Dark
fibre control in sheep and wool
Introduction
Dark fibre in white wool is a serious fault that is
heavily penalised when identified (Fleet et al. 2002). Australia has a valued reputation for supplying wool that generally has low dark fibre
content (Hansford and Swan 2005). This advantage relies on farmers and
woolclassers, with guidance of the AWEX Code of Practice, to ensure that
affected wool is separated and appropriately described. Should dark fibre
content go unrecognised in manufacture then the costs of batch rejections and
product mending that result can be prohibitive and this affects the value and
competitiveness of wool fibre.
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Fig 1. Fabric
containing dark fibres.
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There are
several sources of dark fibre of which sporadic “clumpy” types are unable to
be reliably sampled (e.g. small spot) in baled wool. Distinct spots of dark
wool and urine stain can usually be readily seen and controlled on-farm while
for inherited isolated pigmented fibre there is information about related
visible pigmentation types on sheep that can be used as indicators of risk.
In general, Merino wool is of relatively low risk of pigmented fibre though
potential for improvement remains.
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Australian Wool
Innovation Ltd (AWI) initiated a research program (AWI 2003) to develop
technology for presale measurement of wool bales for dark and highly medullated
fibre (AWTA et al. 2004; Fleet et al. 2007) together with a
complementary information system for specifying these risks. The priority for
this program developed with industry concerns about changes in Merino
crossbreeding that involves high-risk coloured and highly medullated types
(e.g. Awassi, Karakul, Damara and Dorper). The Federation of Australian Wool
Organisations, with support from AWI, introduced vendor declaration and the
Dark and Medullated Fibre Risk Scheme to provide wool buyers and processors
with guidance about the content of dispersed dark and medullated fibres in wool
lots (http://www.woolindustries.org/dmfr.htm ; AWEX 2007).
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Fig 2. White polypropylene
in wool top and dyed fabric.
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Sources and control
NON-SHEEP dark fibre contaminants are
controlled by quality management, minimising the risk of exposure of sheep to
such fibres. This involves avoidance of use or precautions (cleanliness) in
the paddocks, yards and shearing sheds. Potential contaminating fibres
include bale twine, fertiliser bags, feathers, string or rope, rags and clothing
and other animal fibres (e.g. alpaca, goat, cattle, dogs, cats and rodents).
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Non-approved
pack material or white synthetic fibres (except nylon), as contaminants of wool
cause the opposite problem to dark fibres, as they may not accept dye (Figure
2). Vegetable matter can also cause problems but this fault is measured
presale and processors can plan to minimise effects through carbonising,
carding and combing. Detection of non-sheep dark fibres (e.g. twine) within
the presale test for vegetable matter leads to the wool lot being withdrawn
from sale and rehandling costs incurred (passed back to the grower) to locate
the material.
SHEEP dark fibres include those transferred
from coloured or partly coloured sheep and those grown on the white sheep.
Highly medullated white fibres transferred from certain breeds or in Merino
crossbreeding situations can also cause problems as these fibres can have
restricted dye uptake and show as faults in coloured wool products. Dark
fibres produced by sheep may be caused by urine stain or melanin pigment.
Other staining effects (e.g. fleece rot, faeces and blood) will generally not
darken the fibres enough to cause a problem except when there is bulk
discolouration (e.g. colour line). Spray marks and brands can sometimes lead
to dark fibres due to inappropriate application (e.g. close to shearing).
Apply spray marks to the face rather than wool-bearing areas.
Urine stain
All sheep can
produce urine stained fibres. Skirted fleece from wethers or rams is lower
risk (stain on belly wool) than for ewes. CSIRO research has determined that
wool exposed to urine should be removed from all sheep prior to shearing
(Figure 3 and 4). Crutching to remove stained wool carried out within 3 months
of shearing greatly reduces the risk of excessive urine stained fibres. Longer
periods are likely to result in dark fibre concentrations exceeding 100 per kg of
top. If precautions are not taken to effectively control urine stain then
efforts to reduce other dark fibre sources could provide limited advantage for
that clip.
