Abstract

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The amount of wool that a sheep produces depends upon its breed, genetics, nutrition, and shearing interval. Lambs produce less wool than mature animals. Due to their larger size, rams usually produce more wool than ewes of the same breed or type. Wool can account for as much as a 20% of the total gross income. Generally, wool traits are highly heritable and that most directly influence the value of a fleece include fleece weight, fiber diameter, and length of staple. Feed represents the largest single cost in all types of sheep production and thus the rations must be formulated to support optimum production, must be efficient and economical to feed, and must minimize the potential for nutrition related problems. Wool growth is usually maximized irrespective of the stage of pregnancy or lactation if sheep is immunized against nutritional deprivation. In this review attempts are made to highlight nutritional role in maintenance of wool yield and quality and catalytic intervention to maximize output as a whole.

Introduction

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Wool is a natural fibre obtained from sheep and certain other animals, including cashmere and mohair from goats. Wool has several qualities that distinguish it from hair or fur, namely it is crimped, elastic, and grows in staples. The term wool is usually restricted to describing the fibrous protein derived from the specialized skin cells called follicles in sheep. Wool is a protein fibre, composed of more than 20 amino acids. These amino acids form protein polymers. Wool also contains small amounts of fat, calcium and sodium. Nutrition plays an important role on the wool growth of sheep. By nature sheep are grazing animals and nutrients from pasture helps the animals for maintenance and as well as production. During the periods of poor pasture growth or when the quality of pasture is poor, there is a reduction of total fleece yield as well as in the quality of wool per animal and per unit area of grazed land. Proper wool growth requires all the important nutrients like amino acids, carbohydrates, minerals and vitamins.  Adequate nutrition increases both the length and diameter of the wool fibre. Though the nutrition of sheep is reported to affect the quantity of wool production and quality (length, diameter, protein composition and strength) of wool fibre, wool growth has also been reported under the condition of negative energy and nitrogen balances. Therefore nutrients requirement for wool production is also included for maintenance requirement. However some of the wool quality parameters like fibre diameter, staple length and fibre strength can be influenced by the supply of nutrients available to wool follicles during the process of fibre growth (Russel, 2002).

Nutrition can play a key role in the diameter of a wool fiber. Sheep which have been placed on a high plane of nutrition tend to have coarser diameter than yearling lamb.  Fibre diameter is the major determinant of wool price but presence of medullated fibres is also an important quality factor in wool (McGregor, 1990). Although medullation is influenced markedly by genetics and age but a possible effect of nutrition is also to be considered. Protein nutrition is important for wool growth and production, as wool is composed of almost entirely protein with very high level of cysteine and serine compared with other body tissues. Growth of wool requires more protein relative to energy, and draws amino acids, particularly methionine and cysteine, disproportionately from the body pool. This chapter deals in details with the nutrients requirement for wool production in sheep.

Review

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WOOL GROWTH
The wool-growth in responses to nutrition refers to changes in the mean fibre diameter and the rate of elongation of the fibre i.e. staple length. If the nutritional regimen is sufficient, the number of follicles actively producing a fibre increases. The specific gravity of wool fibres is relatively constant and not influenced by nutrition. Under normal feeding conditions, both the length and diameter acts in the same direction and the ratio of length to diameter remain approximately constant for species.
Staple strength is second only to mean fibre diameter as a determinant of wool price and is measured as the force (newtons) required to break a staple of wool, corrected for the linear density (weight per unit length) of the staple (kilotex (ktex)). Several features of the fibres within the staple potentially contribute to the staple strength. These are mean minimum fibre diameter along the staple,  the rate of change in fibre diameter along the staple, variation in diameter between fibres, variation in length between fibres, variation in crimp frequency between fibres, the intrinsic strength of the fibres and follicle shut-down (Reis, 1992).

NUTRIENT REQUIREMENT FOR WOOL PRODUCTION
Indian breeds of sheep have lower body weight and growth rate in comparison to exotic breeds. Besides genetic makeup, inadequate nutrition is the major factor responsible for low growth and production. Although the wool producing capacity of a sheep is determined by its genetic makeup, it can be influenced to a considerable extent by environmental factors like nutrition. Therefore sheep should be fed according to their nutrient requirements. Nutrient requirements vary by the age and size (weight) of the sheep and their stage and level of production. Nutrients like energy, protein, minerals and vitamins plays important role in wool production. Different countries have developed their own feeding standard based on their animals, production level and agro-climatic conditions prevailing in that country like NRC, ARC etc. Unlike other feeding standards, Indian Council of Agricultural Research (ICAR) also has given the nutrient requirement for wool production (Table 1). These requirements are derived from number of experiments that were conducted at Central Sheep and Wool Research Institute, Avikanagar, Rajasthan.

