-By: Swastika Chauhan, B.Sc.Ag., Lamjung Campus, Nepal
Cotton, the ‘White gold’, is one of the important commercial crops playing a key role in the economic, political and social affairs of the world. Cotton is a soft, fluffy staple fiber that grows in a boll, or protective capsule, around the seeds of cotton plants of the genus Gossypium. The fiber is almost pure cellulose. Under natural condition, the cotton balls will tend to increase the dispersion of the seeds.
There are four commercially-grown species of cotton, all domesticated in antiquity (Greek and Roman time):
Gossypium hirsutum – upland cotton, native to Central America, Mexico, the Caribbean and southern Florida, (90% of world production)
Gossypium barbadense – known as extra-long staple cotton, native to tropical South America (8% of world production)
Gossypium arboreum – tree cotton, native to India and Pakistan (less than 2%)
Gossypium herbaceum – Levant cotton, native to southern Africa and the Arabian Peninsula (less than 2%)
The two New World cotton species account for the vast majority of modern cotton production, but the two Old World species were widely used before the 1900s. While cotton fibers occur naturally in colors of white, brown, pink and green, fears of contaminating the genetics of white cotton have led many cotton-growing locations to ban growing of colored cotton varieties which remain a specialty product.
a. Economic importance of cotton can be listed as:
i) It is a cash crop.
ii) Lint fiber (seed surface fiber) is used in textile industry.
iii) Seed oil is used for the manufacture of vanaspathi ghee.
iv) Cotton flour is used for making bread and biscuit.
v) Cotton seed cake is used as organic manure.
vi) Cotton is used in stuffing pillows, cushions, mattresses etc.
vii) Cotton is used in making rubber tyres, carpet, blankets and cordages.
viii) Fatty acid obtained from the oil is used in the manufacture of insecticide, fungicides and plastics.
ix) Cotton is a basic raw
b. Cotton production status in world:
About 20 million tones of cotton are produced each year in around 90 countries. China, United States, India, Pakistan, Brazil and Turkey account for over 75% of global production.
United States 19.9%
c. Cotton production in Nepal:
In Nepal cotton farming was inititated in 1972-73 in banke district in the form of small research. Nepal government and government of Israel took this research jointly.
Harvest is the process of gathering mature crops or parts of crop from the fields. It is the act of obtaining the economic part of crop or crop from the field as raw as it gets maturity. For the efficient and more economic it is necessary to harvest the crop at its proper maturity time and at proper environment as well at proper method. We also can say that harvesting is the action that is done to obtain the crop which is in harvesting maturity. Harvesting maturity is different from physiological maturity. Harvesting maturity means the crop which is now is ready to harvest for proper consumptive use or for proper storage but physiological maturity means the crop which is fully matured beyond which the crop starts to desiccate and damages.
Cotton can be harvested either manually or by mechanized method using machine. Manual picking is slow but better preserves fiber characteristics of cotton. Boll opening is the first action on the fiber which pushes fibers from the place where they were embedded for weeks before being exposed to the external conditions. The boll opening action is gentle and thus has no effect on the fiber quality. However, a longer stay of the open bolls in the field may change the color and also make the fibers shrink, thus affecting the three most important fiber characters, i.e. length, strength and micronaire. One character may be affected more than the other if there is frequent dew. Such an effect cannot be eliminated as all bolls do not open at the same time and some open bolls have to stay in the field for days and sometime even weeks. In the case of hand picking, it is possible to pick open bolls at frequent intervals, and weather effects on the fiber, after bolls have opened, can be minimized. In China (mainland) this effect is minimized as land holdings are so small that the majority of the growers who have planted cotton on about 1/10th of a hectare, can go many times to the field to pick few open bolls. In slightly bigger plots, fiber quality is preserved through a number of pickings during the season. 3-4 pickings/season are very common in many countries where cotton is handpicked. After the first picking the 15-20 days interval is required for 2nd and 3rd picking.
Hirsutum cottons can hold locks for a longer period of time without letting the seed cotton fall on the ground. But, if cotton is not picked for weeks and months, it slowly gets loose and ultimately falls on the ground. In G. arboreum varieties, burs do not possess enough holding force to keep seed-cotton sticking in the burs. It is easy to pick such cotton but requires more frequent pickings. In China (mainland), India, Myanmar and Pakistan, where such cottons are grown on significant area, as many as 8-10 picks are very common. Thus arboreum varieties are not suitable for machine picking as locks fall to the ground quickly. Upland cotton locks which fall to the ground are usually loose, but arboreum locks remain more or less intact. G. barbadense types are almost like hirsutum cottons. Most varieties belonging to G. herbaceum are very difficult to pick because of the position of burs after opening. Bolls are smaller and locks after opening are positioned such that each lock has to be picked separately. Varietal differences do exist within all species.
