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J. J. VENTER, J. W. NEL and W. K. EDWARDS, Agricultural Research Institute of the Karoo Region

Middelburg C.P. 5900 



A large portion of the normal fibre length is chemically and physically damaged by sunlight. In order to obtain a sound length of fibre, the extent of weathering on the sheep must be limited as far as possible. Walls (1974) stated that a large percentage of the weathered tip portions are removed in carding and the rest in the combing process as noil. If it is retained it give rise to uneven dyeing (Kritzinger, 1958). According to Louw, Swart & Mellet (1963), Haly, Feughelman & Griffiths reported the decomposition of a maximum of 25 % of the cystine in wool through irradiation by ultra-violet light of wavelength over 290 mµ within two hours. Louw et al. (1963) also showed that exposure for only seven days decreased the cystine content by about 8 to 10% for raw wool sections, being of the same magnitude found for normal weathering of wool over a period of 12 months on the sheep's back. They found that raw wool was less seriously affected by irradiation than the clean scoured counterparts. This emphasized the protective action of fleece density, staple formation and yolk as binding factors of the wool fibres and in preventing it from falling open on the sheep's back (Truter & Woodford, 1955; Le Roux, 1958).

It is possible, however, that due to the short length of the wool for the first weeks after shearing, no staples are formed, the yolk being insufficient to close the tips into staples. Louw (1960) pointed out that newly grown wool fibres emerging above the skin area, were not yet fully keratinised, thus being more sensitive to irradiation. Swanepoel (1961) stated that it appeared that the modifications in wool, which were responsible for decreasing its contraction, had to take place in the very earliest stages of the weathering process. Weathering is accompanied by the conversion of some disulphide cross-linkages into thio-ethers (Louw, 1960), and the probable formation of intermediary oxidation products of cystine in which the disulphide linkages are not broken but stabilized by weathering so that disulphide exchange is inhibited (Swanepoel, 1961).

A gradual increase in the degree of weathering from three months to twelve months after shearing was noted by Snyman (1960). The high degree of weathering at three months was ascribed to the poor adaptability of the sheep to a new environment and climate. According to Marston (1948) and Phillips & Spencer (1948) individual sheep may differ in their adaptability to a new environment.

In the Karoo a much higher weathering index was experienced in spring than in any other season (Snyman, 1960). However, Bosman (1935; 1937) stated that in the Karoo at least, provided that the nutrition was not changed, there was no seasonal influence on the dimensional attributes of fineness and length, which affected the volume of wool produced by Merino sheep. Coop & Hart (1953) found that more uniform wool was produced in regions showing a small difference in daily temperature between summer and winter. A linear relationship was noted by Steenkamp, Venter & Edwards (1970) between the degree of weathering and maximum daily temperature, as well as hours of sunshine. The degree of weathering increased sharply during the hot summer months. It was found that damaging of short wool was limited to the first centimetre of the tip portion. No deeper weathering in short wool seemed to occur during the cooler winter months of the year. It was evident that the same maximum degree of weathering of the uppermost tip occurred within three months after shearing during summer and after seven months during winter. The final degree of damage to the extreme tip portion seems to be unaffected by the time of shearing. Deeper weathering within the staple proved to be dependent on the season and growing period of the wool.

Wool samples of eight- and twelve-month growth shorn at different times of the year were compared to investigate the seasonal influence on the degree of weathering of the tip portions.



A group of 112 two-tooth Merino wethers were kept in an open pen under favourable feeding conditions of lucerne hay ad lib. in self-feeders and 14 g of maize per sheep per day.

In order to compare wool grown over different periods it was decided to concentrate on two spinning lengths of eight-month and twelve-month growth.


Eight-month wool

Sixty-four of the 112 sheep were divided into four equal comparable groups in respect to body mass. The separate groups were successively shorn at monthly intervals from the beginning of May 1970 and kept for three successive periods of eight months.


Twelve-month wool

The rest of 48 sheep were divided in the same way into four equal groups, but successively shorn at three-monthly intervals from the beginning of May 1970 but kept for two successive periods of twelve months.