Energy intake and wool growth
Energy makes up the largest portion of the diet and is usually the most limiting nutrient in sheep diets. Carbohydrates, fat, and excess protein in the diet all contribute towards fulfilling the energy requirements of sheep. Carbohydrates are the major sources of energy. Concentrates (grain) contain starch, which is a rich source of energy. Forages contain fiber or cellulose, which is not as rich in energy as starch. The major sources of energy in a sheep's diet are pasture and browse, hay, silage, and grains.  Meeting energy requirements without over or underfeeding animals is one of the producer's biggest challenges. Energy deficiency is the most common nutritional deficiency in sheep. An energy deficiency will manifest itself in many ways, as for example, in growing animals, an early sign of energy deficiency is reduced growth, then weight loss, and ultimately death. In reproducing females, early signs of an energy deficiency are reduced conception rates, fewer multiple births, and reduced milk production.  With restricted energy consumption, wool growth slows, fiber diameter is reduced, and weak spots (breaks) develop in the wool fiber. An energy deficiency reduces the function of the immune system. Energy is quantified in the ration in many ways. The simplest measure is TDN or total digestible nutrients. Metabolizable energy (ME) and net energy (NE) values are more accurate measures of energy in a sheep's diet.

An increase in energy intake, except at very low levels of protein content in the diet, usually has direct positive effect on wool growth (Doney, 1983). Although the influence of feed intake on wool growth is well recognized, there is no disagreement on the roles of dietary energy and/or protein. Results from experiments suggest that wool production is linearly related to intake, presumably, digestive energy with protein intake having very little influence. Ferguson (1959) studied the effects of diets containing different levels of CP at different levels of intakes and contended that the wool growth response was due to increased feed intake. A summary from numerous studies (Kempton, 1978) suggests that about 2 g of wool is produced for every 100 g of digestible dry matter (Allden, 1978). This value is low as it is associated with net efficiency and is confounded with maintenance costs. Graham and Searle (1982) reported marginal (partial) efficiencies of metabolizable energy for wool production of 16 to 19%. Using these values, Shelton (1998) suggested that the amount of good quality feed (50% TDN) required to produce 453.6 g of fiber is about 11.3 to 13.6 kg. For most of diets consumed by grazing sheep (6-11 MJ/kg DM), at a rates of 800-1500 g DM day, wool growth rate will be limited by the supply of protein to the intestines.

Protein intake and wool growth
Protein is usually the most expensive part of the diet of sheep. Since the rumen synthesize protein from amino acids, the quantity of protein is more important than the quality of protein. Protein requirements are highest for young, growing lambs and lactating ewes that are producing milk proteins. Although it is clear that wool growth is primarily determined by feed intake, understanding the true nutrient requirements or costs of wool production is far more complex. When the rumen is bypassed, or protein passes through the rumen un-degraded, there are clear-cut responses in wool growth to protein and only small responses associated with energy reversal of the effects noted for diets digested in the rumen (Kempton, 1978) is observed. Predicting the rate of wool growth depends on an understanding of the quantitative relationships between diets, and the composition and amounts of protein available for absorption in the intestine. The lack of wool growth response to dietary crude protein supplementation suggests that it is unlikely that supplemented protein reached the true stomach i.e. abomasums in sheep.

Microbial protein synthesis in the rumen and its availability for digestion and absorption in the intestine is more closely related to the intake of digestible energy by the animal than to the protein content of the diet. Although wool growth increased with increasing digestible organic matter intake (DOMI), its affect is consistent with its probable effect on microbial protein synthesis in the rumen. Thus it would appear that the apparent response in wool growth to an increase in organic matter or energy intake is to the increased supply of microbial amino acids reaching the lower gastro intestinal tract. If the supply of ATP, nitrogen and sulfur in the rumen is non-limiting, microbial outflow from the rumen will provide about 6.6 g digestible protein/MJ of metabolizable energy (ME). Thus microbial protein would provide 0.2 g sulfur amino acids/MJ of ME. In the absence of unfermented or escape dietary protein, it appears that the supply of sulfur-amino acids from microbial protein is the primary factor limiting wool growth. With grazing sheep, large amounts of feed are needed to provide sufficient amino acids for maximum wool growth. The large amounts of feed needed to provide the amino acids for maximum wool growth also provide energy and other nutrients well above maintenance which might be used for other functions (Hogan et al. 1978). Amino acids composition of wool is presented in Table 2.

