Agronomic features and economic importance of the cotton crop

Economic Significance and Yield Potential of Cotton

Cotton is a valuable row crop, which serves as an excellent component in the crop rotation of grains and other crops. The plant is also valued as a good honey plant. In addition to high-quality fiber, the crop provides raw materials for the oil and fat and feed industries. Cotton stalks (guzypaya) are used to obtain tannins or as fuel.

• Oil yield per 1 centner of seed — 17–19 kg
• Cake yield per 1 centner of seed — 40–42 kg
• Protein content in cake — up to 40 %
• Yield of seed cotton in Central Asia — 2.5–2.7 t/ha
• Global sown area — about 32 million ha

Cotton seed hulls are a valuable raw material for the chemical and woodworking industries. After processing, these production wastes are turned into useful products. The following are obtained from seed hulls:

• paper;
• insulation materials;
• ethyl and methyl alcohols;
• citric and acetic acids;
• potash;
• organic fertilizers.

Cotton is cultivated in tropical and subtropical zones. The main sown areas in the CIS are concentrated in Central Asia (Uzbekistan, Turkmenistan, Tajikistan, Kyrgyzstan, Kazakhstan) and Azerbaijan. The average yield of seed cotton in these regions is 2.5–2.7 t/ha, and leading farms obtain 4 t/ha or more. In 1980, the area under this crop in the former USSR reached 5.67 million ha.

Biological Features and Key Crop Species

India, Mexico, and Peru are recognized as the oldest centers of cotton cultivation on the planet. Archaeological excavations in Pakistan have revealed ancient cotton fabrics created in the 3rd millennium BC. Later, the crop spread to China, Egypt, Central and Asia Minor. Attempts to grow cotton in Russia near Astrakhan and in the lower reaches of the Volga, as well as in the south of Ukraine near Odessa, proved unsuccessful.

The crop belongs to the genus Gossypium of the Malvaceae family. This genus includes approximately 20 species, but only five are used in global agricultural production. Of these, three species occupy leading positions in sowings: upland cotton — Gossypium hirsutum L., sea island cotton (fine-fiber) — Gossypium barbadense L. (or Gossypium peruvianum Gav.) and Levant cotton, guza — Gossypium herbaceum L.

Upland cotton, or medium-staple cotton, is the most common species in the world and in the CIS. It is grown as an annual semi-shrub 1–1.5 m high with large 3–5-lobed leaves. The plant has single-tier pubescence, heart-shaped bracts, and cream-colored flowers. Large bolls contain 5–11 fuzz-covered seeds in each of the 3–5 locules, ensuring a high fiber yield (up to 35–40%) with a length of up to 35 mm.

Fine-fiber (sea island) cotton in the wild is a tree-like shrub, but in culture, it is grown as an annual 1–3 m high. The leaves have 3–5 long lobes, and the bracts with 10–15 teeth exceed the length of the boll. Lemon-yellow flowers are distinguished by a red spot at the base of the petals. Small 3–4-locule bolls with a pitted surface contain 5–8 almost naked seeds per locule.

Temperature Conditions and Development Phases of Cotton

Cotton is biologically a perennial crop, but in agriculture, it is cultivated as an annual. For an agronomist, controlling temperature conditions from the first days of sowing is of key importance, as the heat factor directly determines the rate at which the plant passes through the vernalization phase and the formation of vegetative organs.

The light stage of cotton under field conditions lasts from 18 to 33 days. Being a short-day plant, the crop completes this stage faster under conditions of reduced insolation, while high night temperatures play an important role. Vernalization usually ends before the formation of the first vegetative branches, and the light stage ends with the appearance of the first fruiting branches.

• Duration of the light stage — 18–33 days
• Growing season of medium-staple cultivars — 125–150 days
• Growing season of fine-fiber cultivars — 145–160 days
• Depth of taproot penetration — 1.5–2.0 m

The development of cotton from sowing to the opening of the first bolls includes five main phases. The speed of each stage determines the overall ripeness of the crops and the planned harvesting dates.

Bush morphology, branching types, and flowering biology

Seed germination begins with the rapid growth of the taproot, which anchors in the soil, after which the curved hypocotyl pushes the cotyledons to the surface. During the first month after emergence, the root system grows at an accelerated rate, reaching depths of up to 2 meters. The leaves are formed alternately, have 3 to 8 lobes, and 2–3 buds are set in their axils, from which lateral branches develop after 20–35 days (starting from the 3rd–5th leaf).

For yield forecasting, it is important to distinguish between two types of cotton shoots. Vegetative (monopodial) branches grow at an acute angle, terminate in a vegetative bud, and do not bear flowers directly. Fruiting (sympodial) branches grow at an obtuse angle, have a zigzag shape, and bear buds, flowers, and bolls. The lower the fruiting branches are set on the stem, the earlier the cultivar: in early-ripening forms, the first fruiting branch appears in the axil of the 5th–6th leaf, while in late-ripening ones, it is in the axil of the 7th–10th leaf.

