… · Web viewFleece weight and fibre diameter are the two most important wool production traits affecting income. However, secondary wool quality traits such as staple length,

12. Genetics of Staple Strength, Style and Skin-Based Selection

Johan Greeff

Learning ObjectivesOn completion of this topic you should have: an understanding of inheritance of staple strength in both young and mature Merino sheep

in different environments an understanding of potential consequences for staple strength of breeding programs

emphasising a reduction in fibre diameter an understanding of potential for the coefficient of variation of fibre diameter as cost

effective and indirect selection criterion for improving staple strength an understanding of inheritance of measured and visually assessed style traits an understanding of potential consequences for style traits of breeding programs

emphasising improvement of fleece weight and fibre diameter. an understanding of skin-based traits and how they correlate with economically important

production traits an understanding of the potential value of incorporating skin-based traits into a breeding

program an understanding of soft rolling skin type and its potential value as a selection criterion an awareness of the South Australian Merino Selection Demonstration Flocks

Key terms and conceptsStaple strength, coefficient of variation of fibre diameter, variation between fibres within a staple, variation of fibre diameter along the staple, style.

Introduction to the topicFleece weight and fibre diameter are the two most important wool production traits affecting income. However, secondary wool quality traits such as staple length, staple strength and style also contribute to the commercial value of wool, especially for producers of fine and superfine wool types. For example, staple strength and style each account for around 11-12% of clean price variation for Merino fleece wool of 19m diameter and finer. When diameter is finer than 17.5m in diameter, up to 22% of price variation can be accounted for by differences in style. It is therefore understandable that ‘wool quality’ traits apart from fibre diameter and fleece weight are also considered in Merino breeding objectives, with greater priority being given to them in fine and superfine contexts (for style) and in environments where low staple strength is possible. This lecture focuses on the potential to make genetic improvements in staple strength and style. Different methods of selection that are used to achieve genetic potential, such as objective measurement, traditional classing and skin-based selection, will be discussed in the last section on breeding methods.

12 - 1 – WOOL412/512 Sheep Production©2009 The Australian Wool Education Trust licensee for educational activities University of New England

12.1 The genetics of staple strengthGenetic variation in staple strengthThere are three potential sources of genetic variation in staple strength that can be exploited by stud and commercial ram breeders: variation between strains, between bloodlines (flocks) within a strain and within-flock variation. The first two sources of variation are comparatively small relative to those observed for traits such as fleece weight, fibre diameter and body weight. Furthermore, strain and bloodline differences in staple strength are dependent on environment, such that differences may only become apparent when considered within a specific environmental context (Brown et al. 1999).

Within-flock selection offers the only practical means whereby genetic improvement in staple strength could be achieved, assuming that this trait is heritable. As shown in Figure 12.1, there are significant differences between sire progeny groups when managed as a single group and compared in the same environment.

Figure 12.1 Average staple strength in 9 Merino sire progeny groups managed as a single group and compared under the same environmental conditions. Source: Denney (1990).

Inheritance of staple strengthA prerequisite for genetic improvement is that the trait under selection must express variation and that a proportion of that variation must be heritable. A number of studies have shown that staple strength is a variable trait, with a heritability varying from 0.23 to 0.51 (Table 12.1). That is, from moderately to highly heritable in Merino sheep. These differences could be due to different environments, strains, shearing times, sex, age at shearing, length of wool growth and flocks with different selection histories. However, it is clear that staple strength is a heritable trait in all flocks. Weighting these estimates according to their standard errors, provides an average heritability estimate of 0.30. Hence, staple strength will respond to selection. For example, assuming a heritability of 0.3, 8 N/ktex for the phenotypic standard deviation and optimal selection intensities, annual genetic gains of around 1.2 N/ktex per year could be achieved under single trait direct selection for increasing staple strength. Inclusion of other traits in the breeding objective (e.g. fleece weight and fibre diameter) and use of additional selection criteria will impact on the extent to which these gains can be achieved.


Table 12.1 Phenotypic standard deviation (p, N/ktex) and heritability estimates of staple strength in different research flocks maintained at Katanning (WA), Turretfield (SA) and Armidale (NSW). All flocks are spring-shorn except the Turretfield hogget rams, which are autumn-shorn, at 10 months of age. Source: Compiled from Gifford et al. (1995); Greeff et al. (1995); Howe et al. (1991); Lewer and Li (1994); Lewer and Ritchie (1993).Research flock p Heritability

Great Southern Agricultural Research Institute, Katanning WA

Hogget rams 6.40 0.51

Commercial environment 7.32 0.31

Stud environment 8.43 0.40

Mature ewes 10.90 0.25

Turretfield Research Centre, SA

Hogget rams - 10 months age, 6 months wool 10.20 0.25

Hogget rams - 16 months age, 6 months wool 10.40 0.47

CSIRO fine wool flock, Armidale NSW

Fine and superfine hoggets – 10 months wool 9.21 0.35

Staple strength resource flocksTo demonstrate to industry that staple strength was a heritable trait, The Department of Agriculture of WA screened more than 20,000 animals in the early 1990’s and established a Staple Strength Resource Flock at Katanning, WA (Greeff et al. 1997). Sheep were selected on the basis of phenotypic records of staple strength and assigned to either high (sound), average (control) or low (tender) staple strength flocks. Additional constraints placed on selected animals included a below average fibre diameter and within 1 standard deviation from the mean for clean fleece weight, body weight and staple length. Selected animals were eventually managed as a single group, except at mating, at the Great Southern Agricultural Research Institute at Katanning, WA, in 1995. The aim was not only to demonstrate unequivocally that staple strength was a heritable trait, but also to establish a resource for investigating biological and genetic factors controlling staple strength. These animals were selected to differ only in staple strength. Table 12.2 shows that the selected ewes stayed in their different staple strength categories when subsequently run under the same environmental conditions. Interestingly, significant differences were found between the lines for CVFD. This confirmed a strong negative phenotypic relationship between staple strength and CVFD that had been found in other experimental flocks.