A dark fibre
decision scheme, originally proposed by CSIRO primarily for identifying urine
stain risk, has been adapted for Merino fleeces and pieces wools to include
contacts with breeds of high-risk for pigmented or highly medullated fibre. The
information system to inform buyers and processors of these contaminant fibre
risks, through catalogue advice and presale test reporting, was introduced to
wool marketing in July 2004 (further details at http://www.woolindustries.org/dmfr.htm
and AWEX 2007).
Fig 3. Ewe with
heavy urine stained wool
around the crutch.
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Fig 4. Urine stained fibres can be minimised by
the removal prior to shearing of wool likely to have been exposed (crutch of
ewes and pizzle wool of rams and wethers) to urine.
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Pigmented fibres
Melanin is
the natural pigment produced in the skin by special cells called melanocytes
(Figure 5). These pigment cells transfer coloured granules to surrounding cells
via finger-like projections. When pigment cells are present in wool or hair
follicle bulbs then a pigmented fibre usually results. Genes that the sheep
inherit primarily determine the location, number and activity of pigment cells.
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Fig 5. Pigmented cell (melanocyte).
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Sheep breeders have selected genes that inhibit
the development of pigment cells within wool and hair follicles so producing
white coats. The genes involved are currently not confirmed though several
potential candidates causing similar effects in human, mice and other
livestock are recognised (Fleet et al 2004; Fleet 2006). Until researchers
can characterise the DNA, or identify genetic linkages that can be used as
aids in selection, breeders will have to accept the inefficiencies of
reliance on phenotype alone.
It is important to know the types of pigment
most relevant to the risk of dark fibres in wool. Increasing buyer attention
to dark and medullated risks will increase the importance of the reduction of
these contaminant or inherited fibres on-farm. SARDI has identified various
types of visual pigmentation on Merinos on which selection can focus to improve
wool quality (Fleet 1996).
Recessive black lambs
An occasional black lamb can arise in Merino
self-replacing flocks when a ram carrying the recessive gene is used within a
flock. The inheritance appears simple (i.e. from carrier parents on average one
in four progeny will be black).
Pigment primarily develops during pregnancy.
The first evidence of pigment cells in the skin of a recessive black foetus
can be seen between 50 and 60 days after conception (Fleet et al 2004). By
70 days, pigment cells are seen being incorporated into budding wool
follicles (Figure 6).
Different recessive black patterns occur due to
genetic variation within the primary gene and to other genes that have
modifying effects; such as white spots (Figure 7 and 8). Black around both
eyes is a distinct feature of extensively white spotted black lambs (Figure
8). Occasionally a coloured lamb may also be born with a brown (moorit) coat
and this is the effect of another interacting recessive gene.
The
frequency of the gene and carrier sheep responsible for black lambs varies
between flocks. When the gene frequency is low many lambs may be born before
use of a carrier ram is recognised.
About
half of the progeny of a carrier ram will be carriers. In a ram breeding
flock all known carriers and their progeny should be culled. The gene for
black lambs has no effect on isolated pigmented fibres in the wool grown on
white carrier sheep. However, coloured sheep left in white flocks can lead
to transfer of dark fibres to white sheep or wool. Coloured lambs and their
rearing ewes should be separated from the white flock.
DNA parentage testing services can be used to
identify which ram in a syndicate mating has fathered a black lamb. Specific
DNA test(s) could allow the reduction or elimination of genetic variation
responsible for recessive black lambs. Research by Macquarie University, to which SARDI collaborated, determined that this genetic variation involves
chromosome 13. Research by CSIRO (involving the SheepCRC, AWI and MLA) and
others has since found the ovine gene responsible (Agouti or ASIP) to
be complicated; making the development of a specific test to identify white
carriers difficult. The most recent research findings are reported by Royo
et al (2008) and Norris and Whan (2008).
Random spot
Distinct patches of pigmented
fibres can occur anywhere (randomly) on the coat. The spots are usually
black or grey (Figure 9) but can be brown or tan (Figure 10). Affected sheep
usually have only one random spot that can range in size from small to large
and obvious. If there are multiple spots on the sheep, or both sides of the
face are affected (i.e. both eyes), the sheep may be recessive black (Figure
8).
