Cotton is a fibre which protects the seed of the cotton plant, much as the flesh of an apple protects the pips. So, fruit are not only for eating.
In late 18th century England, James Hargreaves’ ‘spinning jenny’ led developments in cotton spinning technology for the textile industry and primed the Industrial Revolution, leading to factories and mass production.
Now cotton is a crop at the forefront of the revolution in biotechnology, being the second genetically modified (GM) crop to be introduced in broadacre agriculture after soybeans in 1997. One of the main traits in GM cotton is tolerance to applications of the non-selective herbicide glyphosate. Other GM cotton varieties provide protection against some of the many insects which can ravage the crop. The choice of herbicide resistance as an early target for biotechnology research indicates the importance of effective weed control in cotton. Although herbicide tolerant cotton has many advantages, it has contributed to enormous increases in the use of glyphosate and the consequent problem of resistant weeds which are no longer controlled by this herbicide.
Paraquat is the alternative non-selective herbicide which, when used in integrated weed management systems, can provide solutions to these weed control issues.
According to WWF, the world's largest and most experienced independent conservation organisation, the most important environmental impacts from the use of agrochemicals in cotton are:
“ ... The quality of soil and water and the impact on biodiversity in and downstream from the fields ...”
Using paraquat in weed control programmes can help to address these environmental issues arising from cotton growing and to combat weed resistance too .
Major environmental issues
WWF cites the major environmental issues in growing cotton as:
- Intensive use of agrochemicals
- Excessive water use in irrigation and processing
- Soil erosion
Displacement of solids (soil, mud, rock and other particles) usually by the agents of currents such as, wind, water, or ice by downward or down-slope movement.
Authoritative On-line References and Resources
http://soilerosion.net/ This site brings together reliable information on soil erosion from a wide range of disciplines and sources. It aims to be the definitive internet source for those wishing to find out more about soil loss and soil conservation.and degradation
- Water contamination by run-off
The occurrence of surplus liquid (like rain) which originates up-slope and is collected beyond the ability of the soil to absorb it. The surplus liquid then flows away over the surface to reach the nearest surface water (pond, lake, river).
Authoritative On-line References and Resourcesand leaching
The natural process by which water soluble substances are carried downward through the soil into groundwater.of agrochemicals
- Loss of habitats and effects on biodiversity
Paraquat is an essential tool for cotton farmers
Paraquat is an extremely versatile tool in the cotton farmer’s battle against weeds.
A video showing paraquat’s unsurpassed speed of action can be viewed here.
It can be used from preparing fields for planting to conditioning crops for harvest. Using paraquat as an alternative non-selective herbicide, with a different mode of action, in integrated weed management systems, is helping to avoid problems of weed shifts and resistance.
The importance of paraquat in fighting weed resistance to glyphosate and maintaining farmers’ options to use conservation tillage systems is discussed here.
Paraquat is deactivated on contact with the soil meaning that it can be sprayed to burndown weeds before planting a cotton crop without risking damage to that crop or indeed subsequent crops in the rotation. There are no leaching, persistence or root uptake problems to restrict its use, unlike many other herbicides which have ‘residual’ properties.
You can read more about paraquat’s unique soil properties here.
Paraquat works well even in cold and rainy weather, unlike most herbicides, making it suitable for use early in the season to control weeds in conservation tillage systems. These systems do not rely on ploughing to control weeds. Not disturbing the soil helps prevent erosion and maintains a healthy soil. Although paraquat is a non-selective, broad spectrum herbicide, if small amounts land on leaves there is little or no crop damage because paraquat does not move through plants systemically like glyphosate. So, paraquat can be used for inter-row weed control to remove weeds growing between the crop rows.
Paraquat can be used effectively and safely as a harvest aid to avoid green leaves staining the lint, control entangled weeds, and speed harvest by cotton pickers.
Paraquat has a very robust environmental profile. It does not leach and is degraded in soil. Further details of paraquat’s safety to the environment, spray operators and consumers can be found by visiting the different sections of the Paraquat Information Center or referring to the Paraquat Fact Sheet.
You can read more about the benefits of using paraquat here.
Where does cotton come from?
Cotton yarn is spun from the fine downy fibres, or lint, which surround seeds of the cotton plant. There are many species within the cotton family, but only two are grown commercially. The most widely grown is Gossypium hirsutum (upland cotton) accounting for 97% of production. The other is Gossypium barbadense (creole or sea cotton).
The principal difference between Gossypium hirsutum and Gossypium barbadense is the length of the cotton staple (individual fibres). Gossypium barbadense has very fine, premium, long staple cotton 5 cm long compared to Gossypium hirsutum’s typically shorter staple length of 2-3 cm.
Staple length (average and uniformity) is an important factor in the quality of the yarn or thread produced. Other quality aspects are staple strength, colour and content of leaf trash. All these attributes may be affected by how and where a crop is grown, eg soil fertility, temperature, rainfall and irrigation, pest and diseases, and the conditioning of the crop for harvest. Although white cotton is the norm and most prized, there are varieties which produce green or brown lint. These are usually excluded from white cotton growing areas to preserve genetic integrity.