Wool protein contains a high proportion of the high-sulphur amino acids cystine, and it has been shown that variation in the availability of the sulphur containing amino acids (SAA) to the follicle can affect both fibre growth rate and fibre composition (Corbett, 1979). Sulphur containing amino acids play a major role in wool growth. The supply of sulphur containing amino acids cystein, cystine and methionine often limits wool growth, and the supply of lysine is also reported to be important. The wool fibre is primarily protein, grouped into three main type of sulphur containing protein, the low sulphur protein constitute 60-70% of total protein, the high sulphur protein accounts 20-40% of total protein and contains very high level of cysteine (10% in wool) and no methionine, and high tyrosine protein that make up to 1-2% of the total protein and characterized by high levels of lysine, isoleucine, histadine and glutamate (D’Mello, 2003). Thus, a large amount of cystein and cystine is required to synthesize wool proteins, and much of these can be provided by conversion of methionine. However methionine plays a specific role in stimulating wool growth (Reis, 1989). Infusion of cystein into the abomasums or into the blood may double wool growth. Infusion of methionine increases wool growth by supplying sulphur for the synthesis of cystine. The optimum protein: energy ratio for wool growth is about 1.88 g digested SAA per megacalorie of digestible energy. This ratio indicates that the supply of absorbed amino acids, particularly SAA, is the major component of feed intake that determines the rate of wool growth. This ratio also implies that an increase in protein absorption increases wool growth. The most common protein supplement for sheep are oil cakes/meals  like soybean meal, ground nut cake, sesame cake, mustard cake, sunflower meal, cottonseed meal etc, however, in general grains are usually low in protein.

Scanty reports are available on feeding of urea as nitrogen source on wool production. Urea is the cheaper source of protein or dietary nitrogen. Urea is converted to protein in the rumen. It has an equivalent crude protein value of 280 percent. Most of the studies reported no improvement on wool production in diets containing supplemental urea. Peirce et al. (1955) provided urea supplements, either with molasses or in pellets with grain and other feeds, to sheep grazing dry pasture in tropical Australia, no improvement in wool production was observed in their study. However, Coombe and Tribe (1962) reported an improvement in wool growth in sheep supplemented with urea and molasses fed diets containing low quality roughages. Supplementation of urea at 1.5% in the concentrate mixture resulted in higher wool production in Chokla rams that were allowed to graze for 6-8h daily (CSWRI, 2010; unpublished work).

MINERALS
Sixteen minerals have been identified as nutritionally essential in sheep diets. Macro-minerals are required in large amounts. They include sodium (Na), chloride (Cl), calcium (Ca), phosphorus (P), magnesium (Mg), Potassium (K), and Sulphur (S). Micro-minerals are required in small amounts and include iodine (I), copper (Cu), iron (Fe), manganese (Mn), zinc (Zn), molybdenum (Mo), cobalt (Co), selenium (Se), and fluoride (Fl), are required in small amounts. Minerals can influence wool growth by affecting feed intake (Na, K, S, P, Mg, Co and Zn), by altering rumen function and hence the supply of nutrients flowing from the rumen (S, Na, K and Co) or by directly disrupting metabolism within the sheep (Zn, Cu, Se, I and Co). The wool matrix contains significant quantities of calcium, potassium, sodium, zinc, copper, manganese, iron and selenium (Lee and Grace, 1988), but only copper, zinc, iodine and possibly selenium alter follicle function and wool growth directly. The mineral composition of wool is presented in Table 3.

The macro mineral S plays an important role in wool production. Clean wool is composed of complex protein keratin which contains about 20 amino acids in many polypeptides and has a sulphur content varying from 2.7 to 5.4% of the fibre weight. Most of the S is present as cystine, with smaller amounts as cysteine and methionine (McGuirk, 1983). Infusion of 2.0 g L-cystine or 2.46 g DL-methionine (the sulphur equivalent) per day increased wool production by 35 to 130% and the S content by 24 to 35% (Reis and Schinckel, 1963). Infusion of methionine into the rumen can increase the protein synthesis of rumen bacteria. Sulphur containing amino acids from bacterial proteins and those infused into the abomasum stimulate the anabolism of animals, which is evidenced by weight gain. Starks et al. (1953) demonstrated that lambs could utilize inorganic sulphur and that supplemental sulphur could increase nitrogen balance. Hale and Garrigus (1953) showed that sheep could synthesize cystine from sulphate, and to a lesser extent from elemental sulphur. Johnson (1971) reported digestibilities of 36, 70, and 78% for sulphur, sodium sulphate and L-methionine in lambs and retentions of 27, 56 and 70%, respectively. Seljeanski (1959) stated that lambs given 5.9 g/day of potassium sulphate produced 6.6% more wool than lambs not receiving sulphate. Supplementation of 0.25% S as sodium sulphate produced 17% more wool than un-supplemented sheep. In that study body weight gain and wool strength also increased (Qi, 1986). Studies on sulphur supplementation as sodium sulphate at graded levels (0.04, 0.13, 0.21 and 0.32%) in Chokla rams resulted higher wool yield at 0.21% (CSWRI, 2008). Sulphur supplementation at 0.3% as sodium sulphate was studied in Bharat Merino ewes fed concentrate containing two levels of protein (high or low). There was no difference in wool yield and other quality parameters at either protein level (CSWRI, 2009).  In another study with Chokla rams, sulphur supplementation at 0.3% in the concentrate containing 19.6% CP improved digestibility of nutrients, immune status and resulted in higher wool production with lower hetero, hairy and medullation % in the wool fibre.