Table 12.2 Least squares means of wool traits of mature Merino ewes and of their hogget progeny in the sound, tender and control flocks of the Staple Strength Resource Flocks. The significance (P-value) of the differences between the three flocks is also given. Source: Greeff (1997).

Staple Strength Resource Flocks P-value

Tender Control Sound

Performance of mature ewes

Clean fleece weight (kg) 3.79 3.74 3.52 n.s.

Staple length (mm) 102 104 100 n.s.

Staple strength (N/Ktex) 22.8 30.6 34.4 P<0.01

Fibre diameter (m) 21.2 21.1 21.3 n.s.

Standard deviation of FD (m) 4.88 4.21 4.02 P<0.01

Coefficient of variation of FD (%) 23.1 20.0 19.1 P<0.01


Table 12.2 continuedPerformance of hogget progeny

Clean fleece weight (kg) 3.36 3.29 3.17 n.s.

Staple length (mm) 105 106 104 n.s.

Staple strength (N/Ktex) 21.1 27.8 33.5 P<0.01

Fibre diameter (m) 18.9 18.9 18.8 n.s.

Standard deviation of FD (m) 4.63 4.24 3.97 P<0.01

Coefficient of variation of FD (%) 24.6 22.5 21.2 P<0.01

Progeny of selected animals (sound x sound, control x control, tender x tender matings) performed the same as their parents (Table 12.2). This confirmed that staple strength was a heritable trait, the phenotypic relationship between staple strength and CVFD was strongly negative and the genetic relationship between staple strength and CVFD was also strongly negative. Thus parents with high CVFD wools produced progeny with relatively low staple strength wool, whereas parents with low CVFD wools produced progeny with relatively high staple strength wool.

This research provides evidence that staple strength is a heritable trait and that it can be improved by selection. Also, genes controlling staple strength are related to genes controlling fibre diameter variation. More detailed information on these selection lines is given by Greeff et al. (1997).

Relationships between staple strength and other wool production and quality traitsStaple strength is not the only economically important trait in Merino breeding programs. Before staple strength is included into a breeding objective, it must be known how this trait relates to other traits of economic importance, to ensure that undesirable responses in these other traits do not occur.

Genetic and phenotypic correlations between staple strength and wool production traits are demonstrated in Table 12.3. The phenotypic correlation between staple strength and clean fleece weight and between staple strength and fibre diameter is moderately low but consistently positive across all experimental flocks. This is good news from the point of view of fleece weight, but clearly undesirable in terms of diameter, implying that a genetic reduction in fibre diameter may cause a reduction in staple strength. Relationships involving the two measures of diameter variation, these being standard deviation of fibre diameter and coefficient of variation of fibre diameter (CVFD), are moderately and consistently negative, the stronger relationship being with CVFD. Genetic correlations are high in all flocks except those at the Turretfield Research Centre. Turretfield flocks were shorn at 6 monthly intervals in spring and autumn and produced sound wool (>40N/ktex) whereas the Katanning flocks were shorn in spring with 12 months wool growth and produced relatively tender wool (<32N/ktex). Fine wool flocks were shorn in spring at 10 months of age with an average staple strength of 37N/ktex. These results imply that CVFD or SDFD could be used very effectively as an indirect selection criterion to improve staple strength in a range of environments.


Table 12.3 Phenotypic and genetic correlations between staple strength and clean fleece weight (CFW), fibre diameter (FD), standard deviation of diameter (SDFD) and the coefficient of variation of diameter (CVFD) in different research flocks maintained at Katanning (WA), Turretfield (SA) and Armidale (NSW). Source: Compiled from Gifford et al. (1995); Greeff et al. (1995); Howe et al. (1991); Lewer and Li (1994); Lewer and Ritchie (1993).

CFW FD SDFD CVFDPhenotypic correlationsGreat Southern Agricultural Research Institute, Katanning WA

Hogget rams 0.08 0.27 -0.29 -0.50Commercial environment 0.09 0.18 -0.23 -0.42Stud environment 0.11 0.26 -0.25 -0.44Mature ewes 0.03 0.21 -0.27 -0.45

Turretfield Research Centre, SAHogget rams - 10 months age, 6 months wool 0.10 0.21 -0.04 -0.19Hogget rams - 16 months age, 6 months wool 0.22 0.32 -0.11 -0.36

CSIRO fine wool flock, Armidale NSWFine and superfine hoggets – 10 months wool 0.16 0.17 -0.25 -0.36

Genetic correlationsGreat Southern Agricultural Research Institute, Katanning WA

Hogget rams 0.42 0.37 -0.29 -0.62Commercial environment 0.03 -0.07 -0.74 -0.82Stud environment 0.10 0.45 -0.33 -0.66Mature ewes -0.14 0.22 -0.56 -0.86

Turretfield Research Centre, SAHogget rams – 10 months age, 6 months wool 0.15 -0.15 -0.30 -0.27Hogget rams – 16 months age, 6 months wool 0.37 0.46 -0.13 -0.46

CSIRO fine wool flock, Armidale NSWFine and superfine hoggets – 10 months wool -0.19 0.27 -0.29 -0.58

Referring back to Table 12.2, progeny of the sound line also had a significantly lower CVFD than the progeny of the tender line. This provides further evidence for both the strong negative genetic correlation between staple strength and CVFD and the potential for CVFD to be used as an indirect selection criterion to improve staple strength.

