Improving the value of strong wool

There are six particularly important traits to focus on when improving strong wool quality:

• Micron (MFD)– the average cross-sectional width of wool fibres has the biggest impact on the end use, and the value, of the final product.
• Staple length– Mainly affected by the length of time between shearings; also affected by genetics and nutrition. Secondary to MFD in importance for determining the use of the end product.
• Crimp– the waviness of the wool fibres. Higher crimp wools may produce textiles with better elasticity and memory.
• Variability in MFD– a high variability is undesirable; it results in irregularities in yarns and stiffer textiles. 
• Coarse edge– ‘prickle factor’. If too high a percentage of fibres are over 30 micron, the fabric will be uncomfortable to wear against the skin.
• Medullation– medullated fibres are hollow and poorly formed. They tend to be coarser and create difficulties at the processing stages.

With strong wool still struggling to command prices that contribute a meaningful return to dual-purpose sheep farmers, the industry is increasingly looking for new ways to reinvigorate the market. One way of achieving this is by developing new products using strong wool as the raw material. This is a driving reason behind New Zealand Merino recently opening Studio ZQ in Christchurch- the space aims to work as an incubator, accelerator and think tank for wool-focused ideas and ventures.

But developing new products isn’t the only way in which the value of strong wool can be increased. Flocks can also be bred so that the wool they produce more closely resembles the higher-value fine wools which are currently the crown jewel of New Zealand’s wool industry. Wool traits are highly heritable, which means that breeding programmes can quickly improve selected traits, and an increase in the value of the raw product can rapidly be seen.

The most important of these traits is the micron, or fibre diameter, of the wool- it is, after all, the defining difference between strong wools and fine wools. However, micron is not the only wool trait which can be bred for in order to increase wool value. Here’s a closer look at micron and 5 other traits, and their respective importance to increasing the value of a wool flock. 

Micron/ fibre diameter/ MFD

What is it?

Micron, also known as fibre diameter, refers to the average width of a cross section of a wool fibre. It is most commonly measured in micrometers (μm), hence the common use of the term ‘micron’. Finer fibres allow finer yarns to be spun, which can be incorporated into softer, lighter fabrics which are more comfortable to wear next to the skin.

What affects it?

Mean fibre diameter (MFD) is influenced by the secondary to primary follicle ratio (S:P ratio). All wool grows out of hair follicles on the skin of the sheep. Primary follicles produce larger wool fibres than secondary follicles. Each primary follicle has between 5 and 25 secondary follicles clustered around it. The S:P ratio is genetically and nutritionally controlled, and varies between sheep breeds. The higher the S:P ratio, the lower the MFD will be.

A single fibre has variation in diameter along its own length. This is caused by a variety of factors, including disease, nutrition, pregnancy and lactation. Significant reductions in MFD are bad, because they cause weak points in the fibre that can cause fleece tenderness. Tender fleeces break during processing, and so yield shorter fibres than sound fleeces.

What’s the range?

Staple length

What is it?

A wool staple is a naturally formed cluster of wool fibres. Staple length comes in a close second to micron in determining the use and value of wool.

Why is it important?

Staple length is measured in millimetres, and is used to get a rough idea of the wool fibre lengths. The reason for its importance is fairly intuitive- the length of the wool fibres that go into one end of the wool processing system is closely related to the length of the wool that comes out the other end. In particular, it is the length after carding that is important for determining end-use and price. Carding is the step in both the woollen and worsted processing systems where the wool staples are broken up, and the fibres aligned parallel to one another.

What affects it?

Staple length is mainly determined by the length of time between shearings. It is also affected by genetics and nutrition. Longer staple lengths are only desirable to a point. Most methods of wool processing require all fibres to be under 150mm.

One study looking at Corriedale ewes found that there’s more variation in staple length between staples on an individual sheep than the amount of variation that there is between different sheep.

Crimp/mean curvature

What is it?

Crimp is the waviness of the wool fibres. Higher crimp is connected to lower MFD and positively related to bulk. Bulk refers to the ability of wool to occupy space.

Mean curvature is closely related to crimp. It is the measurement of how much a fibre twists in a given length, and is measured in degrees per millimeter. Values might exceed 100 degrees/millimeter for fine Merino wools, and be less than 40 degrees per millimeter for coarse wool sheep.

Why is it important?

Crimp has a major impact on processing performance. The optimal amount of crimp differs with the end-use of the wool. Higher crimp wools tangle less during scouring and have lower wastage via nops (tangled balls of wool), but have higher noil (short fibres removed during combing), produce more irregular yarns, and are stiffer than lower crimp wools. Low crimp wools are easier to spin, whereas higher crimp wools are less inclined to pill.

It has been suggested that increased crimp results in fabrics which have better ‘memory’, are more elastic and are less prone to creasing.

Wool bulk is a characteristic which significantly affects the end-use performance of wool. It is strongly positively related to mean curvature. Higher bulk wool is desirable for fabric and carpet manufacture.

Variability in MFD

MFD variability is usually given as the coefficient of variation of mean fibre diameter (CVMFD). It doesn’t matter how it’s calculated, but it’s good to know that a single farm’s values are typically 20 to 30%. A higher CVMFD is undesirable as it affects spinning performance, causes irregularity in yarns and results in stiffer textiles. 

Coarse edge

What is it?

Coarse edge, also known as prickle factor, or comfort factor, is the percentage of fibres over a certain MFD threshold (typically 30 microns). It is directly related to the CVMFD. It is important for determining the suitability of wool for garments to be worn against the skin.

Wool prickle is the uncomfortable itching sensation that some wool garments cause, which tends to get worse when it’s hotter, or when the person wearing the wool is exercising. If the coarse edge value is less than 5%, wool prickle is reduced enough to not be noticed by most people under most conditions.

What causes it?

The most common line of thought as to why wool prickle occurs is based on the ‘needle’ analogy. The reasoning is that wool fibres over 30 micron are stiff enough to poke into skin, whereas thinner fibres bend more easily and therefore offer less resistance.

It has also been suggested that medullated fibres increase wool prickle due to their non-round cross section. The thin longitudinal edge of some medullated fibres may cause skin irritation by ‘slicing’, rather than ‘stabbing’ the skin, like a blunt serrated knife.

Medullation

What is it?

The last trait for today is medullation. Medullated fibres are ones which are poorly formed with a hollow core. The proportion of medullated fibres in a fleece tends to increase as the CVMFD increases. 

Why is it important?

They tend to be coarser than non-medullated fibres, and because of this medullation is undesirable when aiming for lower MFD. Medullated fibres also create difficulties at the processing stage: they are more difficult to spin into yarns, and the hollow core means that they don’t take up dye as well as non-medullated fibres.