Wool Technology and Sheep Breeding
Volume 44, Issue 3 1996 Article 1
Wool follicle and skin characters-their
potential to improve wool production and
quality in Merino sheep.
PI Hynd RW Ponzoni† R Grimson‡
KS Jaensch D Smith†† R Kenyon‡‡
Copyright
c 1996 Wool Technology and Sheep Breeding. All rights reserved.
ISSN 0043 - 7875/96 Wool Tech. Sheep Breed., 1996, 44 (3), 167 - 177
Wool Follicle And Skin Characters -Their
Potential To ImproveWool Production And
Quality In Merino Sheep
Phil I. HyndA, R. W PonzoniB, R. Grimsonc, K. S. Jaenschc, D. SmithC and R. KenyonB
A Department of Animal Science, University of Adelaide, Waite Campus, Glen Osmond
South Australia 5064
South Australian Research and Development Institute, GPO Box 397 Adelaide South
Australia 5001
South Australian Research and Development Institute, Turretfield Research Centre,
Rosedale, South Australia 5350.
Summary
This paper investigates the relationships between objectively assessed skin and follicle
characters, and objectively measured fleece characters, in the South Australian
Strongwool Merino strain. The relationships between objectively-assessed skin
characters and classer assessed skin and staple characters were also investigated.
The results of these and other studies indicate that genetically high producing sheep
with low fibre diameter, tend to be those with high follicle density, high secondary1
primary follicle ratio, evenly-seated follicles arranged in distinct, large follicle groups.
The follicles of these sheep tend to have a low degree of curvature and produce fibres
with low crimp frequency, low paracortex content, low sulphur content, but with welldefined
crimp. Somewhat surprisingly, we found that sheep with light (thin?) skins
tended to produce more wool of lower diameter than heavy (thick?) skinned animals.
This is in direct contrast to previous findings of moderate positive genetic correlations
between skin thickness and both fleece weight and fibre diameter.
Skin quality, subjectively assessed by a classer, had a moderate to high heritability
(0.36 and 0.24 at 10 and 16 months of age, respectively) and was closely genetically
associated with clean fleece weight (r = 0.65 and 0.57 at 10 and 16 months of age,
respectively). As such, skin quality could be a useful indirect indicator of fleece weight.
It also means that selection for clean fleece weight should result in an improvement in
skin quality. The latter is contrary to the belief of some in the industry, that selection
for fleece weight will result in a deterioration in skin quality.
It is recognised that a number of skin characters (objectively and subjectively assessed)
are associated with economically important wool traits. However, it is doubtful that an
increased (or sole) emphasis on them will bring about rates of genetic gain greater than
what can be achieved by direct measurement of, and selection for, the economic traits
in question.
Paper presented at the June 1996 Wool Forum on "Demands of Wool and
Woolgrowers Beyond 2000" convened by the SA Stud Merino Sheepbreeders' Association,
SARDI (Turretjield Research Centre) and Rampower Wool Breeding Services.
Hynd et al. 167
Wool Technology and Sheep Breeding, Vol. 44, 1996
168 Wool Follicle And Skin Characters
It is recognised that a number of skin characters (objectively and subjectively assessed)
are associated with economically important wool traits. However, it is doubtful that an
increased (or sole) emphasis on them will bring about rates of genetic gain greater than
what can be achieved by direct measurement of, and selection for, the economic traits
in question.
Keywords: Wool follicle, skin characters, Merino sheep
Introduction
The breeding objective of many wool producing enterprises is to increase the
production of finer, stronger fibres with improved style characteristics, on sheep which
have high live weight and which are resistant to fleece rot, fly strike and internal parasites.
The selection criteria used to achieve these objectives should be as direct as possible
because the closer the association between the selection criteria and the traits in the
breeding objective, the more accurate will be the selection of genetically superior animals,
and consequently the faster will be the genetic progress.