Components of variation of fibre diameter within a stapleCVFD is the sum effect of variation between fibres and variation along the staple. It has therefore been suggested that changing the fibre diameter profile could improve SS and reducing fibre diameter variation along the staple could also reduce fibre diameter blowout (FD blowout) during the growing season. Yamin et al. (1999) estimated the heritability of CVFD along the staple using individual wool snippets on mature Merino ewes. They estimated the heritability of CVFD along the staple to be between 1 and 20% and found a low to moderate phenotypic correlation of 0.15 to –0.43 with SS. As the OFDA2000 is capable of measuring variation between fibres and fibre variation along the staple, it is possible to measure the extent that between and along fibre diameter variation from the OFDA2000 contribute towards selection of sounder wool.

Greeff (2001) showed that CVFD between fibres and CVFD along the staple are both heritable (0.4 and 0.20, respectively) but not as heritable as CVFD on a minicored sample (0.67). CVFD between fibres is genetically strongly (-0.7) correlated with SS to nearly the same extent as total CVFD (-0.65). Thus there would be no advantage of using CVFD along the staple or CVFD between fibres to improve staple strength. Faster genetic progress would be achieved by using total CVFD tested on either a minicore midside sample or on an intact wool staple because of its higher heritability and stronger genetic relationship with SS. In addition, this will result in a small positive correlated response in FD blowout mainly because CVFD along the staple is genetically the same trait as FD blowout.


Predicted responses in staple strength when selecting for improved fleece weight and fibre diameterIdeally, ram breeders and commercial woolgrowers should give high priority to fleece weight and fibre diameter in their breeding objectives given the importance of these two traits in terms of wool income. But what would happen to staple strength over time if fleece weight and fibre diameter were the only characters in the breeding objective? Table 12.4 shows the predicted responses in clean fleece weight, staple strength and CVFD after 10 years selection for increase fleece weight and decreased fibre diameter, with varying emphasis on diameter reduction and ignoring staple strength in the breeding objective. Results indicate that genetic gains in staple strength will be reduced as more selection pressure is placed on fibre diameter reduction, while increasing fleece weight. Over the same time CVFD will increase slightly.

Steps should be taken to prevent possible deterioration in staple strength when diameter reduction is of high priority in the breeding objective. This can be done relatively cheaply and simply by using CVFD as an indirect selection criterion to improve staple strength. As shown in the last column of Table 12.4, inclusion of CVFD in the selection index can prevent staple strength from deteriorating with only small penalties in genetic gains for fleece weight and fibre diameter. Lewer and Li (1994) have shown that using CVFD to improve staple strength can be between 50 and 80% as effective as using staple strength itself. Other advantages of CVFD are:

a heritability of 0.33 to 0.74 across the range of studies reported in Table 12.3

a flock testing cost of around $1.20 per animal compared to more than $9 per animal for ATLAS measurements of staple strength

a repeatability of 0.62 between measurements taken at 10 and 16 months, compared to 0.23 for staple strength.

Table 12.4 Predicted responses in clean fleece weight (CFW, grams), staple strength (SS, N/ktex) and coefficient of variation of fibre diameter (CVFD, %) to breeding for improved clean fleece weight while decreasing fibre diameter by different amounts over a 10 year period. Source: Greeff (1997).Breeding Objective Changes in 10 yrs for:

Maximise CFW and maintaining body weight while reducing FD by:

CFW (grams)

SS (N/Ktex)

CVFD (%)

0.0 micron +478 +1.8 +0.45

1.0 micron +354 +0.4 +0.46

2.0 micron +62 -1.7 +0.32

Maximise CFW, maintaining body weight and increase SS by 2N/Ktex using CVFD as an indirect selection criterion while reducing FD by:

0.0 micron +476 +2 +0.33

1.0 micron +323 +2 -0.06

1.8 micron +16 +1.9 -1.16

Spring versus autumn shearingTime of shearing can influence staple strength by determining the position of break. For example, shearing in spring in a Mediterranean environment will position the point of minimum fibre diameter towards the middle of the staple, whereas an autumn shearing places this point towards the end of the staple. Thus a spring shearing allows the weakest point to contribute towards the staple strength measurement whereas an autumn shearing potentially removes its effect from the measurement, by placing the weakest part of the staple in the clamps of the staple breaker. The determinants of staple strength variation in spring-shorn wool are likely to differ from those associated with autumn-shorn wool.


So to what extent is staple strength in spring-shorn wool controlled by the same genes controlling strength in autumn-shorn wool? In other words, if a stud undertakes a spring shearing and aims to genetically improve staple strength, are there any genetic gains to be made in the commercial clients’ flocks if they undertake an autumn shearing?

Ponzoni et al. (1995) found a genetic correlation of only 0.68 between staple strength at 10 and 16 months of age for sound wool (>42N/ktex), with the two staple strength measurements corresponding to a spring and an autumn shearing respectively. While this result indicates that the two measurements may not be entirely controlled by the same genes, it is not possible to partition this between age and shearing time effects. An alternative experimental design was considered at Katanning, WA, whereby half of the male progeny of a group of 121 Merino sires were randomly allocated to be shorn in either spring or autumn, while a third of the female progeny were shorn in the spring and the remainder in autumn. Using full pedigree information, an estimate close to 1 was obtained for the genetic correlation between measurements of staple strength taken under both shearing scenarios. This indicates that traits measured in different seasons were genetically the same. However, it also showed that the genetic correlation between staple strength and CVFD was –0.65 in spring-shorn wool but only –0.30 in autumn-shorn wool, suggesting that CVFD may be less useful as an indirect selection criterion if an autumn shearing is undertaken. Greeff et al. 92004) investigated this issue further on six month wool shorn in spring or in early autumn and found that the genetic correlation between staple strength in these wools were 0.87, while it was 0.98 for CVFD. This supports previous studies that the same genes control staple strength in autumn or spring shorn wool and likewise for CVFD. Furthermore, the genetic correlation between CVFD measured at any point along the staple varied between –0.37 and –0.67 that supports the average genetic correlation of –0.50 between CVFD and staple strength.