Micro-mineral copper plays a very important role in maintaining quality of wool fibre. A deficiency of copper, either in the ration of sheep or induced by high levels of S and molybdenum in the diet, results in de-pigmentation of the wool, with production of wool that lacks crimp and has low mechanical strength and a lustrous appearance. De-pigmentation of the wool is caused by low activity of the copper containing enzyme tyrosinase. Zinc deficiency on the other hand results in a marked reduction in wool growth, over and above that associated with the reduced feed intake induced by the deficiency (White et al., 1994). Some fibres are shed, and the fibres that are produced lack crimp and are lustrous and brittle. Cell division in the follicle bulb is marginally reduced by zinc deficiency, but the major effect appears to be on the keratinization of the fibre. Selenium also plays a role in wool growth. Its deficiency reduces wool growth without a reduction in feed intake. While the exact mechanisms involved are not known, many of the selenoproteins have key metabolic roles as antioxidants and affect the redox status of cells. Uncontrolled peroxidation during severe selenium deficiency causes necrosis due to oxidative damage to cellular macromolecules. A lesser deficiency may result in a milder oxidative stress caused by increased concentrations of peroxides of hydrogen and lipids. Oxidative stress causes gene repression through modulation of transcription factors. Such changes may induce temporary growth arrest and lengthening of the cell cycle in the follicle (Morel and Barouki, 1999).

VITAMINS
Vitamin plays important role in wool production. Fat soluble vitamins namely Vitamin A, D and E are required in sheep. Vitamin A is absent in plant material, but can be synthesized from beta-carotene. Vitamin D is required to prevent rickets in young animals and osteomalacia in older animals. B-vitamins are not required in the diets of sheep because they are synthesized in the rumen. Vitamin K is essential for blood clotting. Dietary supplementation is usually not necessary. A deficiency of a vitamin may reduce or completely inhibit wool fibre growth, by reducing the feed intake of the animal thereby impairing the supply of substrate to the follicle, by inhibiting the activity of enzymes involved in protein or energy metabolism, by reducing the production of nucleic acids required in the follicle for cell division and finally protein synthesis or by directly inhibiting keratinization (Hynd, 2000). Thiamine (vitamin B1) is a cofactor for the transketolase enzyme required for the pentose phosphate pathway. Pyridoxine (vitamin B6) is required for amino acid metabolism in general and in the trans-sulphuration reaction in which methionine is converted to cysteine. Pyridoxine is also essential for polyamine synthesis and is involved in glycogen metabolism. Biotin may be involved in follicle function, because it is required for nucleic acid synthesis. Folic acid is essential for transferring one-carbon fragments from serine, glycine and histidine to other amino acids, purines and thymidine, thereby contributing to cell division and protein synthesis. Vitamin BI2 is a cofactor in methionine synthetase, involved in methionine conservation and the provision of methyl groups for a range of molecules. Vitamin B12 is also essential for the activity of methylmalonyl coenzyme A (CoA) isomerase, a key enzyme in the production of glucose from propionate.

The only direct demonstration of a vitamin deficiency affecting wool growth occurred in preruminant lambs supplied with diets deficient in folic acid (Chapman and Black, 1981). The wool lacked crimp and in several cases fibre growth ceased completely, despite the fact that the animals were gaining weight. Provision of folic acid alleviated the condition, supporting the notion that this vitamin is essential for wool growth. While microbial synthesis of the B-group vitamins in the rumen means that adult ruminants are unlikely to suffer deficiencies of these vitamins, perturbations to rumen function may reduce microbial supply. The presence of 'antivitamin' compounds in feeds (e.g. anti-thiaminase in bracken fern) may also induce a deficiency. The fat-soluble vitamins A and D3 probably have direct effects on follicle function, as both have specific receptors in various parts of the follicle.

Conclusion

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Nutritional augmentation of wool production can never be economical in real terms, although it significantly affects wool yield and quality. Moreover, any supplemental nutrition has got concurrent bearing on growth, reproduction, lactation and harvest of more sheep per sheep and these in totality has influenced economic return from sheep production through narrower input: output ratio and increased profitability.

Source(s) of Funding

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none

Competing Interests

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