About 30% of world production is machine picked. Australia, Israel and the USA are the only countries in the world where all cotton is picked by machines.
e. Harvest aids programs:
The harvest aids programs or harvest aids activities should be conducts to aid the harvesting efficiency. The harvest aid program is the act that is done prior to harvesting the plants so that the plant would be ready to harvest economically and physiologically. Fertility, water management and weed control play an important role in the success of a harvest aid program. The success of a harvest aid program is highly dependent on having adequate plant activity as leaf defoliation and boll opening as an active process. Harvest aid programs often include the use of compounds that result in leaf defoliation, boll opening or tissue desiccation. The two primary categories of harvest aid products, hormonal and herbicidal, are based on their mode of action.
Harvest aids are applied to facilitate leaf removal or enhance boll opening prior to mechanical harvest. Harvest-aid chemicals hasten harvest of a mature crop and reduce potential pre-harvest loss of yield or fiber quality. When cotton is properly defoliated, trash content is reduced and less cleaning of the lint is required at the gin, minimizing fiber damage and maintaining quality. Improper choice of harvest-aid materials can result in poor preparation for harvest and may lead to reductions in yield and quality. Ideally, the harvest aid material chosen should defoliate the entire plant and open all mature bolls with minimal drying or desiccation.
“Harvest aid” is a general term used to describe chemicals applied to terminate cotton growth, open bolls, defoliate, or desiccate the cotton plant. Defoliants are applied to remove leaves from the cotton plant and enhance the formation of an abscission layer at the base of the leaf petiole, resulting in leaf drop. For maximum leaf drop, defoliants require healthy, active leaves that are not drought-stressed. Warm temperatures generally enhance activity. Contact-type or herbicidal defoliants slowly injure the leaf. The “wound response” causes ethylene to be produced, eventually leading to leaf drop. A similar response often is observed with other types of stress, such as drought, disease, insect injury, or mechanical damage. Hormonal or plant growth regulator (PGR) materials directly enhance ethylene production, which again leads to leaf abscission. Both types of harvest aids can cause leaf drop without injury to the leaf, thus avoiding “leaf sticking.”
Desiccants are harsher treatments than defoliants. Desiccants dry the plant by causing the cells to rupture and lose cellular contents and water due to leakage. These chemicals lead to rapid moisture loss, resulting in leaf and stem desiccation.
Boll openers affect natural plant processes associated with boll opening but do not increase the rate of boll or fiber maturation. Defoliants can be tank-mixed with boll openers to provide improved overall harvest-aid performance.
For machine picking it is necessary that there are no green leaves on the plant. Under natural conditions, leaves are shed with age but formation of the abscission layer between the main stem/branch and leaf petiole is stimulated by low night temperatures. If the carbohydrate accumulation is slow and leaves are not shed naturally, application of defoliants becomes necessary. Some varieties are more susceptible to low temperatures and have a greater tendency to shed leaves compared with others. If the leaves are not shed and a time has come to pick cotton, it becomes necessary to shed leaves artificially through application of defoliants. Defoliants also need to be applied if there are green bolls along with open leaves.
Deciding when to defoliate a crop is an important decision from several stand points. If the crop is defoliated too soon, yields, quality and profits suffer. On the other hand, depending on the location and the field condition, delaying defoliation may increase likelihood of additional insect problems, or delaying harvest into bad weather which will affect yield and profits. That is why defoliation decisions must be based on the crop and the crop environment. Plant maturity is perhaps the most important factor, but other factors such as picking capacity, custom harvesting, and weather are also important.
Defoliation is an important management practice associated with high yields and high quality cotton. The decision as to how and when to remove the leaves and open the bolls appears to be one of the more difficult tasks confronting a cotton grower. There are so many variables involved that the results of harvest aid applications are often unpredictable and sometimes even undesirable. You would think that after more than 40 years of research in this area we could obtain desirable results under all circumstances. However, this is not always true, and we often have failures. Thus, defoliation has come to be considered as much art as science.
There are many benefits that can be expected from a good defoliation job. Many experiments have shown that defoliation improves picker efficiency in fields with large green plants. Additionally, defoliated fields tend to dry out faster, permit more picking hours per day and allow picking sooner after rain. Defoliation also cuts off the food supply to late season insects that are entering diapause. Under certain conditions, defoliation has reduced boll rot by creating better drying conditions in the field. This is especially true in rank cotton.
