Wool Biology and Production

CSIRO Livestock Industries, Wembley, Western Australia

INTRODUCTION

Wool is a generic description of hair from various breeds of domesticated sheep (Ovis aries). Wool appears to be the earliest material man used to spin and weave into clothing, with evidence of shears for harvesting wool being used around 1000 b.c. Requirement for shearing implies development of sheep with a continuously growing fleece. These developments associated with domestication have continued until this day, although wool is no longer a dominant textile fiber.

BIOLOGY

The gross morphology of a wool fiber is shown in Fig. 1.[1] The fiber is surrounded by cuticle cells that overlap in only one direction, leading to directional frictional characteristics and wool felting. The cuticle has four layers with a combined thickness of 0.5 to 0.8 mm, occupying between 6 and 16% of total fiber weight.

The cortex, composing 90% of fiber weight, consists of two cell types, ortho- (60 to 90%) and paracortex cells (10 to 40%), the latter containing higher quantities of sulphur than the former, resulting in a tougher cell with more cross-linkage. Cortex cell-type arrangement changes with increasing fiber diameter. In fine-wool Merinos, the cortical cells are arranged in a bilateral manner, and the border between cell types is arranged in helical pattern along the fiber axis. This helical pattern results in fiber crimp, with paracortex being situated in the inner part and orthocortex in the outer part of the crimp. Cortex cells have a spindlelike shape, being 45 to 95 mm long and 2 to 6 mm wide. Ortho-cortex cells rarely contain nuclear remnants and cytoplasmic residues.

At its widest point, each cortical cell contains 5 to 20 clearly separated macrofibrils embedded in intermacrofi-brillar matrix material, in a hexagonal array. Macrofibrils are composed of bundles of 500 to 800 microfibrils. Microfibrils, or intermediate filaments, are composed of alpha-helical proteins of comparatively low cystine content that are linked by both disulphide and hydrogen bonds. A matrix of intermediate filament-associated proteins surrounds microfibrils and is composed of two families of nonhelical proteins, one being cystine-rich and the other, glycine- and tyrosine-rich.

Wool is almost entirely composed of a family of proteins known as alpha-keratins. Merino wool has higher cystine content than coarse wools as a result of having a larger proportion of high-sulphur alpha-keratin proteins. Amino acid composition can vary between sheep, with the growth phase of the wool follicle cycle and with the nutritional status of the sheep.

Wool fiber diameters range 10 to 80 mm and have a density of 1.304 g/cm3, with slightly and imperfectly elliptical cross-sections. Wools with higher fiber diameter tend to be hairlike and medulated. Proteins in wool have the ability to adsorb water. At standard atmosphere of 65% relative humidity and 20°C, water regain ranges from 14 to 18%.[2]

Wool fibers are highly elastic, and if not strained by more than 30% of length for longer than one hour, they can return to their original state by soaking in water. The intrinsic strength of wool is low, varying between 50 and 300 megapascals.

Sheep wool follicles have a very long anagen phase, with 1 2% of follicles inactive at any one time. The general morphology of anagen follicles is shown in Fig. 2.[3] A connective tissue sheath surrounds the tubular down growth of epithelium, and there is a dermal papilla responsible for cell division. Blood vessels are found in the connective tissue sheath and, except in the smaller secondary follicles of Merino sheep, the dermal papilla.

Primary and secondary follicles are distinguished by their appendages and time of initiation in fetal skin. Primary follicles form first, at about 60 days postconception, and secondary follicles start 14 to 20 days later. Variable numbers of secondary follicles may form either as separate follicles or as outgrowths of other secondary follicles. Sweat glands and arrector pili muscles are appendages of primary follicles. Both follicle types have sebaceous glands. The ratio of primary to secondary follicles varies between sheep. Merino sheep with 19 mm

Morphology Woolwww.dekker.com.)"/>
Fig. 1 Gross morphology of a Merino wool fiber. (From CSIRO Livestock Industries.) (View this art in color at www.dekker.com.)
Fig. 2 Diagram of skin and wool follicle groups showing primary follicles, with arrector pili muscles and sweat glands, and secondary follicles. (From CSIRO Livestock Industries.)
Table 1 Principal greasy wool producing countries and world production from 1988 to 2002 (million kg)

Year

Australia

New Zealand

China

Eastern Europe

World

1988

916

346

209

613

2,919

1989

959

339

222

627

2,967

1990

1102

311

237

623

2,970

1991

1066

305

239

592

2,953

1992

875

296

240

526

2,989

1993

869

256

238

456

2,913

1994

829

284

240

439

2,817

1995

731

289

260

409

2,689

1996

725

275

298

263

2,541

1997

700

266

255

229

2,418

1998

684

252

277

198

2,379

1999

678

253

283

188

2,348

2000

652

237

291

187

2,303

2001

600

246

294

188

2,239

2002

565

253

293

189

2,217

(From the International Wool Textile Organization.)

(From the International Wool Textile Organization.)

wool have 80 follicles/mm2 and a secondary to primary follicle ratio of 20:1, whereas Drysdale sheep (43 mm) have 13 follicles/mm2 and a secondary to primary follicle ratio of 5:1.