However, the use of indirect selection criteria can be useful in the following
circumstances:
*When the heritability of the indicator trait is sufficiently greater than that of the
trait of interest and the genetic correlation between both traits is also sufficiently high.
*When an indirect measure is a cost-effective alternative to a directly measured trait
(e.g. coefficient of variation of fibre diameter appears to be a cost-effective alternative
to staple strength measurement).
*When early selection of animals is required, and fleece measurements at an early
age are poor indicators of adult performance (Atkins and Mortimer 1987).
*When additional information not available by any other means can be obtained by
an indirect trait (e.g. fibre diameter variability as an indirect indicator of fleece rot
susceptibility).
*When a final selection decision is required between animals which have similar
objective measurements.
Often classers use visual assessment of characteristics believed to provide an
alternative to, or adjunct to, objective measurement. Characters such as handle, lock,
crimp definition, crimp frequency, dust penetration, tip structure and skin quality are
frequently part of the assessment.
This paper examines the genetic relationships derived in the South Australian
Strongwool Merino Resource flock experiment at Turretfield, between objectively
measured skin traits, subjectively assessed skin and fleece characters, and objectively
measured fleece characters. First an examination is made of the logic behind skin
measurements.
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168 Wool follicle and skin characters-their potential to improve wool
Wool Technology and Sheep Breeding, Vol. 44, 1996
PI I. Hynd et al. 169
Why Skin Characters?
Over the years, sheep classers have emphasised the importance of considering skin
characteristics as indirect criteria in sheep selection programmes. This emphasis on
skin is well-placed in that it is the skin that nourishes and supports the massive population
of fibre-producing follicles. A wide range of skin characters has thus been measured
and their relationships with important economic characters determined. Follicle density,
secondaqdprimary (SP) ratio, skin thickness, primary follicle density, follicle curvature,
follicle depth, and follicle group size have been measured in a number of selection
flocks, and the genetic correlations determined between these traits and fleece traits
(Brown and Turner 1968; Gregory 1982; Davis and McGuirk 1987).
Despite moderate to high heritabilities of many of the follicle characters and moderate
to high genetic correlations with fleece characters, selection based on individual follicle
characters such as SiP ratio has not resulted in the anticipated changes in fleece characters
(Jackson et al. 1975; Rendel and Nay 1978). We hypothesised that compensating
changes in other characters not under selection were negating the positive movements
in the character under selection. For example, selection for high S/P ratio would be
accompanied by corresponding changes in the size of follicle bulbs, the result being no
overall gain in the total amount of follicular tissue in the skin, hence no change in the
total amount of fibre produced per unit area of skin. Similarly, selection for increased
follicle depth would be expected to result in increased bulb size with a concomitant
decrease in follicle density, the result again being no change in total follicle tissue per
area of skin. Interestingly, tandem selection for follicle depth and density did increase
fleece weight, supporting our contention that the only programmes which will increase
fleece weight are those that result in more follicle tissue in the skin.
Skin thickness assessment was considered for some time to be a useful indirect
indicator of the productive capacity of the sheep. Indeed, Gregory (1982) provided
data which indicated that there was a moderate, positive genetic correlation (0.39)
between skin thickness and clean fleece weight in SA Merinos. He concluded that skin
thickness could be auseful early selection criterion for high producing animals. Recently,
Ponzoni et al. (1995) showed that there was a high genetic correlation between
subjectively assessed skin quality and clean fleece weight, suggesting that skin quality
might be a useful indicator of fleece weight. There is currently considerable interest in
the idea of selecting sheep which display the so-called 'soft rolling skins'. It is claimed
that breeding from these animals results not only in dramatically increased fleece weights,
but also in large fibre diameter decreases, and in an improvement in staple structure,
staple character, and processing performance (J. Watts, personal communication). This
issue is discussed further in other papers in this Wool Forum.