A wool sample of 100 g was cut from approximately 400 cm2 of the midrib area by means of small curved scissors.



The following physical and chemical properties were determined on the wool samples by means of standard techniques as described by Venter (1976).

(i) Staple length and straight fibre length

(ii) Mean fibre diameter

(iii) Crimp frequency

(iv)  Ratio i.e. the actual diameter (d) divided by the estimated diameter (d') according to the number of crimps per 25,4 mm based on the standards of Duerden (1929).

(v) The degree of weathering was determined by means of the methylene blue method as described by Steenkamp et al. (1970) on three successive portions of one centimetre each designated as Tip A, Tip B and Tip C, cut from the tip end of a sufficient number of staples.



The mean and coefficient of variation of all measured fibre properties as well as the percentage of methylene blue absorbed in Tip A, Tip B and Tip C are given in Table 1 for the different eight-month and twelve-month groups and corresponding growing periods.



Fibre properties

Although the sheep received ad lib. feeding for the whole experimental period an increase of two micrometer in fibre diameter and a corresponding decrease in crimp frequency was observed over the three successive eight-month and twelve-month growing periods.

The degree of crimpiness as indicated by the  ratio, showing the relationship between the crimp frequency and fibre diameter, however, remained the same for all sub-groups. Even the crimp ratio, indicating the relative depth of crimp, showed a small change over the whole experimental period. A difference in resistance to compression (Van Wyk, 1946) and resistance to extension and bending of individual crimped wool fibres (Van Wyk & Venter, 1954; 1971) can be ascribed mainly to a difference in crimp ratio and of crimp-fineness ratio ( ratio). As no change in crimp ratio and did' ratio was observed over the three eight-month periods, it was reasonable to accept that the difference in degree of weathering in Tip A and deeper into the staple could only be due to a difference in climatical conditions.

It was further apparent that the weathering indices of the Tip A portions of all eight-month and twelve-month groups were relatively high (> 75 %) irrespective of the season of growth. The coefficients of variation within groups were also small and showed that the effect of fleece characteristics such as density and wool yolk were too insignificant to prevent weathering of Tip A. A marked difference was, however, noted between and within groups, in respect to deeper weathering. The degree of weathering in Tip B of certain groups, was nearly three times higher than of some other groups. This could be ascribed to a difference in season as stated by Steenkamp et al. (1970). Deeper weathering seemed to be more dependent on the fleece and fibre properties according to the higher coefficient of variation within groups. The same applies to weathering in Tip C.

The weathering indices of wool grown over different periods were compared and the differences tested by means of an analysis of variance carried out in respect of each tip portion. The wool of eight-month growth and that of twelve-month growth were compared separately. The a and b groups of twelve-month growth was, however, compared separately because a year effect was observed. The eight-month and also the different twelve-month groups were ranked according to their weathering indices of Tip A, Tip Band Tip C. The rankings and calculated F-values, as well as the Least Significant Differences (LSD) for each comparable group and respective tip portions, are given in Table 2 for the eight-month groups and in Table 3 for the twelve-month groups.

Table 2, 3


Comparing the eight-month groups grown and shorn over different seasons as given in Table 4, it was evident that Group 4a (August I 970-April 1971), Group 2a (June 1970-February 1971) and Group 3a (July 1970-March 1971) were highly significantly (P<0,0I) more weathered in Tip A, Tip B and Tip C. In contrast, Group 2b (February 1971-0ctober 1971) and Group 3c (November 1971-July 1972) were the least weathered.

Comparing the twelve-month groups, the degree of weathering in the Tip A portions was high for all growing periods. For Group 7a (November 1970 to November 1971) the weathering in Tip A was, however, significantly less than for all other groups. The same tendency was apparent in comparing the twelve-month b groups. Shearing in August 1970 and again in August 1971 (Group 6a) showed the highest weathering in Tip A. The same tendency was apparent in Group 6b (August 1971-August 1972). Data of the other shearing periods were intermediate in both cases.