Prediction studies have shown that using CVFD is about 65% as effective as using staple strength measurements in breeding programs. This has been supported in the Western Australian Staple Strength Resource flocks which showed that animals with high staple strength have low CVFD and vice versa. The results also showed that the progeny of high and low selection lines, performed in a similar way as their parents.

12.2 The genetics of styleGenetic variation in styleStyle of wool is generally defined as the visual and tactile appearance of wool. The industry classifies wool into five grades, Spinners, Best, Good, Average and Inferior topmaking quality. The ram breeding industry does not have a standardised system into which wool is classed. They generally classify sheep into three grades, i.e Tops, Flock and Culls using a subjective assessment of the handle (softness), dust penetration, staple thickness, definition of crimp, lustre, greasy colour (whiteness) and amount of wool wax (nourishment). There are only limited amounts of variation (1-6%) between bloodlines within a strain for most of the component traits of style, relative to that for traits such as fleece weight, fibre diameter and body weight. Between-bloodline variation in greasy colour is larger (16%) and comparable to other production traits of economic importance. This suggests that genetic gains in greasy colour could be achieved in part by exploiting genetic differences between bloodlines, but limited scope exists for other style traits.

12.2.1 Fine wool merinosThe CSIRO Fine Wool Project (CSIRO FWP) was a resource flock run by CSIRO at Armidale, NSW, comprising sub-flocks representing 11 industry bloodlines (studs), including 9 fine wool and 2 medium wool studs. Style data was collected from 5100 hoggets, representing 260 sires, based on mid-side samples taken when animals were 10 months of age. Both subjective assessment and objective measurement of the components of style were recorded. Additional data recorded as part of this project included all traits of economic importance to commercial wool growers as well as skin traits and susceptibility to skin conditions such as fleece rot and mycotic dermatitis. One of the goals of this project was to investigate genetic mechanisms operating in relation to both assessed and measured wool style components. More detailed information on the CSIRO FWP is given by Swan et al. (1997).


Table 12.5 Definition of subjectively assessed style traits in the CSIRO Fine Wool Project. Source: Lax et al. (1995).Trait Score of 1 indicates: Score of 5 indicates:

Handle Very soft Very harsh

Crimp definition Well defined Little / no visible crimp

Greasy wool colour White and bright Dull and yellow

Staple thickness Wire thin Very thick

Dust penetration Restricted to tip Penetrates to skin level

Subjective assessments of handle, crimp definition, colour, dust penetration and staple thickness in the CSIRO FWP were made using a 5 point scoring system as summarised in Table 12.5.

The objective measurement of the components of style involves image analysis instrumentation developed by CSIRO Division of Wool Technology. The objectively measured style traits considered in the CSIRO FWP are defined in Table 12.6.

Table 12.6 Definition of objectively measured style traits considered in CSIRO Fine Wool Project. Source: Lax et al. (1995).Trait Definition Decreasing measures / scores imply:

O-crimpf Objectively measured crimp frequency

decreasing crimp frequency

O-crimpr Objectively measured crimp regularity

improved crimp regularity (desirable)

O-dust Objectively measured dust penetration

reduced dust penetration

O-colour Objectively measured greasy wool colour

whiter colour

Table 12.7 Phenotypic variances (Vp) and heritabilities (in bold on diagonal) for measured and assessed style traits, as well as phenotypic correlations (above diagonal) and genetic correlations (below diagonal) between objectively measured and subjective assessed traits in the CSIRO Fine Wool Project. Source: Swan et al. (1997).


Objectively measured (O) Subjectively assessed (A)

Crimpf Crimpr Dust Colour Handle Crimpd Dust Colour

Vp 1.01 0.13 18.63 3.72 0.47 0.12 0.40 0.37

O-crimpf 0.27 -0.16 -0.10 0.08 0.01 -0.05 -0.17 -0.05

O-crimpr -0.30 0.43 0.01 0.00 0.22 0.08 0.29 0.03

O-dust 0.01 0.03 0.10 -0.17 0.02 0.03 -0.05 -0.09

O-colour 0.00 0.03 -0.18 0.40 0.25 0.11 0.08 0.50

Handle 0.18 0.51 0.34 0.55 0.22 0.16 0.15 0.25

A-crimpd -0.22 0.03 0.04 0.36 0.45 0.09 0.16 0.08

A-dust -0.55 0.49 -0.24 0.19 0.26 0.42 0.25 0.17

A-colour -0.11 -0.01 -0.23 0.90 0.44 0.28 0.30 0.29

Table 12.7 summarises heritabilities and genetic correlations obtained for objectively measured and subjectively assessed style traits. Objectively measured style traits were moderately heritable, apart from dust penetration with an estimate of 0.10. Estimates for the other objective traits ranged from 0.27 to 0.43. Estimates of genetic correlations between objectively measured traits were not significantly different from zero, with the exceptions of crimp frequency and crimp regularity (-0.30), dust and colour (-0.18). Of the subjectively assessed style traits, crimp definition was lowly heritable (0.09) while the other subjectively assessed style traits were moderately heritable, with estimates ranging from 0.22 to 0.29. Genetic correlations were moderately positive, ranging from 0.26 to 0.45.