Random spots appear to have a
complex determination but affected sheep can increase with other pigmentation
types (e.g. leg fibre pigment). These spots, including affected wool that is missed
at shearing, can be important sources of dark fibres. The wool fibre spots
can fade in early life, making subsequent detection difficult, yet still
present a large number of hidden pigmented fibres. Affected sheep should be
shorn separately and culled (for slaughter) as soon as possible from the
white flock. The entire fleece or at least the pigmented spot(s) should then
be placed in a container for pigmented wool (BLK). The white wool from
these fleeces has a risk of residual pigmented fibres so needs to be kept
separate from low risk wools and the Y suffix can be used instead of BLK.
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Fig 6.
Developing pigmented wool follicles.
Fig 7. Twin lambs
(one black and one white from carrier parents.
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Fig 8. Black
lamb with extensive white spotting.
 Fig 9. White lamb
with black random spot on the side. Also note the tan halo-hair on the back-of-neck.
Fig 10. White lamb
with a tan random spot.
Note also the tan ear tips and tan halo-hair back-of -neck.
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Fig 11.
Isolated pigmented fibre in a wool staple.
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Isolated pigmented fibres
Some white
fleeces contain isolated pigmented wool fibres (Figure 11 and 12). Even when
these fibres are numerous, identification is unlikely from visual inspection
of wool. In Merino sheep visible hair pigmentation is usually less frequent
than skin or hoof pigmentation. Culling types of visible hair pigmentation has
been shown by SARDI to be effective at minimising the risk of isolated
pigmented wool fibres in the hogget fleece. In some of the flocks, one or
two types of hair pigment (e.g. pigmented birthcoat halo-hair and leg fibres)
may be important, while in others several types of fibre pigmentation may
have additional effects. These types of hair pigment in, or bordering, the
fleece can also be direct contaminants.
AWI and MLA (2007) produced a Visual Scores
booklet to assist breeders in recognising and recording pigment and that
information may ultimately help develop DNA tests to assist sheep selection.
(http://www.sheepgenetics.org.au/merinoselect/)
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Fig 12. Isolated pigmented wool follicle in skin.
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Fig 13. White
foetal lamb skin at 85 days after conception showing developing wool
follicles (F) and pigment cells (M). In this case, isolated pigmented wool
fibres could develop. Note the differences with Figure 6 for a black Merino
foetus.
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In contrast to
the recessive black foetus there is usually an apparent complete absence of
pigment cell precursors in the developing foetal wool-bearing skin of a white
Merino lamb. Pigment cells were usually not evident in the skin of white
Merino lambs until late pregnancy or after birth and miss inclusion in
developing wool follicles. In certain cases, some pigment cells reach the
skin while primary and original secondary follicles are initiated (64 to 100
days) and isolated pigmented wool fibres may then develop (Figure 13).
However, the route of entry via the epidermis is restricted since
colonisation in this tissue is inhibited at that time. The genes affecting
pigment cell migration, colonisation and longevity in sheep remain to be
confirmed and these advancements would likely assist the development of
diagnostics for specific selection of the most effective and acceptable
genetic variation.
Risk of
isolated pigmented wool fibres in Merino sheep is confined mainly to young
sheep of less than 2 years of age. Sheep between 2 and 6 years of age seem
likely to have a low risk of isolated pigmented wool fibres present from
early development. This change with age involves the loss of pigment cells
from wool follicles (“greying”).
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Birthcoat halo-hair
Lambs may be
born with pigmented halo-hairs. These hairs tend to be concentrated on the
back-of-neck making visual recognition practical for newborn lambs. There
may be single, few or many hairs (Figure 14 and 15) evident on affected
lambs. The risk and level of isolated pigmented wool fibres increases with
degree of pigmented halo-hairs evident on the back-of-neck.
