Improving cotton yields

Cotton is highly sensitive to excess water and water logging, which causes a reduction in yields through boll shedding, even when the plant appears to be unaffected. Cotton plants prefer deep, well-drained soils with good nutrient content. Ideal soils are clay-rich, vertisols (also called ‘black cotton soils’). This ensures that the tap roots of cotton plants can penetrate up to 3 meters in such soils and hence are able to sustain short periods of drought. However, cotton is also grown on less ideal sites with shallow, sandy soils, both under irrigated and rainfed conditions. This will, however, require well-adapted varieties and solid management practices.

Land preparation should be done early while incorporating green manure or animal manure before planting. Land preparation should also ensure that weeds are substantially removed to prevent an excessive growth of weeds during the early growth phases of the cotton crop. Planting should be done as soon as the rain season begins to ensure that the planted seeds get adequate moisture for germination and growth. Cotton is either planted on flat rows or ridges. Ridges are used in soils that are difficult to drain, and in regions with little rainfall, as this helps to conserve water under dry conditions and aid drainage under wet conditions.

In small-holder cotton production, cotton planting is done by hand and about 3 to 4 seeds are planted per hole in flat rows or ridges. Thinning is done when the plants are 6 to 10 cm high, leaving the strongest two plants per hill. The optimum spacing depends on the size and fruitfulness of the plant permitted by local conditions. The optimum spacing ranges from 20 to 50 cm within rows and 60 to 90 cm between rows, with one or two plants per hill.

Cotton is by nature a perennial crop, but it is commonly grown as an annual crop. Contrary to most cereal and leguminous crops, cotton is deep rooting. Its vertical tap root gives the plant access to water and nutrients in lower soil layers. This makes it a good rotational crop, and one that is relatively tolerant to drought and variable rainfall.

a. Crop rotation

Cotton must be grown in rotation with other crops to maintain fertility of the soil, break the development cycles of soil-borne pathogens and prevent the spreading of weeds. Cotton should not be grown after cotton in the same field. Instead another crop should be grown at least for one, but preferably for two seasons, before the next cotton crop.

Cotton rotations should include legume crops such as mung bean, cow pea or chick pea for harvesting or a green manure crop such as sun hemp or cow pea, to be cut and ploughed back into the soil before flowering. Pulses like mung beans, soybean, chickpea, pigeon pea and groundnuts increase the nitrogen content in the soil by fixing nitrogen from the air. But they can also be marketed to increase farm income. Suitable crop rotation patterns with cotton depend on the climatic conditions, market requirements and availability of land.

b. Green manures and intercropping

When cotton is intercropped with maize, sorghum, beans or peanuts, pests find it more difficult to move from one host plant to another, and they are controlled by a number of beneficial insects hosted by the intercrops. Some examples are:

• Maize planted in every 2 rows of cotton attracts the African bollworm.
• Sunflower or cowpea sown in every 5 rows of cotton attracts moths when planted as trap crops.
• Castor bean (Ricinus communis) also attracts caterpillars.
• Rice when rotated with mung bean and cotton disrupts the life cycle of pests attacking these crops.

For best distracting effect, planting of intercrops, trap crops and border crops should be timed such that they flower at the same time with cotton. Intercrops are usually allowed to mature and are cut and used as mulching material after the seeds are harvested.

Diversification of crops furthermore reduces vulnerability to crop failure and to fluctuating prices. It also helps to prevent shortage of labour in peak seasons, as labour requirements are more evenly distributed throughout the year.

Green manures improve soil fertility and can act as trap crops. Green manure crops like sun hemp (Crotolaria juncea), jack bean (Canavalia ensiformis), Lablab (Lablab purpureus) or velvet beans (Mucuna pruriens) are usually sown between the rows after cotton seedlings emerge. They are cut before or at the time of flowering, and are either used as mulch or incorporated into the soil.