Defoliation may also have some disadvantages and limitations. When plants are defoliated, the fiber and seed development essentially stops. Therefore, if too many bolls are immature at the time of application there can be a reduction in yield and quality associated with the treatment.
i. Timing of defoliation:
Harvesting cotton as early as possible increases the likelihood of more ideal weather conditions and higher lint quality during the first part of the harvest season. It is important to apply harvest aids early enough to take advantage of the benefits of early harvest, while avoiding application so early that it decreases yield and quality of the cotton.
Timing of harvest-aid applications is not exact. There is a relationship between maturation of later-developing bolls and degradation of the earlier bolls that already are open. The correct decision is a compromise between these two factors. Timing of harvest-aid application varies with the area of the country, harvest-aid materials used, type of harvest, and individual preferences.
When harvest aids first were introduced, they were applied according to historical harvest dates; however, factors such as weather, heat unit accumulation, and cotton varieties made this technique largely undependable. Currently, timing is determined by a combination of techniques, each of which further confirms and verifies the others. These techniques are Percent Open Bulls, Cut Boll Technique, and Nodes Above Cracked Boll (NACB). These techniques will be discussed individually and, later, together as they relate to each other.
- Percent Open Bolls was one of the earliest techniques developed; it was used extensively prior to the introduction of hormonal boll openers. Decisions for timing of defoliation were made by counting the total number of bolls on the plant that would contribute to harvest and calculating the percentage of these bolls that were open. The primary problem with this technique when used alone is that it does not allow for differences in boll development throughout the plant. If there is a gap in the fruiting pattern, some harvestable bolls may not be allowed to mature. Recommendations vary, but, for timing of defoliants, 65 to 90 percent of bolls should be open; for timing of desiccants in stripper cotton, 80 percent or more of bolls should be open. This technique should not be used alone, but rather in support of the other techniques described below.
- The Cut Boll Technique is used to determine the maturity of the seed inside the boll. This technique has been used extensively since development of hormonal defoliants and boll openers. Cutting a mature green boll is roughly equivalent to cutting a one-inch diameter, wet cotton rope, and the knife must be sharp to obtain usable results. Be careful with this technique:
Immature green bolls are sliced easily and lack of resistance may cause an accident! Mature green bolls are difficult to slice; when sliced, the seed inside the mature boll will have a dark seedcoat and a fully developed pale green embryo inside. Seeds that are not yet mature will have a light-colored seed coat and will contain a gelatin-like substance.
The Cut Boll Technique is straightforward, but the difficulty in making harvest-aid timing decisions involves determining the approximate nodal position of the uppermost harvestable bolls. If the cotton clearly has “cut out,” the topmost full-sized boll typically is regarded as the uppermost harvestable boll. Usually there is a visible size difference between this and the smaller bolls near the top of the plant. Missing fruit often make it somewhat more difficult to identify the average nodal position of the uppermost harvestable boll, but, once this boll (or nodal position) has been identified, it should be monitored and harvest-aid applications made when it attains the maturity criteria noted above.
- Nodes Above Cracked Boll (NACB) is a relatively new technique that uses the principles of plant monitoring to determine the proper time for harvest-aid application. This technique can use average heat unit accumulations to determine whether the plant is ready for harvest-aid application or approximately how long it will be until the plant is ready. Square initiation, flowering, and boll development proceed up the main stem in an orderly manner during the life of the cotton plant. At first-position fruiting sites, the difference in age for each node is approximately three days, or 55 heat units. This relationship occurs in theory throughout boll development in the plant. As the end of the season approaches and daily heat unit accumulation declines, allowance will need to be made for the three-day rule. The difference between nodes may be four – even five – days as the season end nears and cooler temperatures are present.
ii. Types of Defoliants:
Defoliants fall into two general types: those with herbicidal activity and those with hormonal activity. Def, Folex, Aim, Blizzard, ET, and Resource are herbicidal defoliants that injure the plant, causing it to produce ethylene in response. Ethylene promotes abscission, or leaf drop. If these defoliants are applied at rates too high for the existing temperature, the plant is killed too quickly, before ethylene can be produced. This results in desiccation or leaf stick instead of leaf drop.
Thidiazuron (Dropp, etc.) and ethephon (Prep, etc.) are hormonal defoliants that cause the plant to produce more ethylene. Ethephon releases ethylene, which stimulates further ethylene production in the plant. The increase in ethylene causes abscission zones to form in the boll walls and leaf petioles. The abcission zone is the point at which the plant tissues dissolve, allowing the fruit to open or leaf to fall from the plant. Thidiazuron is a type of hormone called cytokinin. In most plants, cytokinins promote leaf health. However, in cotton and related species such as velvetleaf, cytokinins promote the production of ethylene and act as a defoliant. Because these hormonal defoliants do not cause the injury that the herbicidal type does, they rarely cause desiccation.
