General Information and Agronomic Aspects
The cotton plant belongs to the Malvaceae family. Over 30 species have been discovered (among others: Gossypium hirsutum L., G. barbadense L., G. arboreum L.). Cotton as a tropical crop originates from several locations (southern Africa, south-east Asia and Peru). There are annual and biennal species, as well as perennial varieties. It produces a yellow, white and purple-red blossom, and is a hermaphrodite.
World wide there are about 33 million hectares of cotton crops. Economically, the most important varieties are Gossypium hirsutum and Gossypium barbadense.
- Upland cotton
- 80-90% of world market
- short to medium fibres (2-3 cm; middle stapled variety)
- Sea Island cotton
- 10-20% of global market
- high-quality, long to very long fibres (3-4 cm; long stapled variety)
In addition, the annual G. herbaceum or the hardy cotton bush G. arboreum produces fibres with a length of 1.8-2.2 cm. Long stapled varieties are cultivated mostly in Egypt and Peru. The middle stapled varieties from the USA, short stapled varieties in AsiaG. barbadense is more susceptible to pests due to its long vegetation period G. hirsutum, ripens far quicker (some varieties after only 150 days).
All of the cotton varieties with coloured fibres, formed from crossings between wild varieties (from Peru) and crops, have provoked a certain interest in the natural textile processing industry. Until now, mostly brown, green and beige varieties had been cultivated.
Organic cotton production is most widely spread in the USA (about 4000 ha). Yet ecological cotton projects also exist in Egypt, Argentina, Brazil, Greece, India, Mali, Nicaragua, Paraguay, Peru, Tanzania, Turkey and Uganda.
Cotton can be seen as a classic dual purpose plant (fibres and oil). A harvest yielding 1000 kg of cotton/ha can be broken down thus:
- about 320 - 420 kg fibres (raw cotton)
- about 200 - 250 kg seed cakes or flourabout 100 - 150 kg oil
- about 200 kg shells
- about 20 kg retained seeds
- about 40 kg dirt
Climate conditions, soil and water management
Cotton performs best in desert climates, under irrigation. It grows on lowland below 1000 m. The optimum temperature for germination is 34degC, for the growth of seedlings 24-29degC, and for later continuous growth 34degC. Low temperature increases the production of vegetative branches and extends the cropping period. Cotton is susceptible to frost. High temperature increases the number of fruiting branches and reduces the cropping period. Cotton is a sun-loving plant and cannot tolerate shade, particularly in the seedling stage. Reduced light intensity, caused by long-lasting overcast weather, shading from inter planted crops or too dense a stand, retards flowering and fruiting and increases boll shedding. Shedding of over 50% of squares, flowers or young bolls, due to early bollworm attack, drought or water logging, is normal. Upland cottons are day-neutral.
The crop will not tolerate very heavy rainfall and, where grown as a rain-fed crop, the average rainfall is usually 800-1200 mm. Adequate, but not excessive, moisture is required for early vegetative growth. The first flowering period requires relative dryness to speed up formation of fruiting branches. An increase in moisture is required for boll setting and renewed growth, followed by dry weather for ripening and harvest. Sufficient soil moisture is essential during the flowering period.
In the tropics most cotton is grown by smallholders who sell their seed cotton to the ginneries. Ginneries may be privately, cooperatively or state owned.
Cotton can be grown on a variety of soils from light sandy soils to heavy alluvium and calcareous clays. Soils must be permeable to water and to roots to a depth of at least 100 cm, preferably over 150 cm, with pH 5.5-8.5. Cotton is one of the salt-tolerant crops.
In its early stages of growth, cotton requires an arid climate with a plentiful supply of water. Afterwards, the weather needs to be dry, especially after the capsules have opened, for if water can enters the capsules it will have a detrimental effect on the quality. The vegetation period generally lasts around 180-220 days (varieties such as G. hirsutum that mature rapidly can be harvested after only 150 days). Very high yields have been reported from the arid areas of the CIS and in Egypt using irrigation.
Because cotton loves the heat, yet is also highly susceptible to frost, temperatures of around 26-28degC are ideal for its development. Lots of sun has a very positive effect during the blossoming and fruit setting periods, in cases of 50% and more cloud during the vegetation period it makes little sense to plant cotton. Cotton cannot withstand shade. Cotton is also a short-day plant, and such conditions will accelerate its growth. The correct climatic conditions are generally found between the 28deg northern and 47deg southern latitudes.