Subjectively assessed traits were not strongly correlated with their equivalent objective measurements with the exception of colour (0.90 between O-colour and A-colour). O-crimpr was not genetically correlated with A-crimpd (0.03), and there was a negative genetic correlation between O-dust and A-dust (-0.24). This indicates that there is a poor agreement between the style machine's view of style and that of a sheep classer. The style machine has been shelved but more work is necessary to develop and to get a standardised style system adopted in Australia.

12.2.2 Medium and strong wool merinosMedium and strong wool Merino resource flocks maintained by the Department of Agriculture of WA at Katanning, the South Australian Research and Development Institute at Turretfield and NSW Agriculture at Trangie, have also been used to collect genetic information on subjectively assessed style and in some instances, objective measurements of style components. It should be noted that some of these research groups have used scoring systems in the reverse order for the subjectively assessed traits as used in the CSIRO FWP. It is important to remember this when comparing results across studies.

In general these studies show results similar to those from the CSIRO Fine Wool Project. That is, style traits are moderately heritable and can be improved by selection. However, some estimates differ from those reported for the fine wool bloodlines, which may reflect differences in the environment. For example, the Trangie results showed a moderate heritability for dust penetration (0.42) but low heritabilities for greasy wool colour (0.05) and crimp regularity (0.09). Under assumptions of optimal flock structure and selection intensities, single trait selection is predicted to achieve annual rates of response in measured style traits comparable to those responses predicted for clean fleece weight and fibre diameter (1.5-2% per year).

Relationships between style traits, fleece weight and fibre diameterEstimates of the correlation between style traits and clean fleece weight and average fibre diameter in the CSIRO FWP are shown in Table 12.8. At the genetic level, higher clean fleece weight was associated to a moderate degree with reduced crimp frequency (-0.21), better objective crimp regularity (-0.27), whiter colour (-0.22), and softer handle (-0.38). There was little or no association with dust penetration (0.09) or assessed crimp definition (-0.03). Reduced fibre diameter was moderately associated with better crimp definition (0.21 with measured crimp regularity and 0.19 with assessed crimp definition), and increased dust penetration (0.34), and more strongly associated with softer handle (0.58).

Table 12.8 Estimates of phenotypic and genetic correlations between measured and assessed style traits, clean fleece weight (CFW) and average fibre diameter (MFD) in the CSIRO Fine Wool Project. Source: Swan et al. (1997).

O-crimpf O-crimpr O-dust O-colour Handle A-crimpd

Phenotypic correlations

CFW -0.03 -0.18 -0.03 -0.17 -0.13 -0.10

MFD -0.02 0.16 0.04 0.03 0.32 0.04

Genetic correlations

CFW -0.21 -0.27 0.09 -0.22 -0.38 -0.03

MFD -0.05 0.21 0.34 0.08 0.58 0.19


In the Trangie study, favourable genetic correlations were evident between fleece weight and the measured style traits. For fibre diameter, the correlations were also favourable except for crimp regularity and greasy wool colour, with finer diameter being correlated with a moderate deterioration in crimp regularity and a small increase in yellowness. Based on both studies, however, it can be concluded that the associations between the style traits and clean fleece weight and average fibre diameter are generally favourable.

Relationship between dust penetration and dust contentBreeders generally use dust penetration as a measure of dust content when selecting indirectly for higher yielding wool and better style wool. Ladyman et al. (2003) measured dust penetration and dust content on 1053 ewe and wether hoggets from the Katanning Merino resource flocks to es-timate the genetic parameters of dust and wool production traits. Dust penetration and dust con-tent (% dust per kg clean wool) had a moderate genetic correlation (0.54), but a low phenotypic correlation (0.32) with each other. Dust content was moderately heritable (0.36), while the heritab-ility of dust penetration was low (0.21) which confirms previous studies. It was concluded that dust penetration and dust content are not genetically the same trait and that dust penetration should not be used as a measure of dust content. As there was a positive genetic correlation of 0.4 and a phenotypic correlation of 0.4 between wax content (% wax/kg clean wool) and dust content, and a phenotypic correlation of -0.2 and a genetic correlation of -0.5 between suint (% suint/kg clean wool) and dust content, selecting for an increase in clean yield would be a better option to reduce dust content. Thus the perception that selecting for white wool with a black tip, which implies re -duced dust penetration will decrease dust content, is not totally true. Breeders would make faster genetic gains in reducing dust content by selecting animals for higher clean yield rather than for lower dust penetration. However, style grades might get worse as the wools would look dirtier but have better yields. Thus this issue needs further investigation.

Including style in the breeding objectiveIn the traditional world of Merino breeding, selection is for a multi-trait objective that is often not formally specified. In fact, many breeders have only a vague idea of their goals. When this is the case, the foggy objectives result in very slow progress in any of the components.

By knowing the phenotypic and genetic relationships between style and other measures of interest to breeders, it is possible to calculate the changes in the components of style that should accompany selection on these other traits. For example, presented in Table 12.9 are the predicted changes in subjective and objective components of style that will result from selection on fleece weight and mean fibre diameter, based on genetic parameters derived from the CSIRO FWP. Three objectives are shown, reflecting differing degrees of selection emphasis on fleece weight and fibre diameter. Further details are given by Swan et al. (1997). The results show that under each of the three scenarios, selection on fleece weight and mean fibre diameter will result in favourable changes in style traits.

Taylor et al. (1997) also demonstrated that selection for reduced average fibre diameter and increased clean fleece weight should result in genetic improvement in most measured style traits in medium and broad wool types.