Before indicating the relationships between skin and follicle characteristics from the
Turretfield flock, we need to examine the biology underlying follicle development in the
foetus and the determinants of fleece weight.
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170 Wool Follicle And Skin Characters
Follicle Initiation In The Foetus
Follicles are formed from epidermal cells which have been stimulated by, as yet
unidentified, chemicals produced largely under genetic controls. The positioning and
size of the primary follicles is determined as early as day 60 in the foetus, but this
appears then to determine the subsequent positioning and size of the secondary follicles
which form from day 86 to 90. Again genetic controls operate to instigate branching of
these original secondaries from day 100 to birth. The extent to which follicle branching
occurs is a significant determinant of follicle density, S/P ratio, fibre diameter, fibre
length, and clean fleece weight (discussed below).
Determinants Of Fleece Weight
The quantity of wool produced per annum (W) by a sheep is determined quite simply
by the formula:
L = fibre length growth rate (prnld)
N = follicle density (follicles /mm2)
CSA = mean fibre cross section area (pm2)
S = specific gravity of wool
A = the fleece-bearing skin area (mm2)
By far the greatest determinant of CFW, within a strain or flock of Merino sheep
producing wool of a similar type, is wool per unit area (i.e. the product of CSA, L and
N) (Williams 1987). We need to examine how these 3 components relate to each other,
and how we might manipulate the relationships to increase output without increasing
fibre diameter too greatly. However, first it is necessary to know to what extent the skin
and follicle characters are determined genetically (i.e. what are the heritabilities of the
characters). These are indicated in Table 1.
Table 1 Heritability and standard error estimates for skin and follicle
characters measured at ten months of age in the Turretfield sheep.
Character
Skin weight
Follicle density
Mean bulb area
Total bulb area
Bulb area standard deviation
Bulb area coefficient of variation (%)
Paracortex %
Mean fibre area
Fibre area standard deviation
Heritability
0.17
0.18
0.25
0.26
0.22
0.09
0.33
0.45
0.25
standard error
0.06
0.06
0.07
0.07
0.06
0.05
0.07
0.08
0.07
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170 Wool follicle and skin characters-their potential to improve wool
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l? I. Hynd et al. 171
The heritabilities of skin weight, follicle density, and bulb areacoefficient of variation
were low (<0.20), while mean bulb area, total bulb area, bulb area standard deviation,
fibre area standard deviation and paracortex percentage were low to moderate (0.20 to
0.35). Only mean fibre area was highly heritable (0.45).
Relationships Between Density, Diameter, Length And
Clean Fleece Weight
Table 2 shows the genetic relationships between follicle density, fibre diameter, staple
length and clean fleece weight in our experimental flock at the Turretfield Research
Centre.
Table 2 Genetic correlations between follicle density and objectively
measured fleece characters at 1 OA and 1 6B months of age
- -- - - --
Character Genetic correlation
Yield 0.37A
0.24B
Clean fleece weight 0.54
0.21
Fibre diameter -0.37
-0.43
CV of fibre diameter -0.09
0.06
Staple length 0.12
0.00
I Staple strength 0.07
A Upper value represents correlation with fleece character measured
at 10 months of age
B Lower value represents correlation with fleece character measured
at 16 months of age
Selection of sheep on the basis of increased follicle density would be expected to
result in an increase in the clean scoured yield, increased clean fleece weight and reduced
fibre diameter. Clearly these are beneficial outcomes but how might one select high
density sheep?
Table 3 shows the relationships between follicle density and some subjectively
assessed characters 10 and 16 months of age. High density sheep can apparently be
identified by selecting sheep with good skin quality (as assessed by the professional
sheep classer involved in our project), good handle, and very well-defined crimp (a low
score for crimp definition represents better crimp definition). Similarly selection for
high follicle density will probably result in an improvement in all of these style
characteristics.
