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Yolk in Wool


Dr J J Venter

Agricultural Researcher: Wool Research

Grootfontein College of Agriculture



The sebaceous and sudoriferous (sweat) glands associated with the wool follicles coat the fibres with their secretions as the new, growing fibre is pushed along the follicle shaft to the skin surface. Wool follicles do not all possess sweat glands, and sebaceous glands may vary both in number and size from follicle to follicle (Sutton, 1931).

A variation in yolk over the body of the Merino was noted by Bonsma & Starke (1934). They pointed out that the yolk in wool is an inherent characteristic.

Bonsma (1930) stated that the wool yolk comprises all the impurities of animal origin adhering to the fibre. This description of the term is hardly satisfactory, since it fails to distinguish between wool fat (grease or wax) and suint. Bonsma & Starke (1934), however, showed that wool fat and suint were in no way correlated, but that wool fat is negatively correlated with yield.

According to Lang (1945), the appraiser and wool buyer do not place a value directly on the yolk itself, although they take note of its nature and use it as a guide in assessing certain characteristics of wool as well as the clean yield.

It was established by Winson (1929) that yolk influences the characteristics of the fibre, viz. wool of more greasy samples of the same quality being distinctly finer and more uniform in fineness, i.e. showing less variation in size of cross-section and a more circular cross-section.

Sutton (1931) postulated that the products of the sebaceous glands and sweat glands might have some influence on the fibre while still within the follicle. There is a popular belief that the yolk nourishes the fibre. The penetration of liquids into the capillary cavities of the wool fibre has been shown by Speakman (according to Sutton, 1931) to influence the pliability and elasticity of the fibre and that yolk functions in a similar manner. However, the yolk may simply have a protective function, serving to prevent weathering of the fibre after it has emerged from the follicles. Yolk is regarded as a substance, which cements the fibres closely together, so keeping the fleece closed on the sheep (Le Roux, 1958; Louw, Swart & Mellet, 1963; Kruger, 1964). Uys (1966) and Van Rensburg (1966), however, could find no significant difference in grease content in large and small staples of fleeces. Truter and Woodford (1955) stated that wool grease has to increase about tenfold to be of any significance in preventing the fleece from falling open on the sheep's back.

Sutton (1931) stated that, whatever the true relationship between yolk and fibre may be, it would seem that empirically, at least, a plentiful supply of fluid (mobile) yolk is desirable, if only as an indication of the good health of the sheep.

Daly & Carter (1955) pointed out that wax production was positively related to an increase in feed intake and negatively to a rise in atmospheric temperature. Suint production was positively related to both feed intake and fleece weight. Further, wool wax and suint production per unit feed intake decreased with an increase in feed intake. Wax per unit feed intake decreased with a rise in temperature and suint per unit feed intake increased with increase in fleece weight.

A negative regression of wax production and atmospheric temperature was found by Bowstead & Larose (1938). No definite relation between suint production and average atmospheric temperature was found.

In New Zealand Romney Marsh sheep under local climatic conditions, Sutton (1931) found that the effect of sheep covers on yolk production was quite insignificant as compared with the normal variations between individual animals.

According to Bonsma & Starke (1934) the feeding of ordinary maintenance production rations, either high or low in protein, did not seem to have any influence upon the secretion of yolk. Raymond & Mandell (1955) postulated that the quantity of grease not only increases with the fineness of the wool, ranging from 5 to 20 percent of its grease weight, but also varies in the wool of sheep of different ages, and also depends on the breed and the climate and to a lesser extent on the age and sex. There is even a substantial difference in the amount of grease found in wool on different parts of the body.

Venter (1966), however, found that the percentage of wool wax in Merino wool under ad lib conditions remained practically constant up to the age of 3½ years. From 3½ to 6½ years a gradual increase was noted in the percentage of wool wax in the wool of wethers and ewes, but a sharp increase was noted in the wool of rams, consequently affecting the yield percentage of the wool.

The increase in the wool wax in the wool of the rams with increasing age was accompanied by a decrease in the percentage of suint.

Venter, Steenkamp & Edwards (1973) found that wool produced on a ration low in protein but with adequate energy had markedly more wool wax than wool produced on a ration high in protein but low in energy or wool produced on a ration with sufficient protein and energy.

A comparative study was undertaken by Venter, Cloete & Edwards (1969) on the productivity of Merino sheep kept under different climatic and nutritional conditions on three sites differing largely in altitude, topography, geology, vegetation and veld type. Climatic and environmental factors, in terms of the penned groups receiving the same ad lib feeding on each site, had no influence on wool wax, fluidity and suint content of the wool. The wool of the sheep on the open veld, and particularly those at a higher altitude and whose grazing was deficient in protein during both the growing and dry season, had less suint and a lower pH than normal. The penned groups had more suint and a generally high pH value.

Bonsma & Starke (1934) stressed that it is futile to try to influence the yolk content of a sheep through feeding. In attempting to increase yolk secretion by feeding, there is a possibility of overheating the body and thus causing an undesirably golden discolouration of the fleece in individuals with a natural tendency to discolouration.

Contrasting statements were found in respect of the stimulating effect of shearing and the use of sheep blankets on the secretion of yolk in wool. Since 1931 sheep blankets were used as a protection for sheep against cold and it was noted indirectly that the wool was cleaner and less weathered (Sutton, 1931; Austin, 1936; Duncan, 1938, 1939; Montgomery, 1938; Lipson, Ellingworth & Sinclair, 1970).

Although the amount of yolk in the wool does not appear to be the main factor contributing to protection against weathering, a sufficient amount of yolk is still essential. It must however be fluid and spread over the entire fibre in order to cement the fibres together within the staples. It is also responsible for a good colour.

A deficiency of yolk is associated with a dull appearance and the falling open of tips, which again is responsible for greater weathering. Excessive yolk in wool, on the other hand, affects the handle by increasing its harshness, and the appearance by causing an undesirable discolouration, and also results in a lower yield. It is therefore evident that either extreme in yolk content will have a detrimental effect on the price obtained for wool.



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Karoo Agric 2 (2), 35-38