Both green manure and intercrops have the following benefits:

• They distract pests from the cotton crop (especially sucking pests), attract and host beneficial insects.
• They take up nutrients from the soil and build-up organic matter, which would otherwise be lost through leaching. The added organic matter builds better soil structure, water retention and overall fertility.
• Leguminous green manure crops and intercrops fix nitrogen from the air and make nutrients available to the cotton crop.
• They suppress weed growth and protect the soil from soil erosion through rain or wind.
• They provide additional yields or fodder for livestock.
• On the other hand, green manure and intercrops do compete with the cotton crop for water, light and nutrients. Thus, appropriate timing of the sowing and cutting is very important in order to get maximum benefit with minimum competition.

c. Agroforestry

In areas with heavy wind and frost, agroforestry with wind breaks should be considered. Different trees are needed to break the wind, protect from strong rains, and provide shade, mulch and fodder. Useful species are Leucena leucocephala, Moringa oleifera, Faidherbia albidia, Neem and other adapted trees, which do not consume too much water and ideally provide additional food and fodder as well as serving as mulch. However, the trees need regular pruning because the cotton plants do not tolerate much shade.

Cotton has high water requirements for boll setting and renewed growth, but dry conditions are also needed for ripening and harvest. In rainfed cotton production, major emphasis should be given to increasing the infiltration of rainwater into the soil and to its conservation. The following measures contribute to increased availability of water:

• Application of compost and organic manures helps increase organic matter content in the soil, which is known to improve soil structure, hence increasing water infiltration and water retention.
• Minimum tillage and shallow soil cultivation (hoeing) reduces water evaporation from the soil. Ridging around the growing plants is also a common practice to conserve water.
• Covering the soil with mulching materials helps to preserve humidity in the soil and to prevent water loss, while enhancing increased biological life in the upper-parts of the soil.
• Active rainwater harvesting through pits or trenches leading to wells can help to recharge groundwater levels and thus to improve the availability of irrigation water.

Where little irrigation water is available, alternate furrow irrigation can still help irrigate the crop. If rain fails after the seedlings have germinated, it can even be worth saving them through bucket irrigation, plant by plant.

Virtually all cotton cultivation in sub-Saharan Africa is rainfed. Irrigated cotton would technically be possible in some areas in very dry regions with less than 600 mm of rain per year. The resulting extra yield from irrigation covers the cost of irrigation.

The right strategy to improve and maintain soil fertility in cotton depends first and foremost on the soil types present in a farm.

Light or shallow soils usually have a lower water retention capacity and lower nutrient exchange capacity than deep or heavy soils. The application of compost or other measures in increasing soil fertility is important to increase water holding and nutrient supply and to improve the structure of the soil. Intercropping of more drought resistant crops like sorghum, sunflower, sesame or castor can help to reduce the risk of crop failure. Soil cultivation should be shallow and kept to a minimum in order to avoid soil erosion and enhance build-up of organic matter.

In deep or heavy soils such as black cotton soil, an intensive production system can be established with sufficient inputs of organic manures, intensive crop rotation and green manuring. Frequent shallow soil cultivation helps to improve soil aeration and nutrient supply. It also reduces evaporation and suppresses weeds. When the cotton crop is well established after 6 to 9 weeks, it is recommended to apply additional organic manure such as vermi-compost or oil cakes and to earth up ridges in order to accelerate decomposition and to bury weeds.

Attention must be paid that the soil is not prone to water erosion. In case of the risk of eroding contour lines or terraces, other soil conservation measures are highly recommended as the basis of improving soil fertility.

Manure or compost should also be applied before soil cultivation such that during ploughing, the organic materials are brought into soil. Well composted manure or other organic fertilizer can replace the extracted nutrients and contribute to maintenance of soil fertility. Availability of nutrients to the crop, however, depends on other factors as well, such as sufficient soil organic matter and active soil organisms, deep rooting of the crop and humid soil conditions.  Cotton’s nutrient requirements are highest between first flowering and boll formation. A second small peak in nutrient demand is during the second flush, after the first picking. A top-dressing of organic manure during this critical growing period contributes to increasing yields.

