Tender wool is a major problem for processors (particularly scourers and combers) because it increases entanglement during scouring and fibre breakage during carding and combing. This results in lower hauteur (average fibre length in the top), greater variation in fibre length and a higher percentage of noil (the short fibres removed during combing). These add to the cost and result in a less valuable top, causing tender wool to be heavily discounted in the market place.
Causes of tender wool
The main cause of tender wool is large fluctuation in wool growth caused by differing feed availability and quality throughout the year. Wool growth is closely related to fibre diameter so variations in growth will be translated into variations in diameter along fibres.
The rate of wool growth is not constant throughout the year but varies according to the amount and quality of feed available (least in autumn and most in spring). Feed quality is poorest during autumn, particularly after summer rainfall that leaches the nutrients out of dry feed without resulting in pasture germination.
After the break of season, emerging green feed will stimulate wool growth and fibre diameter will begin to increase. During this short period, it is not uncommon for sheep liveweight to fall while wool fibre diameter increases.
The weakest point will usually correspond to the minimum diameter and this is often at the break of season (or after heavy summer rainfall that results in substantial green feed). There is no evidence that the stress of a rainfall event itself causes a weak point in the wool.
Another, less common cause of tender or 'rotten' wool, is fibre shedding caused by adrenal hormone secretions in response to stresses (for example, flystrike, acidosis, lack of water or disease). This results in a well-defined 'break' or window in the staple where fibres cease to grow. The strength of these staples is usually below 10N/ktex.
The genetic variation in diameter between fibres also affects the staple strength. Mostly, this is associated with variability in individual fibre lengths and crimp. Fibres that vary in length will break at different times during the process of measuring staple strength, resulting in lower staple strength.
Most susceptible sheep
Ewes
Pregnancy and lactation place additional burdens on the nutrition of ewes. If lactation occurs in autumn, tender wool will almost certainly result as the stress of lactation, poor feed quality and limited quantity occur at the same time. Large, usually uneconomic, amounts of supplementary feed may reduce the risk.
The incidence of tender wool can be reduced if lambing is delayed to late winter or spring because:
- the minimum diameter along fibres will not be as low as if ewes were nutritionally stressed in autumn
- there is less of a 'blow-out' in fibre diameter due to the spring flush of pasture. Consequently, there will be less variation in diameter along fibres, leading to increased staple strength.
Weaners
Young sheep tend to produce a higher proportion of tender fleeces than adult sheep. This is because they are still growing and body growth is competing strongly with wool growth for the available protein in the diet. Fibres tend to have a lower minimum diameter in autumn leading to lower strength. However, during the spring pasture flush in the following year, the weaner’s body weight and fibre diameter increase dramatically. This 'blow-out' increases the variation in diameter along fibres and produces tender wool.
There are two approaches to reducing tender wool in weaners:
- Increase the minimum diameter along fibres in summer and autumn by increasing nutrition.
- Limit increases in fibre diameter during the green pasture phase in winter and spring by restricting intake.
The thinning of the wool fibre in summer/autumn can be reduced by decreasing stocking rate, or by supplementary feeding. If shearing in spring, the minimum diameter of wool by growing sheep (medium wool types) can be increased to reach between 30-35 kilograms (kg) liveweight going into summer.
The fibre diameter of wool from heavier animals will still fall towards the break of season but the minimum diameter along fibres will be higher than that from lighter animals. This means that management to increase weaner weights prior to summer (for example parasite control, careful selection of weaning paddocks and treatment of trace element deficiencies) may increase minimum fibre diameter and staple strength in autumn.
Weaners (and wethers) can be grazed hard as soon as winter feed becomes available by increasing stocking rate. This will reduce the 'blow-out' in fibre diameter normally associated with the onset of green feed. Lighter weight sheep at the break of season (particularly weaners) are more likely to experience fibre diameter 'blow-out' as green feed becomes available. A further bonus of increased stocking rate is the reduction in mean fibre diameter and the production of more wool per hectare (despite lower fleece weights).