Cotton is an annual plant with broad lobed leaves. Its woody stem and flowering characteristics, however, show its former perennial nature. Cotton flowers over an extended period and mature bolls (seed capsules) may be present on the same branch as new flowers. However, plant breeders have managed to develop varieties which are more synchronous flowering. The development of suitable harvesting machinery enabled mechanised single harvests to replace several rounds of hand-picking.
Nevertheless, cotton is not as domesticated as other major crop plants and in intensive production systems chemical growth regulators are widely used. Cotton plants can grow up to 3 m high, but excessive vegetative growth is controlled chemically. This improves the yield of cotton fibre. Despite more synchronous flowering, growth regulators are used to encourage the opening of less mature bolls. The cotton lint needs to be exposed for picking or stripping from the bolls by harvesters.
After harvest, cotton lint, which is still attached to seeds, is taken for ‘ginning’. In cotton gins (from en’gin’e), the lint fibre is carefully mechanically combed through a sieve to separate lint and seeds, and packed in bales. Cotton is then ready for transport to textile mills.
Apart from the use of cotton fibre in textiles, the pure cellulose is a feedstock in the production of chemicals for use in products as varied as cosmetics, films and plastics. Lint accounts for 30-45% by weight of cotton harvested, but 85% of the commercial value. However, the 55-70% that is seed also has its uses. The crop is the world’s fifth most important oilseed. After the seed has been crushed to extract the oil, the remaining meal is a useful livestock feed.
Where is cotton grown?
China, USA and Africa each accounted for a further 16%, 12% and 10% of the total, respectively. Significant areas were also grown in Asia Minor and South America, mainly in developing countries.
Although native to the tropics, cotton crops are grown as far north as the Ukraine and as far south as South Australia. It does, however, remain vulnerable to frost.
Some harvest data for main producing countries are shown in Table 1.
Table 1. Cotton production in leading countries in 2009 (FAO data).
|Harvested Area (ha)||Cotton Seed Yield (t/ha)||Lint Production million bales*|
* 1 bale = 480 pounds (218 kg)
How is cotton grown?
Cotton is a ‘row crop’.
Cotton is sown in late spring to early summer and harvested in early autumn. Cotton seed is planted in rows of varying widths. Traditionally, wide rows (about 75 cm apart) have been used which enabled weed control by mechanical cultivators while the crop was growing. These still by far the most popular in cotton, but narrow rows (25-50 cm apart) and ultra narrow rows (less than 25 cm apart) are also used..
Cotton followed closely on soybeans as the second genetically modified (GM) broadacre crop to be introduced in 1997 in the US. In 2009, 16 million ha of GM cotton were grown in the world (53% of the total area). In the US, herbicide tolerant cotton now accounts for 75% of the area grown. About half are varieties which are ‘stacked’ with resistance to insects and herbicides. The vast majority are tolerant to glyphosate. Although this has simplified weed control there are issues with crop damage if glyphosate is applied as crops prepare to flower, adverse changes in the weed flora and resistance to glyphosate developing. Paraquat has an important role to play in avoiding these problems by its use in integrated weed management systems.
Soil erosion and conservation tillage systems
Broad-spectrum herbicides, led by the introduction of paraquat in the 1960’s, have allowed the adoption and growth of soil cultivation systems which do not rely on controlling weeds by burial from ploughing. Abandoning the mouldboard plough in no-till systems saves cash, time and fuel, improves soil structure, reduces erosion and provides havens for wildlife. More recently, conservation tillage systems have been developed as a wider approach to crop production than just tillage. These include the use of cover crops and mulches of residues of previous crops to ensure at least 30% of the soil surface is always covered to prevent soil erosion.
Popular conservation tillage systems in the US include growing a cover crop such as wheat or rye over winter which is burned down before planting cotton; and strip tillage where a narrow strip of land (about 40 cm wide) is tilled into which the rows of cotton are planted, leaving stubble in between the rows.
These techniques help to reduce soil erosion. Cultivating soil to control weeds can lead to soil erosion which removes nutrients and organic matter and decreases water holding capacity. In the fields of Alabama, a major cotton growing state in the US, an average 25 tonnes/ha of soil are estimated to be lost each year. This equates to about 25 mm of soil depth. Crop scientists estimate that this would lead to a yield loss of up to 0.6 tonnes of lint per hectare after 20 to 30 years of erosion at that rate.
Pests & diseases
Cotton crops often come under heavy pressure from insects. An estimated 15% of the global crop is still lost to insects even after control measures. Pests and diseases also affect cotton quality, including staple length and strength, and lint colour. Apart from the use of chemical insecticides, control methods may include management techniques such as the careful use of irrigation and GM varieties engineered to express insecticidal proteins found in the bacterium Bacillus thuringiensis (BT).