The cotton flower has a large corolla of 5 petals fused at the base with a staminal tube bearing 50–80 anthers. The ovary is usually 3–5-loculed; pollination occurs primarily through self-pollination. The life cycle of an open flower lasts only two days and follows a strict schedule.

1. Early in the morning, the flower opens (petals are pale yellow or cream-colored, with some species having a red spot at the base).
2. After noon, the petals begin to turn pink, and by evening they wilt and close.
3. On the second day, the corolla turns purple, finally dries out, and falls off.

After fertilization, a fruit is set—a round-ovoid boll with a smooth or pitted surface. As it ripens, it opens along the seams into 3–5 valves, releasing seeds with short fuzz. Fruit size and opening quality directly depend on the conditions of mineral nutrition and soil moisture availability.

Fiber formation and mineral nutrition

After flower fertilization, the ovary begins to develop, which eventually turns into a boll. The fruit formation period is divided into two main stages, on which the commercial properties of cotton directly depend. It is important for an agronomist to monitor these phases to obtain conditioned raw material.

Cotton fiber begins to grow immediately after fertilization and develops almost until the boll opens. It is an elongated single cell of the seed coat epidermis with a thickness-to-length ratio of 1:1500–2000. In each boll locule, 5–8 seeds are set. They are covered with long fibers and short fuzz (linter) of white, greenish, or brown color, which is absent only in naked-seed forms.

1. Length growth. The fiber actively elongates during the first 20–25 days after flower fertilization.
2. Cellulose accumulation. Gradual deposition of the substance occurs from the inside onto the walls of the fiber.
3. Ripening. The inner lumen narrows, the fiber contracts and acquires the strength, convolution, and fineness characteristic of the cultivar.

The quality of the raw material is evaluated by four main indicators: fiber output, length, strength, and fineness. Fiber output is 25–40% of the dry raw mass and is determined by the cultivar, agricultural practices, and weather. Fineness (fiber diameter) is measured under a microscope and is 10–25 µm. Tensile strength ranges from 3 to 8.5 g: the thicker the deposited cellulose walls, the stronger the fiber, whereas immature fibers remain thin and weak.

The length of cotton fiber is usually in the range of 10 to 50 mm, most often 25–40 mm. Longer fiber is valued higher as it allows for the production of fine yarn. Fiber convolution is also important for production: it ensures better cohesion when twisting the thread. By fiber length, cultivars are classified as follows:

• Fine-staple: fiber length from 35 to 40 mm and above;
• Long-staple: fiber length from 30 to 35 mm;
• Medium-staple: fiber length less than 30 mm.

To form 1 ton of raw cotton along with the above-ground vegetative mass, plants require a substantial volume of mineral nutrients. It is important for an agronomist to balance fertilizer application rates to avoid yield shortfalls and a reduction in raw material quality. The average nutrient uptake per ton of raw cotton is:

• Nitrogen (N) — 50–60 kg
• Phosphorus (P₂O₅) — 10–15 kg
• Potassium (K₂O) — 50–60 kg
• Calcium (CaO) — 50 kg

Peculiarities of crop rotation, sowing, and harvesting

Continuous cotton cultivation in the same field leads to a rapid accumulation of pathogens and pests, especially the causal agents of Verticillium wilt. Alfalfa is recognized as the best predecessor in cotton crop rotations. It not only provides high-quality feed but also radically improves the condition of the soil. Alfalfa increases organic matter reserves, improves the water-physical properties of the soil, and reduces its salinity.

Tillage system of the soil">The tillage system for cotton is structured with mandatory consideration of the predecessor. Sowing begins in the spring when the soil at the seeding depth warms up and stably maintains a temperature of 12–14 °C. Sowing is carried out using a wide-row method with row spacings of 45, 60, or 90 cm. With the precision drilling method, 1–2 seeds are sown in a cluster every 10 cm, which ensures uniform distribution and a density of 100–150 thousand plants per hectare without thinning.

The seeding rate depends on the row spacing, sowing scheme, and climatic conditions during the work period. One can be guided by standard rates for popular sowing schemes. seeding depth">The seeding depth is strictly tied to the soil type in a specific field.

On sierozems, the seeding depth of seeds">seeding depth should be 4–6 cm, and on meadow soils, it is reduced to 3–4 cm. Harvesting of raw cotton is carried out by mechanical means using cotton pickers. Harvesting is performed in two stages as the bolls open.

• The first harvest begins 8–10 days after defoliation. By this time, 50–60% of the bolls have opened and at least 80% of the leaves have fallen.
• The second harvest is carried out 12–15 days after the first harvest.