A strong wind can suck the fibres out of the capsules and blow them away. Today's varieties are tolerant with regard to salinity (up to a salinity of 0.5-0.6%). The soil's pH value should be between 6 and 8. In addition, cotton also requires deep, well-drained and ventilated soil, in order to properly develop its system of tap-roots (resistance to drought).
The site's elevation plays a large role when planting coloured varieties of cotton, because the intensity of the sun's rays has a strong effect. At least the green varieties tend to bleaching when the intensity increases too much (Peru).
Seeds and shoots
The fertilisation process is generative. Most wild Gossypium varieties are perennial. Annual varieties are most generally used for cultivation, which conclude their development cycle during one single vegetation period. Local and regionally produced seeds should be used, which will have developed a tolerance or resistance against the pests most commonly found in the region. Because the sale of seeds is usually controlled by government authorities, it is important to try to acquire untreated seeds of the desired variety early enough.
During the last few years, hybrid seeds have been developed that provide high yields. Yet this method makes it impossible to use self-produced seeds from the crop, and a new supply of seeds must be bought for the next season.
Because cotton can be affected by various root and damping off diseases, in certain cases, it is worthwhile considering pre-treating the seeds. In contrast to conventional cotton crops, only micro-organisms, which work antagonistically, are used. In Egypt, for example, the seeds are treated with Bacillus subtilis, Gliocladium penicilloides and/or a suspension of trichoderma. In order to improve the nutrient availability for the young plants in India, the seeds are additionally treated with Azotobacter and bacteria strains capable of breaking down phosphorous.
If the cotton is to be planted by machines, then the seeds need to be rid of the fluff that surrounds them (otherwise the seeds stick together). This is not necessary when the seeds are sown by hand. The temperature should not fall below 18degC. Temperature of 35degC is optimal. The seeds should be sown at a depth of maximum 5 cm.
The density depends upon the method utilised (manual or by machine). Bio-dynamic farmers in Egypt sow several seeds by hand every 20 cm into the prepared planting rows (distances between the rows 60-70 cm). Between 3 and 4 weeks later, the plants are inspected and all but the 2 strongest specimens removed. The plant population of rain-fed cotton of small-scale farmers in Tanzania is much lower. The farmers sow cotton by broadcasting the seeds on the flat land or in rows at a spacing of 90 cm between and 30 - 50 cm in the rows. Some farmers sow also on ridges with the same spacing.
Mechanical methods usually leave 70 cm (50-120 cm) between the rows and 20 cm (20-60 cm) distance between the seeds. When the harvest is done mechanically, varieties such as G. hirsutum that produce few branches are sown every 8-10 cm with a distance between the rows of 15-20 cm.
Cotton is either planted on flat soil, ridges or in furrows. Furrow drilling is employed mainly as a protection against quicksand. Ridges are used in soils, which are difficult to drain, and in regions with little rainfall, as this eases irrigation and facilitate the seeping in of the water into the soil. Its disadvantages are more difficult sowing and tilling of weeds. The cotton is sown in the lower third of a ridge in high-content soils and the upper third for low content soils. The seeds should be watered as soon as possible after sowing.
During the first 3 weeks, the shoots can offer little resistance against weeds, but this improves until the thick crop growth has no more problems in the area. For this reason, a suitable position in the crop rotation, suitable soil cultivation method and preparation of the seed beds should be taken care of to prevent an excessive growth of weeds during the early growth phases. On irrigated soils, irrigation is carried out prior to sowing in order that the weed seeds germinate and grow. The resulting growth of weeds can then be easily removed by appropriately cultivating the soil, before the cotton is sowed. The final soil treatment before planting should include the spreading of compost.
For cultivation done using animal-drawn (oxen or water-buffaloes) implements time requirements are: 15 animal days and person-days/ha, or by hand: 50 person-days/ha. By 2 or 4-wheeled tractors (150 kWh/ha).
The land should be prepared early and to a depth of at least 15 cm. To maintain soil organic matter, liberally apply or incorporate plant residues and animal manure during land preparation. Planting should be early, as soon as rainfall is adequate for the germination and growth of the crop. Cotton grown by smallholders is commonly planted with a delay, because the food crops are given priority. In hand planting, cotton is usually sown at a seed rate of 11-14 kg/ha and at a depth of about 25 mm with 3-6 seeds per hole in rows or ridges. Ridges are an advantage as they can be tied to conserve water under dry conditions and aid drainage under wet conditions. Thinning is done when the plants are 6-10 cm high, and two plants per hill are usually left. The optimum spacing depends on the size and fruitfulness of the plant permitted by local conditions. It also depends on the interactions between variety, soil and climate. The optimum spacing ranges from 80-20 to 100-40 cm, with one or two plants per hill. Plant densities may vary between 40,000 and 100,000 plants per ha, but are generally between 50,000 and 60,000 plants per ha.