Purvis and Swan (1997) included assessed style grades of fine wool into a formal breeding objective, so as to direct changes in these traits towards a particular target. But formal incorporation of such measures into a breeding objective must await the calculation of relative economic values (REVs) for style and its components. Relative economic values provide a means for comparing the value of a single unit of change in each trait in the breeding objective. This is a relatively straightforward procedure for traits such as fleece weight and fibre diameter. In relation to style, though, consideration needs to be given to the REVs for each of the component traits, and the different classes of Merino enterprises. This is not an easy task, particularly as there are no direct market signals for component traits, only for style grade.


Table12.9 Predicted genetic changes in clean fleece weight (CFW), fibre diameter (MFD), and measured and assessed style traits after 10 years selection on the CFW and MFD only, for micron premiums of 0.3%, 5% and 10%. Genetic changes are expressed as the percentage change relative to initial trait means. Source: Swan et al. (1997).Micron premium

CFW MFD O-crimpf O-crimpr O-dust O-colour Handle A-crimpd

0.3% 18.9 0.0 -4.0 -16.6 0.5 -23.1 -12.6 -0.7

5% 7.5 -15.9 -1.0 -25.1 -5.5 -24.1 -28.2 -3.2

10% 2.9 -17.7 0.0 -23.2 -6.3 -20.1 -27.8 -3.4

Trait mean 1.63 16.88 6.15 0.99 24.50 2.86 2.77 2.14

Style is important because more stylish wools attract higher premiums especially at the finer end of the clip. Fortunately the objectively measured fleece traits such as fleece weight, fibre diameter, staple strength and coefficient of variation have a favourable genetic relationship with style. Thus selecting on these objectively measured traits will also result in an improvement in style. These traits are also more accurately measured and therefore will result in faster genetic gains than using the subjectively assessed style traits.

12.3 Methods of selectionThroughout the history of Merino breeding in Australia, considerable interest has been shown in the relationship between skin-based characteristics and productivity of the animal. During the late 1800’s and early 1900’s, ram breeders and wool growers debated the relative merits of skin wrinkle as a means for increasing wool productivity, culminating in the importation of and subsequent opposition to the highly-developed Vermont Merino. Today, interest in skin-based traits is perhaps even stronger, being fuelled by the more detailed specifications of the fibre product that many woolgrowers are striving to meet. The current interest in skin-based traits (described comprehensively by Maddocks and Jackson 1988) has arisen predominantly from debate about the soft rolling skin approach to sheep selection. This involves a visually-identifiable skin and fleece ‘package’ which brings with it, an individual animal reported to be superior in terms of wool cut per head and all components of wool quality.

Two questions thus arise. Firstly, to what extent can genetic progress in the breeding objective be achieved by use of skin-based traits as within-flock selection criteria? Secondly, is selection based largely on an assessment of the skin and fleece ‘package’ more efficient in achieving the designated breeding objective than is selection based largely on objective fleece measurements relating to wool quantity and quality, optimally combined in a selection index?

The genetics of skin-based traitsHeritabilities and genetic correlations: Skin traitsOverall, skin-based characteristics are under low to moderate genetic control (Table 12.10), indicating that each could be changed over time by selecting appropriate animals. These estimates are derived from different flocks, representing fine through to strong wool Merino genotypes, as well as the measurements being made at different ages. There is evidence, at least for follicle density and S/P ratio, that the level of inheritance varies with Merino strain, suggesting that the response achieved to selection for any one of these traits may differ between strains.


Table 12.10 Heritability estimates for a number of skin-based traits Source: Barton et al. (2001); Jackson et al. (1975); Heydenrych et al (1977); Hill et al. (1997a); Hynd et al. (1996, 1997); Purvis and Swan (1997) and classer traits Source: Brown et al. (2002b).

Skin Trait Heritability

Skin-based traits

Follicle density 0.18 - 0.62

S/P ratio 0.21 - 0.52

Skin thickness 0.60

Follicle depth 0.37

Follicle curvature 0.40

Follicle bulb area 0.25 - 0.26

Variation in follicle bulb area 0.09 - 0.22

Classer type skin traits

Softness/Handle 0.44

Crimp definition 0.47

Lustre 0.29

Nourishment 0.24

Bundle size 0.50

Skin quality 0.33

Genetic correlations between skin and fleece traitsBased on the genetic relationships estimated between skin and fleece traits, predictions can be made as to the direction of change in fleece traits following a genetic change in any one skin-based trait (Table 12.11). Increased clean fleece weight is expected to result from single trait selection for increased follicle density, S/P ratio, follicle depth and skin thickness, or decreased follicle curvature. Reductions in fibre diameter are expected following selection for increased density and S/P ratio, or decreased follicle curvature. Correlated changes in staple length, diameter variation, style and crimp frequency are also expected following single trait selection for some of these skin traits. Note, that there is currently no complete set of estimates pertaining to any one strain or age, and not all published estimates reflect the trends in Table 12.11. There is evidence to suggest that underlying genetic mechanisms may differ markedly between strains and possibly even flocks, indicating that different outcomes could be achieved even when the same single skin trait is being used.