PRODUCTION

Wool production normally occurs in areas where the pasture quality is adequate, but insufficient for meat production. Seasonal variation in clean wool growth is similar to the variation in availability of digestible dry matter per hectare, or crude protein available per hectare.[4] The amplitude in seasonal wool growth rate ranged from 20.3% in Armidale, New South Wales, to 650% in Huang Cheng, China. Peak wool growth occurred in the spring for cold, wet winter areas and in the summer in summer rainfall, tropical areas. These changes in wool growth are also seen in follicle bulb diameter, dermal papilla length, skin weight, and the incidence of active follicles in the skin.[5] Wool growth also varies between years, reflecting annual pasture production cycles, with amplitude in fleece weight between years varying from 15.8% to 118.5%.

Only 20% of the protein synthesized in skin is excreted as wool, with the remaining 80% being degraded in the skin or desquamated as epithelial cells. The value of 20% is robust for a wide range of sheep breeds and feeding levels.[6] Thus, genetic selection to increase fleece weight also increases the rate of skin protein synthesis.

Fleece weight is affected by age, pregnancy, lactation, and sex, with rams producing more wool than wethers or ewes.[7] Fiber diameter increases by 3.9 mm from 18 months to 6 years of age in rams, but for ewes the increase is 0.4 mm over the same period. Pregnancy and lactation reduce fleece weight by 30 to 600 grams and fiber diameter by 0.4 to 1.5 mm, and also affect staple strength.[8] Strong seasonal patterns in wool growth occur in many breeds, with annual rhythms synchronized by photoperiod acting through melatonin secreted by the pineal gland. Breeds such as the Merino have a reduced response to photoperiod.

Fiber diameter is the first determinant of wool price, and many wool breeding programs aim to reduce fiber diameter. Fiber diameter is changed by nutrient availability between and within years, but is mainly determined by the strain of sheep.

Seasonal variation in wool growth reduces staple strength, which is the second most important determinant of wool price. Staple strength has a strong genetic component, depending on variation in fiber diameter, fiber shedding, and the strength of individual fibers. For breeding ewes, the most critical time remains the last two weeks of pregnancy.[9]

World greasy wool production peaked in 1990, with a total production of 2970 million kilograms, and declined to 2217 million kilograms in 2002 (Table 1). On a clean wool basis, Australia and New Zealand are the world's two largest wool producers, although China produces more greasy wool than New Zealand. Australia is the dominant producer of Merino sheep in the 18- to 23-mm range for apparel production. New Zealand wool production is dominated by Romney sheep in the 30- to 38-mm range, which is suitable for carpets. China has increased wool production from 7.2% of world greasy wool in 1988 to 13.2% in 2002.

World production statistics are provided on a greasy wool basis, and greasy wool contains from 30 to 70% impurities. Wool impurities are wax, suint, dust, and vegetable matter. Sheep coats successfully reduce these contaminants. Low wool yields in countries such as China are a consequence of overnight corralling of sheep for feeding during cold winters and protection from the elements.

CONCLUSION

The average fine-wool sheep produces some 6000 kilometers of a complex protein fiber each year. This fiber is produced from wool follicles that use 20% of the protein turnover in skin. Wool is predominantly produced from grazing lands and is highly seasonal in growth. Australia is the largest producer, with 25.5% of the world production of 2217 million kilograms in 2002.

REFERENCES

1. Hocker, H. Fibre Morphology. In Wool: Science and Technology; Simpson, W.S., Crawshaw, G.H., Eds.; CRC Press: Cambridge, England, 2002; 60 79.

2. Heale, J.W.S. Physical Properties of Wool. In Wool: Science and Technology; Simpson, W.S., Crawshaw, G.H., Eds.; CRC Press: Cambridge, England, 2002; 80 129.

3. Orwin, D.F.G. Variation in Wool Follicle Morphology. In The Biology of Wool and Hair; Rogers, G.E., Reis, P.J., Ward, K.A., Marshall, R.C., Eds.; Chapman and Hall: New York, 1989; 227 241.

4. Schlink, A.C.; Mata, G.; Lea, J.M.; Ritchie, A.J.M. Seasonal variation in fibre diameter and length in wool of grazing

Merino sheep with low or high staple strength. Aust. J. Exp. Agric. 1999, 39, 507 517.

5. Schlink, A.C.; Sanders, M.; Hollis, D.E. Seasonal variations in skin and wool follicle morphology of grazing Merino sheep with low or high staple strength. Asian Australas. J. Anim. Sci. 2000, 13 (Suppl. A), 253 256.

6. Adams, N.R.; Liu, S.; Masters, D.G. Regulation of Protein Synthesis for Wool Growth. In Ruminant Physiology: Digestion, Metabolism, Growth and Reproduction; Cronje, P.B., Ed.; CAB International, 2000; 255 272.

7. Corbett, J.L. Variation in Wool Growth with Physiolog ical State. In Physiology and Environmental Limitations to Wool Growth; Black, J.L., Reis, P.J., Eds.; The Uni versity of New England Publishing Unit: Australia, 1979; 79 98.

8. Hynd, P.I.; Masters, D.G. Nutrition and Wool Growth. In Sheep Nutrition; Freer, M., Dove, H., Eds.; CAB Interna tional, 2002; 165 187.

9. Robertson, S.M.; Robards, G.E.; Wofle, E.C. The timing of nutritional restriction during reproduction influences staple strength. Aust. J. Agric. Res. 2000, 51, 125 132.

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