Cotton seedlings are sensitive to competition from weeds. Weeds should be controlled early to prevent damage to the crop. Control weeds against the first flush of weeds before sowing and plant the seeds closely. Weed early and frequently. Place manure some 6 weeks after sowing. These measures help to reduce hand weeding to some 15 person-days (work of one person done per day) per ha. It also helps to reduce attack by pests harboured by weeds.
Intervals between irrigation should be 2-3 weeks on deep permeable sandy loams to heavy clays and less for very light, very heavy and shallow soils. The irrigation period should be 19 weeks. Excessive irrigation besides being a waste of water increases risk of disease incidence. Most irrigation is by gravity using furrows. Water saving is possible by alternate furrow irrigation or by hand watering with a hose pipe. The latter is only applicable in small plots.
Compost, decomposed animal dung and mulching material are used to supply the soil with sufficient organic material. This supports the cotton's growth. Potassium can be supplied by spreading wood ashes, and phosphorous by adding rock phosphate. Usually, the soil is prepared before the cotton is sown by spreading compost that is mixed with ashes and rock phosphate. In some areas, animal manure that has been composted is used for fertilisation. It should be applied inside the furrow close to the plant lines or in each plant hole. The above is only applicable to smallholder farmers. It is not practicable where cotton is a plantation crop as in Egypt, Sudan and Tanzania.
Cotton should not be grown for more than 3 years on the same field. It is important that cotton is grown in rotation with other crops. This helps to improve and maintain soil fertility. Crop rotation and mixed cropping help prevent build-up of pest population, diseases and weeds. Cotton grows well in rotation with cereals, tobacco and legumes. Particularly good yields can be achieved when cotton is grown after pulses (soybean, chickpea, pigeon pea, groundnut etc.), horticultural crops like chillies or vegetables, and after sugarcane and wheat.
Cotton intercrops such as maize, sorghum, beans, and peanut create a natural balance of pests, natural enemies, and weeds in the cotton field environment. Maize or tobacco planted in every 20 rows of cotton attracts African bollworm. Sunflower or cowpea sown in every 5 rows of cotton attracts moths when planted as trap crops. Castor bean (Ricinus communis) attracts caterpillars. Rice when rotated with mungbean and cotton disrupts the life cycles of pests attacking these crops. However, the timing of planting of inter crops, trap crops, and border crops should be planned to flower at the same time with cotton.
In Africa generally, cotton is hand picked. This creates work for farmers' families or village labourers, usually women. It also produces clean seed cotton that can be ginned easily and cheaply in low-cost ginneries. One picker can harvest 25-40 kg of seed cotton per day depending on the availability of open bolls. Picking is very laborious. It should be done every 3-4 weeks, so that open cotton is not left in the field for too long which may result in a change of the colour and reduced the quality of the lint. It is then sorted into clean and stained cotton before marketing. Harvesting begins about 4 months after sowing, lasts for 2 months and 2-3 pickings are usually done.
Biological methods of plant protection
Preventative or combating methods are already known for all of the important pests and diseases that can occur on organic cotton plantations. In the long-term, it is safe to presume that a high level of pest and diseases will significantly diminish following the introduction of an organic cultivation system.
Yet this requires a successful plant protection management system. The farmer should consider the following:
- Which important infection agents are present in the region?
- Which preventative strategies he wishes to implement against them on his site?
- Which combating measures exist against a heavy infestation?
- Which permitted resources are available for organic systems?
- How these are applied?
- When it is correct time to apply them?
- Whom he can turn to in an emergency (advice)?
The following preventative measures should thereby be strictly adhered to:
Selecting crop rotation
As cotton is not well compatible with itself, it is not advisable to have a larger ratio than 1/3 in the crop rotation. Other mallow plants (e.g. hibiscus, ocra) must be excluded from the rotation, or at least not planted on the same soil. It is also important to check that no cotton is grown on any of the neighbouring plots. On the whole, a diversified crop rotation works best.
Mixed crops with plants that act as a repellent
Mixed or strip cultivation with onions, garlic, chillies, chrysanthemums or hot peppers have proved their worth because of their repellent effect against, among others, bugs, white fly and cotton leafworm (Alabama argillacea). Rotted liquid manure can also act as a repellent (and be simultaneously used as a fertiliser).
