Staple strength is a measure of how much force is required to break a wool staple. Recorded as Newtons per kilotex (Nkt), this refers to the force/pull (Newtons) required to break a staple of given thickness (kilotex).

Staple strength relates to wool processing through influencing the efficiency with which wool is combed, and the amount of fibre breakage and wastage during this process.

Recent changes in the textile market suggest an increased processor preference for longer tops (increased Hauteur) of slightly reduced average fibre diameter, with low short fibre content. This gives spinners an ability to spin fine yarns faster with fewer yarn breakages and with increased evenness and bundle tenacity, while weavers benefit from finer yarns of increased tenacity and evenness which increases weaving speed and reduces fabric faults while producing lighter cloth.

As a consequence from this change in preferences, growers can expect increasing trade recognition and preference for wool of high staple strength and low midbreak. Strength and midbreak percentage are the two most important factors in determining the combing performance of wool. Many mills will specify a strength minimum along with a maximum midbreak percentage for their deliveries, thereby placing increased price pressure on low midbreak types and magnified decreases in wools displaying a high midbreak percentage.

Auction data confirms the above notion and show that prices improve as staple strength increases. When compared to a baseline 35 Nkt type, there are large penalties present for very tender wools (14 Nkt and 21 Nkt), with the discounts becoming less significant as the strength increases. Premiums become evident once the wools test above 40 Nkt, particularly at the finer end where any discounts or premiums are magnified. It is not uncommon for stylish 50 Nkt types at 16 micron to trade between 200 and 500 cents above 35 Nkt types.  This trend has become clearer recently with the super-sound types outperforming lower strength types.

Strength Discounts - Fine Wool

Strength Discounts - Medium Wool

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Wool strength trends

The following chart show historical strength premiums and discounts. They are shown relative to wool measuring 35 Nkt, which is commonly used as a strength benchmarking figure.

Wool strength trends

Commencing in the 1995/1996 season there was a long period where Premiums and Discounts were trading in a range and the trend was generally sideways. The discounts began to drift out however, and around the 2000 season both the 40nkt premium and the discounts for tender wool reached a peak, as the market began to steadily climb off lows made during the 1998/99 season.

From the 2001 season through to 2005 relativities between the strength categories trended towards each other as prices fell and China began to increasingly 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 40nkt types have gradually lifted while tender types have seen a sharp increase in their discounts. This is expected to continue at least in the short term while the affects of the prolonged drought continue to ripple through auction offerings around the country.

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Wool strength tips

Managing diameter variation along fibres

The key to sustainably improving staple strength and reducing the likelihood of mid-breaks in your flock is to manage the diameter profile along fibres. This is because the strength of any fibre is the strength of its weakest point, and this is generally its thinnest point. This 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.

The best tools farmers have to manage diameter variation along fibres is to:

• Control the supply of nutrients to follicles. Supplementary feeding during times of nutritional stress will help to maintain a constant level of nutrition throughout the year, which should help to offset the effects of seasonal breaks.
• Identify the most appropriate time for shearing and joining. Shearing close to the date the nutritional break occurs will put the break in the wool very close to the tip or base. 
• Maintain good sheep health. Sheep will often develop breaks in the wool during times of sickness.

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 to each other.

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

Simple recommendations are to:

• Consider carefully your source of rams.
• Make sensible use of measurements

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