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Strength

Staple strength is a measure of the force is required to break a wool staple of a given thickness (kilotex), recorded as Newtons per kilotex (Nkt).

Staple strength relates to the efficiency of wool processing, particularly the amount of fibre breakage and wastage during combing.

Recent changes in the textile market increased processor preference for longer tops (increased hauteur) of slightly reduced average fibre diameter, with low short fibre content. Spinners can spin fine yarns faster from these tops, with fewer yarn breakages and with increased evenness and bundle tenacity. Weavers using these finer yarns of increased tenacity and evenness can increase weaving speed and produce lighter cloth with fewer fabric faults.

Woolgrowers can expect increasing trade recognition and preference for wool of high staple strength and low mid-break. Strength and mid-break percentage are determine the combing performance of wool. Many mills specify a minimum strength and a maximum mid-break percentage for their deliveries, increasing price pressure on low mid-break types, and reducing competition for wools with a high mid-break percentage.

  wool strength

Auction data show that prices improve as staple strength increases. Compared to a baseline 35 Nkt type, penalties are large for tender wools (14-21 Nkt), with discounts declining as strength increases. Wools testing above 40 Nkt attract premiums, particularly at the finer end where discounts or premiums are magnified. Stylish 50 Nkt types at 16 micron can trade 200-500 cents above 35 Nkt types. This trend strengthened recently with the super-sound types outperforming lower strength types.

Strength discounts - fine wool


Chart of price discounts for strength in fine wool.

Strength discounts - medium wool


Chart of price discounts for strength in medium wool.

Strength trends

The following chart tracks historical trends in strength premiums and discounts relative to the common benchmark of wool measuring 35 Nkt.


Chart of trends in price premiums and discounts for wool strength.

From 1995/1996, premiums and discounts traded in a range and the trend was generally sideways. Discounts began to drift out around the 2000 season when both the 40 Nkt premium and discounts for tender wool peaked as the market recovered from 1998/99 season lows.

From 2001 to 2005, relativities between the strength categories trended towards each other as prices fell and China began to dominate the market. The low point in 2005 coincided with the bottoming of the market and an increase in lower styles and tender types due to drought conditions across most of Australia.

Since then premiums for 40 Nkt types have gradually lifted while tender types have seen a sharp increase in discounts.

Wool strength - management tips

The key to sustainably improving staple strength and reducing the likelihood of mid-breaks in a flock is to manage the diameter profile along fibres. The strength of any fibre is the strength of its weakest (generally thinnest) point. Variation in diameter along fibres throughout the year accounts for around 40% of the variation in staple strength. Strength is maximised by making the diameter profile as smooth and level as possible along the length of the fibre.

Supplementary feeding during times of nutritional stress can help to maintain a constant level of nutrition to wool follicles throughout the year, helping to offset the effects of seasonal breaks.

Shearing close to the date of a seasonal nutritional stress will put the break in the wool very close to the tip or base.

Sheep will often develop breaks in the wool during times of sickness. Maintain sheep in good health. [LINK to wool.com webpage 2.1.1 Sheep health].


Diagram showing stress point where wool fibre is thinnest.

Breeding to reduce between fibre variation in diameter

Variation in diameter between fibres as they emerge from the skin accounts for around 40% of the variation in staple strength. Therefore, a key breeding objective to maximise staple strength is to have all fibres within staples of similar diameter. Consider carefully your source of rams, and make sensible use of measurements.

Clues to look for to identify sheep with low variation in diameter between fibres include:

Visual clue All fibres of similar diameter Fibres of widely differing diameter
Visual Wool brightness Glossy, or bright lustrous wool Dull or flat
Crimp Depth (fibre alignment) Deeply crimped from staple base to tip Flat, lazy, or indistinct crimp
Staple thickness Clear-cut bundles or thin locks Thick staples or blocks
Tip shape Small, rounded tips with evident crimp Pointy dry tips or angular blocks
Visible hair No evidence of hair around breech or poll Hairy breech, poll and harsh, kempy muzzle
Tactile Texture Smooth, silky feeling Rough, harsh, or gritty
Apparent temperature Cool-to-touch  
Measured Shape of Diameter Histogram Narrow, symmetrical Broad, skewed
CV of Diameter Less than 19% for mid-sides More than 21% for mid-sides
% fibre > 30 um < 1%  
Staple strength SS > 40 Nkt