Last update: April 2, 2012 03:54:30 PM E-mail Print





Head: Wool and Mohair Research

Agricultural Research Institute Grootfontein



Being a natural fibre, wool is affected by the vagaries and elements of nature. In this respect the weathering of wool is a serious problem, especially in the semi-arid wool producing areas.

Degradation of wool, especially in the tip portion of the staple, takes place on the back of the sheep and to a lesser extent on the other parts of the body of the sheep due to exposure to sunlight, dust and dirt. Those portions of the fibres thus exposed become chemically and physically damaged - they become brittle, weak and sensitive to alkalis. Weathered wool also has greater felting properties. This means that highly weathered wool will result in a higher degree of fibre entanglement during scouring which in turn causes more fibre breakages in the carding process.

According to Australian figures, less than 18 percent of the original tip portions are found in the top stage, while in the yarn stage less than one percent is found. Weathering can cause losses of more than 30 percent in length.

When evaluating wool, the amount of oil is estimated, thus taking into consideration the damage due to weathering. It is thus of importance to the producer to limit the extent of weathering.

A study of the effect of season and climate on the degree of weathering was recently completed at Grootfontein. It was established by Steenkamp, Venter & Edwards (1970) that damage during the summer period November to February was severest. Directly after shearing the new wool growth is, according to Louw (1960), apparently not fully keratinised and is consequently more liable to weathering. Steenkamp et al (1970) showed that the damage to wool exposed for only one month during this period, may be as high as 70 percent, while during the milder winter period the effect of weathering decreases to less than 10 percent.

It can thus be expected that long wool grown over the period November to February and which is lacking in density, substance and sufficient fluid yolk, may tend to open up and will become even more deeply weathered. In contrast, long wool over the milder winter period will be less affected.

By changing the shearing time, deeper weathering can be prevented if the wool is short over the summer period. Usually short wool of two to three months has already formed staples in which the fibres bind together and also through its inherent stiffness and yolk, thereby creating a fleece, which will remain dense and so protect itself, leaving only the extreme tip exposed.

A comparison was made of eight months' wool grown over different periods during the year (Venter, 1976). Three 10 millimetre portions – Tip A, Tip B and Tip C - were cut progressively down the staple and analysed for damaging. According to the results of Venter (1976) the degree of weathering of the extreme tip portions were relatively high (greater than 79 percent) irrespective of the period in which the wool was produced. The results showed that eight months' wool shorn in February and again in October was less weathered; the best growing period being after the hot summer months and before it again became too hot. Significantly less weathering was found even in the Tip A portion and deeper into the staple, namely 77,2 percent in Tip A, 7,1 percent in Tip 8 and 4,9 percent in Tip C.

The most damage was found in eight months' wool shorn in July and growing over the hot summer months and shorn again in March, the following year, viz. 87,7 percent in Tip A; 20,9 percent in Tip 8 and 11,2 percent in Tip C.

It is clear from the above that the final degree of weathering in the fleece will to a large extent be determined by the season of growth and the time of shearing. Snyman (1960) also showed that the intensity of weathering is largely influenced by the environment and the adaptability of an animal to a particular climatic condition. The weathering in the Karoo and even more so in the Orange Free State was found to be significantly higher than in the Eastern Transvaal and Western Province, mainly due to the higher solar energy of the sun. The seasonal effect even differs from region to region.

Information gathered by means of a questionnaire showed that many farmers in the Karoo Region shear during late summer and autumn, others after the winter to early December. Feeding conditions, mating and lambing seasons and labour problems peculiar to each area were given as reasons.

Before a definite and generally acceptable shearing time, applicable to the various farming regions in the Karoo, can be decided upon, a study has to be made of the degree and intensity of weathering in wool produced in different climatic regions. Such a study is presently being carried out by the Karoo Region.

The Karoo Region is subdivided into different farming areas based on topography, plant distribution, climate, soil type and estimated nutritional potential.

With the approval of the Wool Board a representative sub-sample from the grasp samples of AM lots are taken for analysis. Wool samples are obtained of wool produced by various farmers whose farms are situated within the boundaries of a particular sub-farming area.

In order to eliminate seasonal and yearly effects, samples are taken of wool produced during different seasons of the year as well as over a number of successive years. The degree of weathering over the three distinct ten millimetre portions cut down from the tip, are determined, using the Methylene absorption test as described by Steenkamp et al. (1970).

Preliminary results obtained so far, clearly showed that the weathering of the Tip A portions are high irrespective of the farming region or time of shearing. It varies from 78,7 percent to 91,7 percent.



Weathering deeper into the staple, however, generally proved to be much higher in comparison to wool produced at Grootfontein. The degree of weathering in Tip B varies from 16,1 percent to as much as 73,6 percent within the different farming regions: A figure of 20 percent is considered as normal for twelve months wool.

Even in respect of Tip C, namely 30 millimetres deep into the staple, weathering indexes varying from 6,5 percent to 31,5 percent are found in comparison to an index of 10 percent, which is considered as normal.

This shows that a large portion of the staple of 90 millimetres (AM) wool becomes photochemically damaged by the sun. It is absolutely essential that the farmer should do something to limit the weathering of wool on the sheep. Weathering can to some extent be limited by good management and feeding and by selecting for a dense fleece with a good staple formation and sufficient and fluid yolk.

The most effective means of limiting weathering of the wool - apart from changing the shearing time to avoid having long wool over the summer months - is to cover the sheep with blankets or to supply sufficient shade for the sheep.

It was evident according to the results of Venter (1976) and Venter, Nel & Edwards (1977) that by using blankets for the full nine to twelve months growing period, the weathering could be reduced to less than 20 percent. This can mean an extra 30 to 40 cents per kilogram of wool. By using sheep blankets only during the hot summer months the weathering was not only arrested but a certain extent of recovery was noted. The damage by weathering was substantially reduced by 23 percent in Tip Band 15,2 percent in Tip C in 9 months' wool. In the 12 months' wool it was reduced even further by 58,5 percent in Tip Band 48,4 percent in Tip C.

Though sheep blankets are considered the most effective method to prevent weathering, the provision of shade in camps, by means of trees or open sheds, is recommended as a practical method. The provision of shade in camps can help a lot towards reducing weathering in the Tip Band Tip C portions. Shade has a relatively small effect on Tip A but damage to Tip B can be reduced by as much as 30 percent. A longer length of sound wool for processing will thus be obtained which in turn will result in a higher price for wool in the grease.

A lack of shade in the veld, especially at watering points, is a general phenomenon in the Karoo. By providing shade in some or other way, not only sounder wool could be grown, but it could also be produced more economically.



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

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

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

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

VENTER, J.J., NEL, J.W. & EDWARDS, W.K. 1977. Effect of sheltering on the weathering of wool. Agroanimalia 9, 45-51.



Karoo Agric 1 (2), 37-40