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Factors affecting fiber quality

CONTROLLING THE weather is a lost cause. However, variety selection and management are two areas a grower can take into his own hands to help improve the fiber quality potential of his crop. For instance, FiberMax cotton seed, produced by Aventis, is scoring high marks for quality. Proper management of such a variety can improve return at the gin.

“The current FiberMax cotton seed product line offers a range of maturity, stress tolerance and disease resistance — all with fiber quality better than the current crop average,” says Jane Dever, Ph.D., global coordinator, cotton breeding and product development for Aventis CropScience. “Specific fiber properties determine the value and utility of a bale of cotton for subsequent processing. These properties help define the monetary value of each bale.”

The properties of fiber quality are length, strength, micronaire, length uniformity, color grade and leaf grade. Fiber quality, ultimately determined after the crop is harvested and ginned, is influenced by various factors throughout the season.

Fiber Properties

Fiber Length. The importance of fiber length to textile processing is significant.

Longer fibers produce stronger yarns that allow for more valuable end products. Longer fibers also enable higher spinning speeds.

“The variability in fiber length can be explained primarily by genetics. Fiber elongation begins at bloom and continues for about 21 days,” Dever says. “Starting with a variety that has better genetic potential for fiber length will minimize the probability of producing fiber length in the discount range.”

Moisture stress during this period will reduce fiber length in all varieties. Severe weathering after bolls have opened can reduce fiber length because more breakage can be expected in the ginning process, Dever says.

Fiber Strength. Fiber strength is translated directly to yarn strength. In addition to producing stronger yarns, stronger fibers allow for faster and more efficient processing. Final strength level is achieved early in the fiber development stage; however, weathering can weaken the primary wall and cause strength levels to decline slightly in the weeks after boll opening.

“Fiber strength is the most variety-dependent property.” Dever says. “Any factor that causes either physical or microbial damage to the fiber can reduce strength. Severe drying at the gin also can reduce strength. The best management practice for fiber strength is variety selection, followed by sufficient potassium levels at the root zone in the soil.”

Micronaire. Micronaire is determined both by fiber diameter and the amount of secondary wall development prior to boll opening. Micronaire is an indirect measure of both fiber maturity and fiber fineness (perimeter).

“There must be at least 100 fibers in the cross-section of yarn, so high micronaire fibers can become a limiting factor in spinning fine yarn,” Dever says. “Fine fibers that are immature can break during processing and they do not dye well. This is why there are price discounts for both high and low micronaire. Fiber biological fineness is influenced by genetics, but environment and management can substantially affect fiber maturity.”

Length Uniformity and Short Fiber Content. Length uniformity is now part of the premium/discount valuation of cotton. Short fibers within a process mix of cotton cannot wrap around each other and contribute little or nothing to yarn strength. Short fibers indirectly cause product defaults and directly contribute to higher waste and lower manufacturing efficiency.

“Since short fiber content and length uniformity are derived from length, they are influenced by the same factors as length,” Dever says. “Crop management practices that influence where bolls are located on the plant can impact short fiber content levels. Uniform fruit retention patterns encourage better length uniformity.”

Color Grade. Both yellowness and brightness (reflectance) define color grade. Less total cellulose production during development can lead to lower brightness levels. The direct impact of color is in the bleaching process. Textile mills must control the color of their cotton within narrow limits to achieve a consistent fabric shade. Cotton loses some luster and brightness the longer it is subject to weathering.

“Environmental factors account for 79 percent of the color variability in cotton,” says Dever. “When total rainfall after the cotton opens exceeds 2 inches, loss in color grade will occur. Discoloration can also be caused by the action of microorganisms in connection with boll rot or insect damage. Cotton stored in modules with moisture content above 11 percent, excessive trash, or improper covering can lead to reduction in color.”

Leaf Grade. The amount of leaf grade is not a function of physical fiber development. While burs, stems or weed plant material can contribute to leaf grade, the major source of trash is leaf and bract material. Leaf grade has the most direct effect on manufacturing waste in the textile mill. Excessive leaf trash requires more cleaning and air-handling equipment, and impacts product quality.

“Variety can impact leaf grade when there are differences in the amount of trichomes (hairiness) in the leaf and bract,” says Dever. “Small bits of hairy leaves cling to fibers making their removal more difficult. High plant population, delayed maturity, excessive vegetative growth, lack of weed control and defoliation problems impact harvest efficiency and increase the amount of leaf in a bale.”

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