Fibre quality

Despite our relatively small crop, Australian exports still make up over 10 per cent of the medium/high grade cotton volume in the export market.

The quality of Australian cotton has improved over the last two decades and has earned a very good reputation amongst spinners for its good spinning ability and low contamination.

Nearly all of Australia’s cotton lint is exported for high-quality end use in mills in Southeast Asia. It is used primarily for producing high-quality fine to medium count yarns for use in the woven and knitted apparel sectors.

 

 

The price received for cotton is dependent on the quality of each bale of cotton. Samples are collected from opposite sides of each bale at the gin after bale formation, and then forwarded to a classing facility for classification.

Cotton prices are quoted for ‘base grade’, with the base grade referring to the grade of cotton that is used by cotton merchants as a basis for contracts, premiums, and discounts. Currently the base grade for Australian cotton is Colour 31 (Middling) with Leaf 3, length 36 32nds and micronaire G5 (3.5 to 4.9) and premiums and discounts apply for higher and lower grades, respectively.

Colour: Colour is currently measured by a trained cotton classer using current Universal Grade Standards for Upland cotton as prepared by the Agricultural Marketing Service (AMS) of the United States Department of Agriculture and by a High Volume Instrument (HVI).

The colour of the majority of Australian cotton is assessed visually and considers both major and minor differences in colour. Major colour differences occur between the five classes of ‘white’, ‘light spotted’, ‘spotted’, ‘tinged’ and ‘yellow’ stained cotton, chiefly due to increasing degrees of yellowness across the five classes. Within each of these classes the reflectance or whiteness of the fibre is assessed across another eight levels from ‘Good Middling’ to ‘Below Grade’. There are currently 25 official physical colour grades for Upland cotton and five grades for below grade colour.

Official colour grades applied to Upland cotton: 

* Base grade

The colour of cotton as measured by HVI is determined by a colorimeter and defined by the Nickerson-Hunter colour model, in terms of brightness (Rd) and yellowness (+b).

A very good correlation exists between colour grade as obtained visually and objectionable by HVI instrument. 

Leaf: Also known as ‘trash’, is a measure of the amount of leaf material remaining in the cotton sample. The amount of trash present in a bale of cotton is affected by the variety, harvesting methods and conditions and whist the gin removes most of the trash, some remains in the sample which is removed in the spinning process resulting in a reduction in lint yield and increases cost. Hence, cotton with high levels of trash attracts a discount. Leaf grades range from 1 (lowest amount of trash) to 7 (highest amount of trash).

Staple length: Length is measured on a sample of fibres known as a ‘pull’ when hand classing and is measured to the nearest 1/32 inch. HVI instruments determine length in hundredths of an inch on a ‘beard’ or tuft of lint formed by grasping fibres with a clamp. Australian cotton is all classed using HVI measurements.

Upland length conversation chart:

Length (32nds)	Length (Inches)	Length (32nds)	Length (Inches)
24	0.79 & shorter	36*	1.11 – 1.13
26	0.80 – 0.85	37	1.14 – 1.17
28	0.86 – 0.89	38	1.18 – 1.20
29	0.90 – 0.92	39	1.21 – 1.23
30	0.93 – 0.95	40	1.24 – 1.26
31	0.96 – 0.98	41	1.27 – 1.29
32	0.99 – 1.01	42	1.30 – 1.32
33	1.02 – 1.04	43	1.33 – 1.35
34	1.05 – 1.07	44 & +	1.36 &    +
35	1.08 – 1.10

* Base grade

Micronaire: Micronaire is measured by placing lint in a chamber, compressing it to a set volume and subjecting it to a set pressure. The micronaire result measured in this way is in fact a function of both fibre maturity and fineness (linear density). As the reading is an approximate guide to fibre thickness the trade use the following micronaire ranges to describe samples:

≥ 5.3	G7
5.0 – 5.2	G6
3.5 – 4.9*	G5
3.3 – 3.4	G4
3.0 – 3.2	G3
2.7 – 2.9	G2
2.5 – 2.6	G1
≤ 2.4	G0

* Base grade

The premium range is 3.8 to 4.5 and the base range is 3.5 to 4.9 (G5) and discounts apply for cotton with a micronaire outside the base range. Discounts for low micronaire can be substantial.