Table 12.11 Predicted direction of change in skin and fleece traits following selection on the basis of single skin-based traits. Only correlations outside the range -0.20 to 0.20 are reported. Source: Gregory (1982); Hill et al. (1997b); Heydenrych et al. (1977); Jackson et al. (1975); Barton et al. (2001); Purvis and Swan, (1997).Selection criterion: Predicted to increase: Predicted to decrease:

Increased follicle density

clean fleece weightyieldimproved crimp definition, visual colour, handle and condition

fibre diameterdiameter variationstaple lengthcrimp frequency

Increased S/P ratio clean fleece weightdiameter variationcrimp frequency

fibre diameterstaple length


Table 12.11 continuedIncreased skin quality(traditional classer assessed)

clean fleece weightyieldstaple lengthimproved condition, visual colour, lock and handle

crimp frequency

Increased skin biopsy weight

fibre diameterdiameter variationstaple strengthcrimp frequencyimproved visual colour

clean fleece weightyieldcrimp definition (deteriorate)

Increased skin thickness

clean fleece weightstaple lengthdiameter variation

crimp frequency

Increased follicle depth clean fleece weightstaple length

crimp frequency

Reduced follicle curvature

clean fleece weightyieldstaple length

fibre diametercrimp frequency

Reduced bulb size variation

staple strengthimproved visual coloursoftnessimproved condition

fibre diameterdiameter variation

Amongst skin traits, the most notable genetic correlations involve follicle density, the strongest of which is with S/P ratio (+0.70). Selection to increase follicle density should therefore also give a correlated increase in S/P ratio as well as decreases in follicle curvature and bulb area variation (genetic correlations of –0.38 and –0.23 respectively).

The relationships identified in Table 12.11 oversimplify the complex interactions between the skin and the fleece. For example, selection for increased clean fleece weight or reduced fibre diameter is predicted to increase follicle density and S/P ratio, and decrease follicle curvature. Such a result was demonstrated following 20 years selection predominantly for increased clean fleece weight with restrictions on fibre diameter and/or crimp frequency. Follicle density increased by 8-13%, S/P ratio by 18-28% and follicle depth by 5-9% while follicle curvature declined by 3-18%, compared to a randomly-selected control line. However, selection experiments have shown that selection either to increase follicle density or S/P ratio, or to reduce follicle curvature, does not increase clean fleece weight as predicted. This is due to compensatory changes in other related skin and fleece weight component traits, such that the overall response in fleece weight is negligible. Tandem selection for increased follicle density combined with increased follicle depth has also been investigated, but only achieved marginal increases in clean fleece weight after approximately 9 years of selection.

An experiment to investigate the correlated response in S/P ratio while selecting for clean fleece weight at hogget age (group 1), and vice versa, selection for increase S/P at 3 months of age and correlated response in clean fleece weight at hogget age (group 2), was carried out by Heydenrych et al 1984. They reported a 12 percent increase in clean fleece weight but only a 6% increase in S/P in group 1. In group 2 they reported a 14% increase in S/P ratio and a correlated response of 8% in clean fleece weight. This differential response for clean fleece weight in groups


1 and 2 can be explained by the expectation that selection for clean fleece weight is more likely to cause an increase in its components, especially those contributing to wool growth per unit area of skin, than would the increase of any of these components cause an increase in clean fleece weight.

It is therefore unrealistic to expect that any single skin trait holds the key by which a breeding objective incorporating more than one trait is achieved.

Skin-based selection criteria for within-flock selectionThe potential value of incorporating skin-based traits into the breeding program ultimately depends on the breeding objective, the efficiency with which that objective can be achieved and the costs associated with delivering the skin measurements or assessments.

A number of theoretical studies have been conducted to evaluate potential benefits, in terms of genetic progress in the breeding objective, when including information on skin-based traits in the selection decision. One study was based on strong wool SA Merino genotypes (Hynd et al. 1997), with a range in breeding objectives including hogget and adult clean fleece weight, fibre diameter and diameter variation, with micron premiums of 5%, 10% and 15%. A comparison was made between selection based on an index of fleece weight, fibre diameter and diameter variation and selection based on indexes involving these three fleece traits and combinations of follicle density, skin biopsy weight and traditionally-assessed skin quality (in terms of ‘pliability’ as assessed by a professional sheep classer). Inclusion of skin-based information in the selection index was shown to give additional gains in the breeding objective (Figure 18-1), although the benefit of including the skin traits was substantially lowered as increasing emphasis was given to reducing diameter in the breeding objective. This reflected the tendency for these skin traits, at least in the SA strong wool strain, to influence fleece weight more than fibre diameter. Of the three skin traits examined, both traditionally-assessed skin quality and skin biopsy weight offered relatively inexpensive alternatives to other skin-based traits like follicle density (approx. $150 per measurement) and S/P ratio (approx. $35 per measurement). Inclusion of the skin traits also had little predicted impact on diameter variation, staple strength and staple length over the 10 year period under all breeding objectives.

However, similar studies using fine and medium wool genetic parameter sets have only shown marginal gains being achieved by including information on follicle density and S/P ratio. More importantly, these marginal gains were outweighed by the cost of skin-based measurements. In practical terms, making use of any of these skin traits as the final deciding factor between sires ranked highly on performance was not shown to be cost-effective.

Figure 12.2 Predicted improvements in genetic gains arising from incorporating skin quality (SQ) and/or follicle density (DE) into an index containing fleece weight and fibre diameter. Improvements are expressed relative to a value of 100 representing the gains

made using an index based on fleece weight and fibre diameter only (BASE). The comparisons were made assuming a 5%, 10% or 15% micron premium in the index. Source:

Hynd et al. (1997).


One aspect highlighted by these studies is that the predicted benefits of using additional information on skin-based traits will depend on the values assumed for the genetic parameters. In the SA strong wool study, for example, additional gains achievable when including the three skin traits varied from 1 to 51% - i.e. from almost zero extra gain to substantial extra gain - when the ‘potential’ range of genetic parameters was considered (Hill et al. 1998). Without accurate estimates of the genetic association between skin and fleece traits, it is difficult to formulate recommendations for the inclusion of skin traits in Merino breeding programs. Also, as the underlying genetic mechanisms of the skin and fleece may vary markedly between Merino strains and even flocks, the potential value of skin-based assessment may also vary markedly.