Cotton has a wide range of pests that can lead to 50 to 90 % yield losses. In most of the semi-arid tropical regions, diseases are not a big problem in well-managed organic cotton fields.

Early observation, combined with the required knowledge of the ecology of the insects and the tools for monitoring (pegboard, biopesticides and advice) as well as a professional support from scientifically trained experts will be required in order to manage the risks effectively.

Cotton research has done a great deal to identify the insects that ‘threaten’ the cotton production system. Researchers in almost all sub-Saharan African countries can claim that the list of important cotton pests consists of:

• Early-season pests: Aphids (Aphis spp.), Whiteflies (Bemisia spp.), Cotton leafroller (Sylepta), Lygus bugs (Lygus), etc.
• Mid-season pests: Cotton bollworm (Helicoverpa armigera), Spiny bollworm (Earias spp.), Cotton leafeater (Spodoptera spp.), Red or Sudan bollworms (Diparopsis spp.), and/or depending on the region Pink bollworm (Pectinophora gossypiella) and False codling moth (Cryptophlebia leucotreta).
• Late-season pests: Aphids (Aphis spp.), Whiteflies (Bemisia spp.), Cotton strainers (Dysdercus spp.), etc.

Selected examples of important cotton pests:

American bollworms (Heliothis (or Helicoverpa) armigera) - The young larvae feed on tender leaves, buds, flowers and later bore into the bolls. While feeding, its head and part of the body are inside the boll. They deposit faeces at the base of the entrance hole. Eggs are pinhead-size and yellowish-green in colour. They are found singly laid on the surface of the leaves. Larvae vary in colour from bright green, pink and brown, to black, with lighter undersides. Alternating light and dark bands run lengthwise along their bodies, the heads are yellow and the legs are almost black. Mature larvae drop to the ground to burrow into the soil to pupate. Pupae are yellowish-green and turn brown as they mature. Adult moths are grey to brown in colour and have dark spots on the front wings. They are active at night and hide in vegetation during the day. The total development period from egg to adult is about 34 to 45 days.

Cutworms (Agrotis spp.) - The larvae cut seedlings often at ground level. They can be found in the soil down to a depth of about 5 cm near the host plant. Cutworms always curl up when disturbed. They feed only at night. Eggs are tiny, pearl-white, round and have a ridged surface. The full-grown larva is brown or brownish-black with a tinge of orange. The pupa is black or brown in colour. The adult has dark brown forewings with distinctive black spots and white and yellow wavy stripes.

Aphids (Aphis gossypii and others) - Aphids are important pests in fields with low populations of natural enemies, high manure application, or where crops suffer water stress. Heavy infestations cause crinkling and cupping of leaves, defoliation, square and boll shedding and stunted growth. If the infestation is not too high, the plant can compensate for the damage. Honeydew excretion causes sticky cotton lint and thus problems in spinning. Aphids produce large amounts of a sugary liquid waste called honeydew. A fungus, called sooty mould, grows on this honeydew, turning leaves and branches black. The eggs are very tiny, shiny black and are found in the crevices of bud, stems and bark of the plant. Winged adults are produced only when it is necessary for the colony to migrate.

Cotton stainers (Dysdercus spp.) - Cotton stainers suck sap from flowers, buds and bolls of cotton. In case of high infestation, the bolls open insufficiently and the lint quality is reduced due to stains resulting from fungal infections. When sucking on immature seeds, cotton stainers transmit fungus on the immature lint and seed, which later stains the lint with typical yellow colour, hence the name ‘cotton stainers’. Heavy infestations on the seeds affect the crop mass, oil content, germination capacity of the seed and marketability of the crop. Cotton stainers are, however, usually not a major problem in organic fields.

Cotton strainers lay their eggs into the soil or under plant debris. Nymphs look similar to their adult counterparts, but have no wings, meaning that they can only attack seeds in open bolls. The adult cotton stainers are true bugs with piercing and sucking mouthparts. They can even suck on seeds in closed bolls. Their colour varies from bright red to yellow to orange, depending on the species.