* Boll-Opening and Combination Materials application
Boll-opening materials are often used in combination with defoliation materials to increase the percentage of the crop harvested during the first picking or to possibly eliminate the need for a second picking. Boll maturity is very important when using a boll-opening material. Lint micronaire and strength can be adversely affected if immature bolls are opened. In certain years, cotton micronaire is improved by mixing higher-micronaire cotton from the bottom of the cotton plant with lower-micronaire cotton from the top of the plant. Picking capacity, the number of unopened bolls, and the cost of second picking determine if boll opening is economical.
Sodium chlorate, paraquat. Desiccants are generally not used as harvest-aids for cotton harvested with spindle-type pickers. If desiccation is necessary because of regrowth or weeds, it is best to apply a defoliant, wait until leaf drop occurs, and then apply the desiccant. Desiccants can kill the entire plant and burn immature bolls. Therefore, 90 percent of the crop should be open before applying a desiccant. Plan to pick within 7 days to avoid possible bark contamination.
The important points that should be considered during the harvesting of cotton are listed here:
- Harvesting (picking) of cotton: Kapas is perhaps the most costly and least efficient operation in cotton cultivation.
- Cotton usually harvested in three or more pickings. Number of pickings depends on maturation habit of the variety, seasonal and cultivation conditions.
- June sown cotton pickings taken up between October to December.
- Picking of cotton is a slow and tedious operation. It is more so in case of Asiatic cotton since the boll size is small and number of plants / unit area are more.
- Middle pickings are usually heaviest and most important (except in herbaceums in which the first picking is the principal picking)
- Careless picking, collection and heaping of Kapas makes the cotton dirty – fetch minimum premium. Skip – row planting facilitates easier pickings since the plants grow tall and compact with more concentration of bolls.
- Start picking when bolls are fully mature
- Picking should not done while the bolls are wet from dew or rain.
- Bolls spoiled during rain or damaged by insects or otherwise damaged should be picked separately and discarded for seed purpose.
- Kapas should be picked by removing only the locules from the bolls using only fingers, taking care to see that collection of trash like dried burrs, bracts, leaf-bits etc. is avoided.
- Kapas from open bolls in the lower regions of the plant should be picked first to avoid contamination by dried matter which may fall down while picking bolls in the upper portions of the plant.
- As far as possible, picking should be avoided in hot mid-day as there is a greater likelihood of collecting dried leaf bits etc. picking should preferably be carried out in the early morning (and Evening) when the weather will be cooler but it is essential that Kapas is fairly dry before picking. Wherever early morning dew is a problem, it will be advisable to wait till the dew evaporates. Similarly, if there is rain before harvest, picking should be postponed till Kapas becomes dry.
- During harvesting, the picked kapas should be heaped on a cloth or paper spread in a corner of the farm. The kapas should not come into direct contact with the soil to avoid increase of trash content.
- After picking, the kapas should be allowed to dry in the shade. Excessive exposure to sun can result in lowering of grade (due to yellowing) and should be avoided. The kapas should be stored preferably on a raised platform or at least on cemented floor inside a shed or room and should not come in contact with loose soil.
- Seed cotton should be clean, with a minimum amount of such material such as leaves and bracts.
- Moist cotton in any way should not picked or stored. At a moisture content of twelve percent or more heat may generate and damage the seed and to fibre.
- Picked cotton, when completely dry, should be stored in a dry place and covered if not ginned immediately.
- Use of non-cotton materials like hessian, woven plastics (polythene etc.) jute fibres and threads, synthetic threads, etc. should be avoided during picking, storage and transport of kapas and lint.
Cotton is the cash crop that has high economic importance. We know human has three main basic needs: “Gaas, Bass and Kapas”. This shows that how the cotton is important for us. Globally, China, USA, and India are the mainland for the cotton production. In Nepal cotton production is very less. In the cotton production, cultivation practices only don’t play important role to produce the physical and economic optima of cotton. It may produce physical optima but to produce good quality of cotton the activity that is done during harvesting plays the major role. For good harvesting it is necessary to harvest crop at harvesting maturity and in efficient method that suits it. For cotton, it is necessary to treat the plant for harvest-aids so to make harvesting more efficiency. For this we have described the techniques like defoliation, boll opening and desiccation. After the harvest-aid program we can then harvest the cotton at appropriate time.
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