Strength: Fibre strength is highly dependent on the variety, although environmental conditions can have a small effect. Rain grown cotton strength is usually not adversely affected by growing conditions. Most Australian varieties are of high strength and local plant breeders have agreed to eliminate varieties that do not meet a minimum standard, thus keeping Australian cotton highly competitive in the world market. Fibre strength is measured by clamping a bundle of fibres between a pair of jaws and increasing the separation force until the bundle breaks.

Most Australian varieties are of high strength, which is expressed in terms of grams force per tex with the following classifications:

Descriptive Designation	Strength (grams per tex)
Weak	23.0 & below
Intermediate	24.0 – 25.0
Average	26.0 – 28.0
Strong	29.0 – 30.0
Very strong	31.0 & above

Contamination: Australian cotton is recognised as one of the least contaminated cottons in the world and receives a premium - so any contaminants lower the value of the product and can potentially damage our reputation. Contaminants can either be natural (rocks, wood, leaf, bracts, bark, green leaf, burrs, and honeydew), or man-made (torn cotton tarps and plastic wrap, twine, oil, hydraulic oil, grease, pieces of metal and equipment, food wrappers, drink bottles, mobile phones, and cleaning rags). In Australia, the biggest issue is plastic, since most of the crop is harvested with harvesters that produce round modules covered with plastic wrap.

Stickiness: Cotton stickiness, when it occurs, can present a major problem, in terms of textile processing performance and cost and product quality. If a country gains a reputation for stickiness, they have difficulty selling their cotton, and incur large discounts. The most common and problematic causes of stickiness are those due to excess sugars related to insect secretions, notably aphids and whitefly and are generally referred to as honeydew these being responsible for some 80 to 90 percent of stickiness problems. In addition, a black sooty mould can also grow on honeydew, darkening the lint and adversely affecting grade.

Other important quality characteristics

Neps: Neps are defined as an entanglement of two or more fibres, creating a small knot (pictured below). Neps are primarily caused by immature fibres which affect the appearance of cotton yarns and subsequently the fabric made from them. Neps are usually associated with lower yarn strength, higher ends down in spinning, less uniform yarn, causing a loss in spinning efficiency as well as resulting in stoppages defects during fabric manufacture. The appearance of dyed or printed fabrics is also negatively influenced by the presence of neps, as immature fibres absorb less dye and appear as white spots or flecks on finished fabrics.

Seed coat fragments: Seed coat fragments (pictured below) appear during the ginning process when the lint is removed from the seed. The fibre is not cleanly removed from the seed coat and the fibre will carry a slight fragment of seed coat. This is problematic as seed coat fragments are very difficult to remove and can cause yarn faults or breaks, reducing the overall quality of the final product as they appear as brown/dark spots or flecks on finished fabrics. 

 

 

 

 

 

 

 

 

 

 

Fortunately, most crop management factors which increase/optimise yield will also increase/optimise fibre quality. Below are a few decisions which could affect your fibre quality:

1. Select an appropriate variety for your region according to recommendations from CSD.
2. Choose the optimal sowing date for your area to maximise yield and fibre quality.
3. Effective control of weeds to minimise contamination, staining and reduced harvest efficiency.
4. Meeting the nutritional requirements of the crop as nutritional excess and deficiencies can have a significant effect on fibre quality.
5. Implement good IPM strategies to avoid insect damage and avoid unnecessary pesticide applications.
6. Avoid aphid and whitefly infestations to avoid sticky cotton.
7. Optimise the timing of cut-out to minimise harvest of immature bolls.
8. Apply good defoliation (product, rate, and time) and a timely harvest.
9. Implement good farm hygiene practices to avoid contamination.
10. Ensure harvesters (both spindle and strippers) are regularly maintained and correctly set up.
11. Avoid harvesting seed cotton with moisture content > 12 per cent.
12. Ensure module wrap/tarpaulins are intact and not damaged.