or a mushroom growth substrate. There is an international market for presscake, and most producing countries export it.