For the US, details of the wide range of insect pests, diseases and weeds infesting cotton can be found by visiting the website of the National (US) Information System for the Regional IPM Centers.
Insects & Nematodes
Worldwide, insect pests attacking developing bolls include: pink bollworm (Pectinophora gossypiella), especially in Africa and India; the boll weevil or bollworm (Anthonomus grandis), is common in the Americas; other bollworms are the Egyptian (spiny) bollworm (Earias insulana) and the red bollworm (Diparopsis castanea); cotton stainers (Dysdercus superstitious) may stain the lint and feeding wounds may allow fungi to access the bolls.
White flies (Bemisia gossypiella) suck sap from leaves and are a serious threat in India and Africa. The cotton melon aphid (Aphid gossypii) infests seedlings, sucking sap from leaves and secreting honeydew on the undersides of leaves. Honeydew may burn leaves and interfere with photosynthesis. Aphids are also virus vectors.
Nematodes living in the soil, including the root knot nematode (Meloidogyne incognita) and the reniform nematode (Rotylenchulus reniformis) are serious pests in the US, withdrawing nutrients and causing stunted growth.
Fungal and bacterial diseases affect bolls, leaves and roots from seedlings to mature plants. Warm, wet weather encourages bacterial blight (Xanthomonas malvacearum) which stunts and defoliates plants. Bolls may not open on infected plants. Several fungi attack bolls directly. Diplodia gossyina, Colletotrichum spp. and Fusarium spp. rot lower bolls under warm, humid conditions and may give a brownish tint to the lint. Plants previously damaged by insect pests are especially susceptible.
One of the most damaging and hard to control diseases is cotton root rot (Phymatotrichum omnivorum) attacking mature plants. It survives for long periods deep in soils and becomes active in very hot weather. The soil-borne fungus Verticillium dahliae penetrates though roots and grows up along the stem tissue under cool, water-logged conditions to kill young seedlings. Other seedling diseases include Rhizoctonia solani, Fusarium spp. and Pythium spp.
Weeds are a major problem in cotton, whatever the climate. Some predominant grass weeds found in most growing regions in the US include johnsongrass (Sorghum halepense), crabgrasses (Digitaria spp.) and goosegrass (Eleusine indica). Major broad-leaved weeds include morningglories (Ipomoea spp), teaweeds (Sida spp.), cockleburs (Xanthium spp.), and velvetleaf (Abutilon theophrasti).
Weed control in cotton is highly sophisticated. Because it is grown in wide rows, mechanical cultivation to control weeds growing between the rows is possible and still practised, but most weed control is by using herbicides.
Herbicides can be used at several stages:
- Pre-planting or pre-emergence: to burndown existing weeds and provide residual control
- Post-emergence: by use of selective herbicides
A chemical product used for eliminating certain types of weeds only (ie either grasses or broadleaved weeds) and not affecting specified crops.
Authoritative On-line References and Resources
http://www.weeds.iastate.edu/ An invaluable source of contemporary information about herbicides and weeds from Iowa State University.or inter-row weed control with paraquat based products.
- Pre-harvest: harvest aids such as paraquat are sprayed to control large weeds and desiccate the crop to make harvesting easier and avoid green leaves staining the lint.
In conservation tillage systems weeds or a cover crop such as rye, wheat or oats have to be controlled by a burndown herbicide, either paraquat or glyphosate. 2,4-D is often mixed with both of these to improve control of some difficult weeds. A paraquat-diuron mixture is used in Brazil. Paraquat-based burndown sprays are more reliable than glyphosate in controlling annual weeds and cover crops at earlier growth stages when the weather is cold and rain falls soon after application. Particularly under such challenging weather conditions, paraquat will control weeds in a few days compared to glyphosate’s 2-3 weeks. Glyphosate is a systemic herbicide and gives a thorough kill of perennial weeds.
Integrated weed management
New ways of farming always lead to ‘weed shifts’ where certain species become more dominant in the weed flora: Without plowing annual grasses, small-seeded broad-leaved weeds and perennials increase. More small seeds are left on the surface ready to germinate and perennials survive better as the underground rhizomes or stolons are not disturbed or destroyed. Weeds less well controlled by particular herbicides used will tend to become more dominant and with intensive use of few modes of action resistant plants may become a serious problem.
Glyphosate resistant weeds in US cotton now include horseweed (Conyza canadensis), Palmer amaranth (Amaranthus palmeri) and giant ragweed (Ambrosia trifida). No more than two applications of glyphosate should be applied to any one field over two seasons. Paraquat can provide the alternative means of effective and sustainable weed control.
Defoliants are applied prior to harvest. These remove leaves exposing the bolls for harvesting. As a final defoliation treatment, paraquat is used to desiccate remaining leaves to ensure leaf chlorophyll cannot stain the lint during harvesting which would otherwise reduce its value. A desiccated crop, dry and free from any entangling weeds, is harvested more quickly.
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