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Vegetable matter (VM) in wool is a disadvantage as its removal during processing can be costly. Depending on the severity and type of VM, wool may need to undergo additional processing called carbonising. Carbonising is a relatively complicated and expensive process where wool passes through sulphuric acid bowls and ovens, before rollers crush the remaining brittle VM. Carbonising is slow and usually involves higher fibre loss than other processing methods.
VM testing is a standard pre-sale procedure. The figure published in wool-sale catalogues is the percentage of VM found in the greasy wool core-sample.
VM is allocated into seven main types in the AWEX-ID typing system:
|B||Burr (barrel medic, burr medic).|
|E||Seed (sub clover, carrot seed, scotch thistle).|
|S||Shive (barley grass, wild oats).|
In combing wool, the fibrous types (S) of VM are the most difficult for processors to remove. S types align with the fibre during combing and can pass through to the final fabric resulting in the largest discounts. Burry VM types (B) such as those belonging to the medic group generally sit on top of the staple and are considered easier to remove during processing. Higher percentages of Burr are usually acceptable by processors for this reason.
VM discounts are relatively minor in the market place for types testing less than 1.5%. Penalties apply when fleece wool approaches the 2% level and rapidly increase when VM measures over 2%, reflecting the higher processing costs and lower throughput for the processor.
VM discount trends in the fine and medium wool categories are shown in the charts below.VM discounts - fine wool
VM discounts - medium wool
Graph of trends for c/kg (clean) discounts for VM% of fine wool.
Graph of trends for c/kg (clean) discounts for VM% of medium wool.
In the finer microns, discounts for high VM levels began to contract in the 2001/02 and 2002/03 seasons. By 2003, the penalties had halved compared to the 2000 season levels.
Discounts began to increase in the first quarter of the 2004/05 season, approaching longer term levels. Ongoing dry conditions increased the number of high VM types, resulting in larger discounts for sale lots with 8% VM and greater.
In the medium microns, discounts for 4% VM types tended to remain static, trading in a 40 to 70 cent range. However, 8% VM types fell sharply with discounts approaching those in the finer microns.
There are a number of ways to reduce VM levels in wool clips.Grazing
The conditions in which sheep graze can have a large effect on the amount of VM in the fleece. Location in Australia will play a large part in both the levels and types of VM in wool, however a number of strategies exist to reduce VM.
Pasture management tools include herbicide spraying or slashing at key times of the year, and changing pasture types, although this may have other long-term management implications.
Grazing tools include matching stocking rate to carrying capacity during the year, and strategic grazing techniques such as cell grazingShearing
Shearing can be timed so that sheep are in short wool at the time of greatest exposure to seeds, burrs and other contaminants. Shear sheep before seeds mature.
Frequency of shearing can also have an impact on the levels of seed and burr in the wool, although more regular shearing (more than once per year) is uncommon and generally seen as uneconomical.
While it can be tempting to skirt heavily at shearing time to reduce high levels of VM in fleece wool, studies have shown that this practice is not economically efficient.Sheep coats
An AWI study of 1200 sheep in Western Australian found that VM can be significantly reduced by using sheep coats.
Fleece from non-coated animals had an average VM of 2.0%. Fleece from coated sheep had an average of 0.7% VM. The coats also improved yield and reduced fleece rot.
Sheep coats improved the value of the wool sold by an average of $3.40 per head but in some cases was up to $8.51 per head. Click to download the Economic Viability of Sheep Coats for Wool Production Final Report (PDF 350Kb).