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Hynd et al. 171
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1 72 Wool Follicle And Skin Characters
Table 3 Genetic correlations between follicle density measured at 10
months of age, and subjectively assessed characters* at 10A and 16B months
of age.
Characters Correlation
Lock 0.06*
0.13B
Skin quality 0.34
0.37
Visual colour 0.07
0.38
Handle 0.16
0.20
Condition 0.05
0.20
Crimp definition -0.62
-0.36
A Upper value represents correlation with fleece character measured
at 10 months of age
B Lower value represents correlation with fleece character measured
at 16 months of age
* See Appendix table for the scoring system used by the classer
Is Skin Weight (Thickness) A Useful Predictor Of Fleece
Characteristics?
Given Gregory's (1982) finding that skin thickness was fairly strongly correlated
with clean fleece weight, we expected a similarly strong relationship between skin weight
(an indicator of thickness) and clean fleece weight in our experiment. This was not the
case, and in fact there was a slight negative correlation between skin weight and clean
fleece weight (Table 4).
Sheep with heavy skins tend to be genetically lower yielding, lower in clean fleece
weight, higher in fibre diameter and higher in staple length and strength. Of the classer
assessed characteristics, only crimp definition was moderately related to skin weight
(sheep with heavier skins have poorer crimp definition i.e. a higher score). In summary
these results suggest that sheep with lighter (= thinner?) skins, tend to be superior in
terms of objectively measured fleece characters, and tend to have greater crimp definition.
Wool Technology and Sheep Breeding, Vol44 1996
172 Wool follicle and skin characters-their potential to improve wool
Wool Technology and Sheep Breeding, Vol. 44, 1996
P I. Hynd et al. 1 73
Table 4 Genetic correlations between skin biopsy weight at 10 months of
age, and objectively and subjectively assessed* characters at 10 and 16
months of age.
Character Correlation
Yield -0.29*
Clean fleece weight
Fibre diameter
CV of fibre diameter
Staple length
Staple strength
Lock
Skin quality
Visual colour
Handle
Condition
Crimp definition
A Upper value represents correlation with fleece character measured
at 10 months of age
B Lower value represents correlation with fleece character measured
at 16 months of age * See Appendix table for the scoring system used by the classer
Is The Fibre Cell n p e A Useful Predictor Of Fleece mpe?
There is evidence indicating that genetically high producing animals tend to have
distinct follicle groups comprising primary follicles in straight lines on the margins of
the groups (Williams 1987). The follicles are uniformly seated and are relatively straight
and deep in the skin (Nay 1966; Nay and Johnson 1967; Nay and Hayman 1969; Nay
1970). High crimp frequency arises from highly-curved follicles, and these crimped
fibres contain a high proportion of paracortical cells (Fraser and Rogers 1955). These
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Hynd et al. 173
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174 Wool Follicle And Skin Characters
paracortical cells contain a higher concentration of sulphur, a feature of low-producing
sheep (Williams 1987). It was thought that for these reasons estimation of the paracortex
percentage could provide a useful indicator of not only the genetic fleece growing
potential, but also of some of the "style" characteristics. We took the opportunity to
examine the genetic relationships between paracortex percentage in the fibres of our
experimental sheep at the Turretfield Research Centre, and a range of objective and
subjectively assessed fleece characters (Table 5).
Table 5 Genetic relationships between the paracortex percentage in fibres
measured at 10 months of age, and objectively measured, and subjectively
assessed* fleece characters measured at 10 and 16 months of age.