Cultivation of trap crops
Trap crops manage to keep pests away from the cotton by offering a more attractive source of food. Strip cropping using lucerne (Medicago sativa L.) within the cotton plants is, for example, practised in the USA and Paraguay, in order to keep pests such as different bug species (Dysdercus spp., Lygus spp.), Helicoverpa spp., Spodoptera littoralis, Platyedra gossypiella and aphids away from the cotton.
Sowing sorghum before the cotton (on neighbouring plots) can help to build up a population of useful insects, which can then combat cotton pests when they appear at an early stage (e.g. aphids).
A similar strategy can be followed by planting Hibiscus esculentus against the pest Podagrica spp., planting Lablab niger L. against the pests Helicoverpa spp., Spodoptera littoralis and Bemisia tabaci, or nasturtium against Tetranychus cinnabarinus (these are based on experiences from Turkey and in Sudan). During 9 years of organic cotton growing in Tanzania the experiences have shown that the most important cotton pest Helicoverpa spp. can be controlled with sunflower as trap crop to such an extent that the threshold for an economic application of insecticides is not reached in most cases. The recommended practice is to sow one row of sunflower around the cotton plot as a living fence and one row of sunflower all 10 meters in the plot. The sowing time has to be very close to the cotton sowing so that the sunflowers will be in the flowering stage when the infestation period starts. The sunflowers attracts the moths of Helicoverpa spp. to lay their eggs. The caterpillars can feeding on the sunflower however without destroying the production of sunflower seeds. So the farmers can get an additional income from the sunflower seeds.
The caterpillars on the sunflowers show also the phenomenon of cannibalism so that they reduce their own numbers itself (Source: Dr. Braun, GTZ-IPM project, personal communication, 1997).
The positive effects of sunflowers are also shown in the results of a study that was carried out by an entomologist on behalf of the GTZ-IPM project in Shinyanga, Tanzania. The researcher found out that in organic cotton plots with sunflowers were up to ten times more useful ants compared with cotton plots without sunflower. It is known that these ants are reducing the eggs and larvae of the African bollworm (Helicoverpa armigera). (Source:Varela (1996) "Ants as mortality factors of the African bollworm Helicoverpa armigera in smallholder cotton fields in Tanzania"). A booklet about the natural enemies of the African bollworm names especially the ant species Myrmicaria and Pheidole as important. 'On sunflower, ants were observed to reduce bollworms by as much as 85%.' (Source: van den Berg, H. and Cook, M.J.W., (1993) 'African bollworm and its natural enemies in Kenya', P. 33, CAB International + NRI, International Institute of Biological Control, Kenya Station). Many contracted smallholders in Tanzania confirm the positive effects on the cotton yield by cultivating sunflower as trap crop and even many conventional neighbours in the region started copying this cheap and easy method of preventive pest control.
Pigeon peas (Cajanus cajan) can also be a useful trap crop for pests like Helicoverpa spp. but it is not so easy to synchronise the flowering stage of the trap crop with the infestation period of the pest. The local pigeon pea varieties in Tanzania start the flowering too late to be an efficient trap crop. Early maturing varieties or sowing of pigeon peas in the previous year could resolve this problem. In the bioRe India project the Pigeon peas are successfully used as trap crop in cotton.
Trap crops planted in autumn (e.g. maize) can be used in combination with a pheromone against hibernating boll weevils.
Leaving a strip of natural vegetation around the cotton plot can be useful against aphids and other pests.
Choosing a site
Cotton should be planted in healthy soil wherever possible. In principle, sites that are infested with weeds should not be sown with cotton, but first cultivated with an appropriate rotation crop in order to prepare it. Care should be taken that no cotton is planted in the neighbouring plots.
The choice of when to sow plays an important role. Cotton sown too early will possibly become infested by the previous pest population. In Tanzania the late sown cotton is often attacked by African bollworm (Helicoverpa armigera) that developed on maize or sorghum plots. At the end of the season the risk of the late season pest cotton stainers (Dysdercus spp). is higher on late sown cotton while it is a minor problem on early sown cotton.
Mulching of harvest residues
Careful mulching of the remains of a cotton harvest can help prevent the survival of pests (e.g. P.gossypiella in seeds and boll weevil, Anthonomus grandis). In the case of heavy infestations with wilting diseases, such as bacterial blight (Xanthomonas malvacearum), anthracnose (Glomerella gossypii), verticillium-wilt (Verticillium alboatrum) or fusarium-wilt (Fusarium oxysporum) it is recommended to remove the residue and then apply compost.
Sufficient, balanced supply of nutrients
A plant that receives balanced nutrients is more vigorous, and therefore less susceptible to infestation. As already mentioned, supplying too much nitrogen will lead to an infestation by pests.