Heritability
of this type of pigmentation is high and should be able to be reduced by
selection. However, a problem with reducing this pigmentation and associated
isolated pigmented wool fibres is the temporary presence of halo-hair on the
lamb coat. Gradual shedding of the birthcoat halo-hairs soon after birth
means a later inspection at lamb marking may only reveal high degrees on early
born lambs. The hairs are usually tan coloured. Mark for culling the
affected lambs, at least those with distinct pigmented halo-hair, and
separate for shearing last and send to market (for slaughter) as soon as
practical.
Leg hairs
Pigmented
hair spots and patches occur on the legs of some sheep. The spots tend to be
tan coloured in Merinos and can vary in degree from one or a few spots, and
from speckled areas to large patches of pigmented hairs (Figure 16 and 17).
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Fig
14. New-born lamb with just a few tan halo-hairs on the back-of-neck.
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Fig 15. Lamb with a
distinct patch of tan halo-hairs on the back-of-neck. Note also the tan
margins on ears.
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There is symmetry of spotting so the
pigmentation tends to be evident on all legs or a pair of legs. The spots tend
to be most prevalent on the back of the rear legs. Leg hair pigmentation is
highly heritable in Merinos and an identifiable gene appears to be involved in
the elimination of this type of pigment; presence being the dominant feature. When
leg hair pigment is present other types of pigmentation tend to be increased.
This includes pigment in nose-lips, skin and hooves, random fibre spots and
isolated pigmented fibres in the fleece.
Pigmented leg hair tends to become more
prominent as the lamb matures and then remains stable during adult life. This
persistence of pigmentation and the effects of the identifiable gene can allow
effective culling and a minimal occurrence in Merino flocks. Inspections can
be made at lamb marking or sheep classing, but this pigment is best observed
when the wool is removed from the legs at crutching and shearing. Ask the
shearers to check for this pigment. Mark for culling the affected lambs, shear
them last and send to market (for slaughter) as soon as practical.
Fig 16. Single spot
of pigmented leg hairs.
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Fig 17. Patches of
pigmented leg hairs. Note streaks in hooves.
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Other types of hair pigment
Other types of hair pigment on white sheep
that have shown associations with isolated pigmented wool fibres can occur on
the horn sites, face, ears and eyelids. At least those lambs/sheep with
distinct fibre pigment should be marked for separate shearing and culling (for
slaughter). The heritability of pigmented horn site hairs is high while for
face hairs it may be low. These pigmented hairs tend to arise and disappear
with leg hair pigment. Like leg hair pigment they become more prominent as the
lamb matures and remain stable during adult life (Figures 18 and 19).
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Fig 18. Pigmented
hairs on horn sites of a lamb.
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Fig 19. Pigmented
hairs on the face and horn sites of an adult.
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Merino sheep can have a mixture of tan and
white eyelashes. In some cases, these tan hairs are very prominent (Figure
20). Heritability is high in Merinos. It is the high degree of expression
(>75% pigmented) that has been associated with risk of isolated pigmented
fibres.
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Fig 20. Prominent pigmented eyelashes on a lamb (above) and a ram
(below).
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Fig 21. Tan hairs
on the ear tips of a lamb.
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Fig 22. Black hairs
on an adult ear.
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Two forms of
coloured ear fibres exist on white Merinos. On lambs, the pigmented ear hairs
are tan coloured and affect the tips or margins of the ears (Figure 21). These
fibres fade as the lamb matures. On hoggets and adults, diffuse spots of black
hairs may have developed, especially on sheep with other pronounced fibre and
skin pigmentation, and these are likely to remain during adult life (Figure
22). Heritability of both types of ear fibres may be low to moderate in
Merinos. The association between tan ear fibres on lambs and isolated
pigmented fibres appears to be less than for the other types of hair
pigmentation.
Skin and hoof pigment
Skin and hoof pigmentation of varying
degrees occurs frequently among Merino sheep. There may be some reduction of
pigmented wool fibres arising from selection against skin and hoof pigment.
However, it is considered more efficient to check for hair and wool
pigmentation of primary concern and select against these types. It should be
recognised that the various types of pigmentation are positively related.