Character Correlation
Yield
Clean fleece weight
Fibre diameter
CV of fibre diameter
Staple length -0.17
-0.12
Staple strength
Crimp frequency
Lock
Skin quality
Visual colour
Handle
Crimp definition
A Upper value represents correlation with fleece character
measured at 10 months of age
B Lower value represents correlation with fleece character
measured at 16 months of age
* see Appendix table for the scoring system used by the classer
- -
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174 Wool follicle and skin characters-their potential to improve wool
Wool Technology and Sheep Breeding, Vol. 44, 1996
J? I. Hynd et al. 1 75
Sheep with a high paracortex percentage tended to have lower yield, lower clean
fleece weights (at 10 months only), higher fibre diameters, lower staple lengths, lower
staple strengths, and higher crimp frequencies. These sheep also had lower lock (i.e.
more tippy, hairy staples), lower skin quality score (i.e. tighter, less pliable skin), and
poorer crimp definition (i.e. higher score). We also found that there were high genetic
correlations between paracortex percentage and follicle density (-0.40) and between
paracortex percentage and total bulb area (-0.31).
These results are in accord with the findings of Jackson et al. (1975), that sheep
with less curved follicles and fibres (i.e. less paracortex) tend to be the genetically high
producing animals. The heritability of paracortex percentage was moderate (0.33 ii
0.07), and it's variance high, suggesting that considerable progress should be achievable
in decreasing the paracortex percentage and improving the fleece characters indicated
in Table 5.
What Skin And Follicle Characteristics Should We Be Aiming
For?
Summarising the above results it appears that the following genetic associations
exist between fleece characters and skin and follicle traits:
High clean fleece weight
Low fibre diameter
High staple length
low skin biopsy weight
high follicle density
high total bulb tissue density
low paracortex percentage
low skin weight
high follicle density
low mean bulb area
low variability of bulb area
high skin weight
high mean bulb area
high total bulb tissue density
low paracortex percentage
High staple strength high skin weight
low paracortex percentage
Can Skin Analysis Improve The Accuracy Of Selection Of
Superior Animals?
As indicated in the Introduction, the most accurate means of selecting superior animals
is usually to measure directly the traits in the breeding objective. While a number of
indirect traits (skin and follicle characters) can be shown to be genetically associated
with the important traits of fleece weight and fibre diameter as well as anumber of style
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Wool Technology and Sheep Breeding, Vol. 44, 1996
1 76 Wool Follicle And Skin Characters
characters, unless they provide substantial additional information which is properly
used, selection procedures placing excessive emphasis on them could only serve to
reduce the rate of genetic gain!
Of all the characters measured in this experiment, follicle density is the most promising
in that it is genetically associated with high fleece weight, high clean scoured yield, and
low fibre diameter. Such animals could be selected for indirectly by selecting sheep
with high skin quality, or, more accurately by selecting sheep with good crimp definition
(Table 3). However, either means would slow down the progress in relation to what
could be achieved by directly selecting for high clean fleece weight and low fibre diameter!
It remains to be seen whether or not the additional cost of skin measurement is
beneficial. At present the answer appears to be negative, although skin analysis is
making an important contribution towards our understanding of the combination of
characteristics likely to be desirable in sheep breeding programmes.
Acknowledgments
The authors wish to thank all those who have been involved in the collection and
processing of skin samples in the Turretfield base flock experiment, including Bronwyn
K. Everett, Selena M. Doran, Jenny Bennett, Natasha Penno, Vanessa Brownrigg,
Samantha van Barneveld, Clare Nicholls and Fiona Withers. Dr D. R. Gifford, Mr P.
M. C. Ancell and Mr J. R. W. Walkley left the South Australian Research and
Development Institute in 1994. They made an enormous contribution to the planning,
initiation and conduct of the project until then. This work was funded by the Wool
Research and Development Corporation (now the International Wool Secretariat).
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Appendix
I Scoring System Used For Subjective Assessment Of Characters.
Lock 1 (tippy, hairy), , 5 (square, blocky)
Skin quality 1 (very tight), , 5 (best, pliable)
Visual colour 1 (yellow), , 5 (lustrous, white)
Handle 1 (harsh, brittle, , 5 (very soft)
Condition 1 (very dry), , 5 (greasy)
Crimp definition 1 (very well defined), , 5 (crimp hardly visible)
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Hynd et al. 177
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