In comparing the a groups the weathering in respect to Tip B and Tip C did not differ significantly between the different growing periods but in the b groups a high significance was found.

Groups 8a, 8b (August to August) closely followed by Groups 5a, 5b (May to May) and Groups 6a, 6b (February to February) proved to be consistently more deeply damaged. Less damage deeper in the staple occurred in Groups 7a, 7b (November to November).

Any weathering deeper in the staple is thus dependent on the length of the wool and period of exposure to high temperature during the hot summer months. The best time to shear twelve-month wool appears to be in the late Spring.

Climate and season had a marked effect on the weathering of wool as shown by Snyman (1960) and Steenkamp et al. (1970). During the cold winter months very little weathering occurred as against the high monthly degree of weathering during November to February. The results showed that wool which was subjected to one or more of the hot summer months tended to open up, possibly due to a lack of staple formation, density (substance) and an insufficient fluid yolk, and consequently became even more highly significantly weathered in Tip A and even deeper within the staple in Tip B and Tip C.

In Fig. 1 the weathering indices of Tip A, Tip B and Tip C for each prescribed growing period, are plotted against the monthly mean maximum daily temperature, hours of sunshine and rainfall.



The degree of weathering was clearly dependent on the mean maximum daily temperature and mean hours of sunshine as stated before by Steenkamp et al. (1970). A sharp increase was noted in maximum daily temperature and hours of sunshine for the summer period from September to a maximum from November to March. The increase in weathering was consequently due to the longer hours of exposure.

The direct relationship between the degree of weathering and mean maximum daily temperature and mean hours of sunshine for each separate eight-month periods are respectively illustrated in Fig. 2 and Fig. 3.



A strong linear relationship was found by Steenkamp et at. (1970) between monthly degree of weathering and hours of sunshine and also between degree of weathering and daylight length. In contrast to a curvilinear relationship between the degree of weathering and temperature as was found by Steenkamp et at. (1970), in this study only a weak linear relationship (r=0,31) between the degree of weathering of Tip A and the mean maximum daily temperature was found. A corresponding though highly significant linear correlation (r=0,82) was found between weathering and mean hours of sunshine.

The rainfall over the entire experimental period showed a greater variation with several peak showers in August 1970, January/February 1971 and also March/April 1972. Rainfall apparently did not play any important role as was evident from both studies.

Deeper weathering in Tip B and Tip C was relatively more severe when the wool grew over any of the hot summer months, more so if the wool was relatively long and tended to fall open. This can be seen when comparing Groups 4a, 2a, 3a and 1a with Groups 2b and 3c. The degree of weathering thus appeared to depend on the season of growth.

In order to limit weathering to only the extreme Tip A portion, the time of shearing should be arranged so that the wool on the sheep is as short as possible over the hottest summer months or so that the fibres are bound together in staples to prevent them from falling open and becoming more deeply weathered.



According to the results of previous studies by Snyman (1960) and Steenkamp et al (1970) it is clear that the extreme portion of the tip reaches maximum weathering very soon after shearing especially during summer, while the process is less rapid during the winter months. It was found that weathering deeper into the fleece is more likely to occur in long wool, which falls open and thus exposes a greater part of the tip to weathering. Shorter wool of eight-month growth was found to be resistant to weathering deeper into the fleece. The final degree of weathering will, to a large extent, be determined by the time of shearing.

It was evident that the weathering of wool was affected by the maximum daily temperature and hours of sunshine. The degree of weathering showed a linear relationship with both climatic factors, thus was mainly dependent on the season of wool growth.

Comparing eight- and twelve-month wool grown over different periods during the year, it was clear that the degree of weathering of the extreme tip portions were relatively high (> 75 %) irrespective of the period in which the wool was produced. The wool grown over the hot summer months was nevertheless highly significantly (P<0,01) more weathered than that grown over the colder months.

The results showed that long wool which was subjected to one or more of the hot summer months tended to open up, possibly due to lack of staple formation, density (substance) or insufficient fluid yolk. Consequently the wool became even more deeply weathered within the staple. Fine and undercrimped wool as well as long wool tended to be more severely and deeply weathered.