Selection against fibre pigmentation will tend to reduce the other more common
pigmentation types (skin and hooves).
There seem to be little benefit to be
gained in wool quality, given the current knowledge, from selecting against
skin and hoof pigmentation on Merino sheep that remain in the flock after close
attention and culling of wool and hair pigment. However, each sheep breed
organisation has specific requirements for the amount of skin and hoof
pigmentation desired.
In the Merino
there has been a traditional selection against all types of pigmentation.
Pronounced nose-lips skin and hoof pigment would generally not be tolerated in
Merinos and tends to accompany unwanted hair pigmentation. In contrast, for
other breeds like the Corriedale and Romney, the preference has been for
prominent nose-lips skin and hoof pigment.
Old-age wool fibre spots
As white sheep age there is potential for
changes within the wool-bearing skin that can make epidermal pigment cells very
active and move to wool follicles leading to pigmented fibres. This change can
be induced by additional shearings or repeated exposure of bare-skin to
sunlight or artificial ultraviolet light. The first sign is a round black-grey
skin spot in the fleece region that subsequently produces black-grey pigmented
wool fibres. The increase with age is exponential, with the pronounced rises
occurring after 5 years in the Merino flock studied (Fleet 2006).
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Very old
Merinos sometimes have many age spots and produce fleece wool with high
concentrations of pigmented wool fibres. Shearers could recognise sheep with
black-grey pigmented skin spots in the fleece area (Figure 23) and the fleece
wool should be kept separate from unaffected lines. Cull (for slaughter)
sheep showing black skin spots in wool-bearing areas.
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Fig 23. Old-age – black
skin spots with pigmented wool fibres. The sheep was 8.5 years old and shorn
3 months previously.
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Dark and Medullated fibre risk scheme
The Code of Practice (AWEX 2007) provides
guidelines for the preparation of Australian wool clips. The woolclasser has a
key role in quality assurance and new wool sale lot test certificate changes
aim to identify wool that lacks this attention or is non-complying for other
reasons (AWEX 2008). The Dark and Medullated Fibre Risk (DMFR) scheme
(Hansford et al 2003; AWEX 2007)
together with revised vendor declarations (http://www.woolindustries.org/dmfr.htm)
was initiated by the Federation of Australian Wool Organisations due to inquiry
and concern from the wool textile industry about increased problems with dark
and medullated fibre. The DMFR scheme is based on a previous CSIRO proposal
(Burbidge and McInnes 1994) that made use of farm information to provide
an insight to risk of urine stain and inherent pigmented fibre. The wool
textile industry is making use of the DMFR information to assist in selection
of wool lots for sensitive end-uses (AWI 2008). Increased adoption of the DMFR
scheme (http://www.awta.com.au/en/Home/News2/General-Information/),
together with other test developments in progress, should ultimately provide
price incentives that encourage greater emphasis on producing low risk wool.
Conclusion
It is not
possible to guarantee a complete absence of dark fibres in wool. It is
practical to implement management and breeding schedules that will serve to
minimise the risk for wool processors.
Ø
Non-sheep dark fibres - minimised by avoidance
of contacts with sheep and wool and general cleanliness as part of quality
management schedules.
Ø Urine stain - minimised by removal within 3 months prior to
shearing.
Ø Pigmented fibres – future occurrence is minimised by culling of
affected sheep as soon as practical and perhaps also their parents (e.g.
recessive black). Age is an important factor for inherited isolated pigmented
fibres and associated types of visible pigmentation in young sheep and
development of pigmented fibres in very old sheep.
Research projects
in progress, where phenotypic information about visible pigmentation is being
collected and analysed in conjunction with DNA marker studies, should assist
selection practices in the future (e.g. http://www.agric.wa.gov.au/content/AAP/Rowe_SheepCRC.pdf).
Acknowledgments
Peter Sommerville made this document and
other related information available from AWTA Ltd home page.