Choosing a variety
It is hereby important to choose varieties adapted to the site conditions, and that are resistant to, or can tolerate, the main pests. In addition, general varieties have proven their worth that matures quicker, thereby shortening the time span they can be infested. Gossypol-free are not so well suited to organic plantations because Gossypol (just like other terpenoide chemical compounds) has a repellent effect on certain insects (e.g. against Helicoverpa spp., Spodoptera spp. and Pectinophora spp.)
How easy or difficult it is to choose a suitable variety show the examples from Tanzania and India.
While in Tanzania there is only 1 cotton variety per production zone the bioRe farmers in India use 9 out of more than 50 different varieties according to their specific needs and preferences. It follows a list with the advantages and disadvantages of the varieties used in India, which shows the different aspects that can be important for the decision.
Planting of boundary areas
Planting 2-3 rows of trees or hedges along the boundaries provides a habitat for birds, improves climatic conditions and reduces the amount of water needed for cotton.
Checking the infestation level of cotton pests
In Tanzania the gtz-IPM Project has developed a method to check the infestation level of the key pest Helicoverpa armigera. The method is called 'scouting' and it works by counting the squared buds on 30 plants of 1 acre. If the number of squared buds comes up to 15 the economic threshold is reached and the farmers are advised to spray an insecticide. The organic farmers can apply an oily formulation of neem. This 'scouting' method works much faster than looking for the pests itself and helps to avoid many applications of insecticide (in organic and in conventional farming).
Light traps allow monitoring the start of pests moving into the cotton plot and at the same time reducing the number of moths laying eggs on the cotton plant. In Tanzania, traps are placed in the fields at rate of one per acre for about two hours after sunset. The reduction of moths can help to reduce the number of sprayings.
Direct combating measures
Direct methods of combating pests are also available for organic plantations, yet are only to be used in emergencies (and not as preventatives). It is necessary that the cotton, and any pests which may eventually develop, should be regularly inspected in order to be able to decide whether a direct method is to be used or not (see chapter above).
If the critical threshold is reached and there is an immediate threat for the cotton harvest the organic farmers need to have insecticides available that are allowed in organic farming. There are several botanical insecticides, which have prooved to be efficient against important cotton pests.
In India the bioRe farmers can choose among 3 commercial neem formulations and the self-made preparations made with crushed neem seeds. In Tanzania neem formulations imported from Kenya and another from India are commercially available for control Helicoverpa armigera. Against the late season pest Dysdercus spp. the farmers use a locally formulated Pyrethrum preparation with black wattle extract (Acacia mearnsii) as UV-light stabilisator.
Beside the neem products there are also some other plants that can be useful to produce botanical insecticides. In Tanzania the Ukiriguru Research Institute tested with promising results an emulsion of Jatropha curcas oil against Helicoverpa armigera. In Mali it has been tested against sorghum pests and a report from Malawi states. The oil and aqueous extract from oil (active principle probably phorbol ester) has potential as an insecticide, for instance in the control of 4 insect pests of cotton including cotton bollworm (Solsoloy 1995) and on pests of pulses, potato and corn.' (Malawi Agroforestry Extension Project Marketing & Enterprise Program Main Report, Publication No. 47, page 46, July 2002).
In West African countries like Benin and Mali the farmers are experimenting with mixtures of plant extracts and other ingredients. Experiences from Benin give the following recommendations for 1 ha:
3 sprayings during early season with a preparation of
- 1.5 kg pounded neem seeds in water, fermenting over night and then filtered
- 1 litre cow urine
- 20 papaya leaves
- local soap diluted in water
total of 10 litres
Then later in the season the following preparation without cow urine in order to avoid excessive vegetative growth:
- 2 kg pounded neem seeds in water, fermenting over night and then filtered
- 5 cloves of garlic
- 20 papaya leaves
- local soap diluted in water
total of 10 litres
Since 1999 organic cotton growers in Mali use a mixture of neem and 'Npeku' oil (Lannea microcarpa) as botanical insecticide. The preparation for 1 ha is done as follows:
- 500 g grounded neem seeds in 10 litres of water for 3 to 5 days and then filtered
- Add 40 to 160 ml of 'Npeku' oil (according to plant stage, see table below) and mix well to get an emulsion
Instead of the 'Npeku' oil the organic cotton farmers in Mali take also the oil of 'KOBI' (Carapa procera) in the same way.
Yellow traps, pyrethrum and also sulphur extracts do not work specifically enough (useful insects are also affected). For this reason, these preparations should only be used when absolutely necessary, and when no other alternatives are available.