To limit weathering to only the extreme tip portions, it is recommended that the time of shearing should be arranged so that the wool will be short during the hottest summer months from November to March or so that the fibres are bound together in staples.

Preliminary results of wool samples produced in different farming regions of the Karoo, differing in climatic conditions, showed a dramatic increase in damage particularly in the Tip B and Tip C portions, leaving a smaller sound portion of the fibre. Weathering indices of91,4% in Tip A, 32,0% in Tip Band 24,7% in Tip C have been recorded in medium/strong wool of 75 mm, showing that there is scope for improvement in producing a sound root to tip wool.

A questionnaire circulated to Extension Officers in various regions, showed that many farmers sheared during late Summer and Autumn, being just the wrong time for shearing as far as weathering is concerned. Feeding conditions, mating, lambing and labour problems peculiar to each area, were given as reasons.

In view of the findings of this study, farmers are advised to arrange their shearing times so that the weathering of wool will be limited to a minimum, taking into consideration other management practices.



The technical assistance given by Messrs J. M. Nel, A. J. du Plessis, Miss H. E. I. Vorster and Mrs J. Hansen, is highly appreciated.



BOSMAN, V., 1935. The seasonal influence on Merino wool production. Onderstepoort J. vet Sci. 5, 1, 323-326.

BOSMAN, V., 1937. Biological studies on South African Merino wool production. J. Text. Inst. 28, 1-64.

COOP, I. E. & HART, D. S., 1953. Environmental factors affecting wool growth. Proc. N.Z. Soc. Anim. Prod. 12, 113-119.

DUERDEN, J. E., 1929. Standards of thickness and crimps in Merino grease wools. J. Text. Inst. 20. T 93-100.

KRITZINGER, C. C., 1958. Weathering in wool. S. Afr. industr. Chem. 11,218.

LE ROUX, P. L., 1958. Photochemical decomposition of Merino wool. S. Afr. J. Sci. 1, 273-287.

LOUW, D. F., 1960. Weathering and resulting chemical changes in South African Merino wools. Text. Res. J. 30, 462-468.

LOUW, D. F., SWART, L. S. & MELLET, P., 1963. The influence of artificial weathering on the chemical composition of Merino wool. S. Afr. J. agric. Sci. 6, 633-646.

MARSTON, H. R., 1948. Nutritional factors involved in wool production by Merino sheep. Aust. J. sci. Res. B. 362-375.

PHILLIPS, R. W. & SPENCER, D. A., 1948. Reactions of Southdown sheep to two environments. J. Anim. Sci. 7, 41-49.

SNYMAN, J. G., 1960. 'n Ondersoek na die invloed wat omgewingstoestande uitoefen op die verwering van Merinowol. M. Sc. (Landbou) verhand. Univ. van Stellenbosch.

STEENKAMP, C. H., VENTER, J. J. & EDWARDS, W. K., 1970. Seasonal effect on weathering of wool. Agroanimalia 2, 127-130.

SWANEPOEL, O. A., 1961. The supercontraction of sound and weathered Keratin fibres. Text. Res. J. 31, 618-624.

TRUTER, E. V. & WOODFORD, F. P., 1955. The protective action of wool wax and suint against the photolysis of wool. J. Text. Inst. 46, 641-652.

VANWYK, C. M., 1946. A study of the compressibility of wool with special reference to South African Merino wool. Onderstepoort J. vet. Sci. 21, 99-226.

VAN WYK, C. M. & VENTER, J. J., 1954. The initial resistance of crimped wool fibres to extension. J. Text. Inst. 45, T. 809-819.

VAN WYK, C. M. & VENTER, J. J., 1971. The effect of uniplanar crimps on the bending behaviour of fibres and filaments. J. Text. Inst. 62, 655-676.

VENTER, J. J., 1976. Gehalte-aspekte van Merinowol. D. Sc. (Agric.) proefskrif, Univ. Pretoria.

WALLS, C. W., 1974. Wool characteristics of processing importance. Wool Tech. Sheep Breed. 27-31.



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