Further reading
AWEX (2007) Code of Practice for the AWEX
Quality System. Preparation of Australian Wool Clips. The Woolclasser
(Australian Wool Exchange Limited; Sydney). www.awex.com.au
AWI (2003)
Four-pronged attack on costly issue. In: Beyond the Bale. Issue No. 5, June
2003 (Australian Wool Innovation Ltd.). http://www.wool.com.au/
AWI (2008) Mills
support DMFR scheme. Beyond the Bale, Issue 33, p3 http://www.woolinnovation.com.au/LivePage.aspx?PageId=57
AWI and MLA (2007) Visual Sheep Scores
(Australian Wool Innovation Ltd. and Meat and Livestock Australia). http://www.sheepgenetics.org.au/merinoselect/
AWTA Ltd, CSIRO and AWI (2004). Development
of an improved test for detection of dark and medullated fibres in presale core
samples. International Wool Textile Organisation, Technology and Standards
Committee, Shanghai Meeting Report No. RWG 5. http://www.awta.com.au/en/Home/Publications/Research-Papers1/Wool-Contamination-/
Burbidge A and McInnes CB (1994) Dark fibre risk and
prediction. Seminar Proceedings: Specification of Australian Wool and Its
Implications for Marketing and Processing, CSIRO Division of Wool Technology
& the International Wool Secretariat, November 1994. Available at http://www.awta.com.au/en/Home/Publications/Research-Papers1/Wool-Contamination-/
Fleet MR (1996).
Pigmentation types – understanding the heritability and importance. Wool
Technology and Sheep Breeding 44: 264-280. http://sheepjournal.une.edu.au/sheepjournal/
Fleet MR (1998). Wool contamination –
pigmented and highly medullated fibres. PIRSA Fact Sheet Agdex 437/85(http://www.pir.sa.gov.au). Also available
on Prime-Notes CD ROM; http://www.sardi.sa.gov.au/
and AWTA Ltd. http://www.awta.com.au/en/Home/Publications/Research-Papers1/Wool-Contamination-/
Fleet MR, Mahar TJ and Turk JA (2002).
Merino crossbreeding and objectionable sheep fibres: the problem and potential
solution. Wool Technology and Sheep Breeding, 50: 650-656. http://www.awta.com.au/en/Home/Publications/Research-Papers1/Wool-Contamination-/
Fleet MR, Forrest JW, Walker SK, Rogers GE
(2004). Foetal
development of melanocyte populations in Merino wool-bearing skin. Wool
Technology and Sheep Breeding, 52: 101-123. http://sheepjournal.une.edu.au/sheepjournal/
Hansford KA and Swan PG (2005). Australian
Wool Innovation 2004 Global Survey of Dark and Medullated fibres. International
Wool Textile Organisation – Technology and Standards Committee. Commercial Technology
Forum. Report CTF 02. Biella Meeting, November 2005. http://www.awta.com.au/en/Home/Publications/Research-Papers1/Wool-Contamination-/
Fleet MR
(2006) Development of black pigmented skin spots
and pigmented wool fibres in a Merino flock – causes, field observations, and
wool measurement. Australian Journal of Agricultural Research, 57:
751-760. http://www.publish.csiro.au/nid/40.htm
Fleet MR, Mahar TJ, Alaya-ay AR, Fish VE and Balasingam A (2007). Towards a reliable test for predicting wool
contamination and impacts on processing. SARDI summary of AWI project TD015 http://www.sardi.sa.gov.au/pdfserve/livestock/meat_and_wool/integrated_production/070502_flyer.pdf
.
Norris BJ and Whan VA (2008). A gene
duplication affecting expression of the ovine ASIP gene is responsible
for white and black sheep. Genome Research 18: (in press).
Royo LJ, Alvarez I, Arranz JJ, Fernandez I,
Rodriguez A, Perez-Pardal L and Goyache F (2008). Differences in expression of
the ASIP gene are involved in the recessive black coat colour pattern in
sheep: evidence from the rare Xalda sheep breed. Animal Genetics 39:
290-293.
Last update: May, 2008
Agdex: 437/30
Author:
Malcolm R. Fleet
South Australian Research and
Development Institute, Livestock Systems Alliance, Turretfield Research Centre,
Rosedale SA 5350, Australia.
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