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The impact of defoliation timing on fibre quality and textile performance

In recent years there have been concerns relating to high micronaire and neps in Australian cotton. In the case of micronaire it has been suggested that warmer seasons and improvements in agronomic practices (such as soil and plant nutrition) that encourage better growth and yields along with pest management strategies and the introduction of Bollgard II (Monsanto) that improves fruit retention, have all contributed to this issue.

Growers in Australia are discounted when micronaire is too high or too low (optimum G5 range is 3.8 to 4.9). While currently there is no discount to growers when there is a high incidence of neps, it can affect the overall industry reputation when cotton arrives at the mill.

Micronaire is a measure of the specific surface area of fibre, and is directly proportional to the product of the fibre’s maturity and linear density (fineness). Maturity relates to the degree of thickening of the cell wall during fibre development. Immature fibres will have lower micronaire and may be more prone to nep formation during lint cleaning. Neps typically absorb less dye, appearing as ‘flecks’ on finished fabrics. Fibre immaturity has also been associated with non-uniform dyeing of fabrics.

There are concerns that management practices that force open immature bolls to be included in the harvest to increase yield or to reduce micronaire may increase the incidence of the textile issues described above.

Premature defoliation application may be an example. The generally recommended practice for defoliation application is to apply defoliants when at least 60 per cent or more of the bolls on a plant are open and the remainder mature.

This article details a field experiment that was conducted to systematically vary the timing of defoliation, with the intention of generating different amounts of immature fibre at harvest. The effects were assessed in terms of fibre quality and textile performance. This information will form part of a larger study that aims to develop and test crop management guidelines to optimise crop yield and fibre quality to meet textile production standards.

Methods
The Experiment was conducted at the Australian Cotton Research Institute (ACRI) using Sicot 71BR sown in October 2005. Defoliant and a boll opener were applied at approximately five day intervals from 143 days after sowing resulting in eight defoliation treatments (Table 1). Treatments were replicated four times. Treatment plots were harvested using a single row picker, and seed cotton was ginned (without lint cleaning) using a 20 saw gin located at ACRI.

Table 1: Time of defoliation application and corresponding per cent open bolls, and lint yield 

Values with the same letter are not significantly different

Sub-samples of ginned lint were subjected to high volume instrument (HVI) testing to determine length, strength and micronaire. Samples were also tested for maturity ratio via the CSIRO SiroMat maturity tester and for fineness (linear density) via the CSIRO CottonScan.

Sub-samples of ginned lint were also subjected to one and two passes of lint cleaning with an experimental lint cleaner. Samples and a control (no lint cleaning) were tested for neps using an Uster AFIS PRO instrument.

CSIRO’s new alternate miniature spinning protocol was used to manufacture 20 tex yarns from 170 g un-lint cleaned samples. Yarn strength was measured via an Uster Tensorapid instrument.

Yarns were knitted into fabric which was scoured and dyed with Cibacron blue reactive dye. To determine the amount of dye uptake in fabric, reflectance measurements were taken of fabrics using a spectrophotometer. Colour differences between the dyed fabric samples were measured in terms of delta E (ΔE), which is the mathematical difference between the colour of the control (mature cotton) and the treatment fabrics. Delta E values greater than were significantly different.

Discussion

Fibre yield and quality
Yield of cotton lint from defoliation treatments applied up to 56 per cent open bolls were significantly less than later treatments (Table 1). Fibre length for non lint cleaned lint was on average about 1/32nd inch shorter for treatments with defoliant applied up to 68 per cent open bolls. This is probably due to the less mature fibres in these treatments having a greater propensity to break up during mechanical handling (harvesting and ginning) (Table 2).

Table 2: HVI fibre length, strength and micronaire, and CottonScan fineness (linear density) and SiroMat maturity ratio for machine harvested ginned lint (not lint cleaned) for cotton subjected to different defoliation treatments

Values with the same letter are not significantly different.

Fibre micronaire and linear density were significantly less for defoliation treatments applied up to 42 per cent open bolls, yet no significant differences were noted between treatments from 56 per cent open bolls (Table 2). The range of fibre maturity ratio across treatments was smaller than anticipated, although the earliest treatments (29 and 42 per cent open bolls) had maturity ratios less than 0.90 (Table 2). Indeed, the difference in the amount of cell thickening between the earliest (29 per cent open bolls) and the latest (100 per cent open bolls) defoliation treatments is evident visually (Figure 1).

Lint cleaning and neps
Neps were higher than expected across treatments (over 250 counts per gram). Although not strongly significant, there was a slight trend for higher neps for earlier treatments, but no significant interaction was noted between defoliation treatment application and the amount of lint cleaning. As expected lint cleaning had a strong influence on nep generation, with each lint cleaner passage generating about 100 counts per gram (Table 3). This result showed that changes in defoliation management had little impact on nep generation in this study but rather the mechanical process of lint cleaning had the greater affect.

Table 3: Uster AFIS PRO Neps for machine harvested ginned lint subjected to 0, 1 or 2 lint cleaner (LC) passages for cotton subjected to different defoliation treatments 

Textile performance
No significant differences were noted across treatments for important yarn performance parameters including yarn strength (Table 4). This is in-line with bundle strength results, which were not significantly different across treatments (Table 2).  

Table 4: Textile performance attributes: yarn strength, and the mathematical distance (ΔE) between the colour of knitted fabric dyed with Cibacron blue reactive dye 

 
It seems that even though the earlier defoliated treatments contained more immature fibre, there was a greater total number of fibres in the fibre package (either bundle or yarn) in the immature samples, which contributed to greater inter-fibre friction and strength.

For fabric dye uptake analysis, early treatments (at 29 and 42 per cent open bolls) displayed delta E values greater than 1 and thus appeared visually lighter in hue compared to later treatments. The thicker more mature fibres (Figure 1B) absorbed more blue dye molecules and thus appeared a deeper blue hue.

Overall the results suggest that the management practice of defoliating cotton from 60 per cent open bolls appears to have no impact on textile performance. Studies are continuing to assess this management practice in a broader range of climatic conditions. It is also the intention that as the impacts of management practices on textile performance are detected, management recommendations will be refined to ensure that Australian cotton maintains its reputation for good quality.

The authors acknowledge input from CSIRO colleagues Geoffrey Naylor, Rene van der Sluijs, Stuart Gordon and Greg Constable. We thank the Cotton Catchments Communities Co-operative Research Centre, the Australian Cotton Research and Development Corporation and the CSIRO for financial support, and we greatly thank Jane Caton, Darin Hodgson, Rebecca Giles, Fred Horne, Mark Freijah, Susan Miller, Geni Kozdra, Susan Horne and Colin Brackley for technical assistance. For more information about this work or the Linking Farming Systems project, please contact Dr Robert Long, E: robert.long@csiro.au 1Cotton Catchment Communities CRC, CSIRO Materials Science and Engineering, and CSIRO Plant Industry.