‘Soft rolling skin’ as a selection criterionIt is reasonable to expect that certain combinations of skin-based characteristics might exist that are favourable in terms of wool production and wool quality, i.e. a favourable skin and fleece ‘package’, based on the genetic associations that do exist between the range of skin and fleece traits. Brown et al (2002a,b) showed that the traits in the ‘soft rolling skins’ classing system, softness, lustre, nourishment, bundle size and skin are all moderately to highly heritable traits. In addition these traits are also favourably correlated with fleece weight and fibre diameter.

Pivotal to the soft rolling skin approach to sheep selection is the use of skin type as a selection criterion. As summarised in Table 18-3, there are three basic skin types proposed, including the soft rolling skin (SRS), that are expected to differ in their skin characteristics. If these combinations were achieved, then based on the correlations summarised in Table 12.11, it could be expected that the SRS type would produce:

higher clean fleece weight lower average fibre diameter lower diameter variation (giving a well-defined crimp) longer, thin staples lower crimp frequency than expected for the diameter (giving a soft handle) lower suint content (giving low incidence of yellow discolouration).

However, it is important to notice that selection using subjectively assessed traits alone may not lead to maximum genetic progress in productivity. Selecting directly for fleece weight, fibre diameter and staple strength will also improve visual appearance of the fleece.

Table 12.12 Proposed combinations of skin characteristics giving rise to the three basic skin types of the soft rolling skin, the heavy tight skin and the flat skin. Source: Watts (1995).

Soft rolling skin Heavy tight skin Flat skin

Follicle density high moderate low

S/P ratio high moderate / low moderate

Follicle depth deep moderate moderate / shallow

Follicle curvature low moderate / high moderate / high

Variation:DepthCurvatureBulb size

lowlowlow

moderate / highmoderate / highmoderate / high

low / moderatelow / moderatelow / moderate

Skin loose and pliable tight and wrinkled plain

Note that information detailing the biological differences between these skin types has been largely anecdotal. However, a number of scientific studies have been recently undertaken to obtain objective data on the follicle attributes of these skin types.


One study focussed on an unclassed mob of 18 month old breeding ewes of the strong wool Merino strain, subsequently classed into the three skin types by an experienced SRS classer. Fifty sheep were chosen from each skin type group for comparison of skin and fleece traits (Table 12.13). There was no significant advantage of the SRS type in follicle density and S/P ratio, but they did show less variation in diameter among secondary follicles. There were, however, production differences between the skin types in favour of the SRS type. Clean fleece weight was higher and fibre diameter lower relative to the other skin types, but without any apparent advantages in other wool quality traits except for higher staple strength.

Similar comparisons of the skin types by other researchers indicate that sheep displaying the SRS skin type do not always have higher follicle densities and higher S/P ratios, nor are they always those of highest wool productivity per head and superior in wool quality traits, such as average fibre diameter, diameter variability and staple length. One plausible reason for these inconsistencies is that the different classers used in these studies differ in their assessment of skin type, given that the assessment incorporates a number of indicator traits. Also, the accuracy of assessment (i.e. repeatability) needs to be considered. As yet no estimates of between- and within- classer correlations have been published. Another possible explanation may be that the underlying biology of the ‘skin type’ concept depends on the strain or type of sheep being used, as is mentioned in the following sections.

Table 12.13 Average performance in a range of skin and fleece traits measured in breeding ewes representing soft rolling skin, heavy tight skin and flat skin types. Within each row, mean values followed by a different superscript indicate a significant difference between the skin types. Source: Brown, D. unpub.

Soft rolling skin Heavy tight skin Flat skin

Follicle density (no per mm2)

86.1 a 84.4 a 74.1 a

S/P ratio 23.7 a 25.6 a 17.9 b

CV of diameter (%):Primary folliclesSecondary

follicles

13.1 a

18.2 a

16.8 b

24.3 b

14.0 a

21.6 b

Clean fleece weight (kg) 3.1 a 2.9 b 2.7 c

Yield (%) 68.2 a 68.2 a 67.2 a

Fibre diameter (m) 18.4 a 19.5 b 19.3 b

CV of fibre diameter (%) 26.4 a 28.6 b 27.5 ab

Staple length (mm) 75.0 a 71.0 b 75.6 a

Staple strength (N/ktex) 20.3 a 18.4 b 18.3 b

One of the important questions arising is the extent to which selection according to skin type compares to the outcome that can be achieved by using direct measurements of the important fleece traits, combined into an index? The outcome of both index selection (based on fleece weight and fibre diameter) and selection on the basis of soft rolling skin when applied to the same flock, has been examined in a flock of strong wool SA Merino ewes (Daily et al. 1997). In this study, outcome was expressed in terms of the selection differential established for a range of fleece traits. Overall, selection on the basis of SRS achieved similar selection differentials for average fibre diameter and diameter variation as did index-based selection, but a somewhat smaller differential for clean fleece weight (Table 12.14). These results highlight the potential value of soft rolling skin as a criterion for within flock selection, but not in a manner that is antagonistic to that achieved by index based selection.

Note, however, that selection solely on the basis of assessed skin type, like any other ‘single trait’ selection strategy, may not effectively accommodate any changes to be made in terms of the relative emphasis given to the traits in the breeding objective, like fleece weight and fibre diameter. Basing selection on measurements and/or assessments of a number of traits will more


readily accommodate changes in the breeding objective by varying the emphasis given to each selection criterion, as shown when comparing the 5% index and 10% index in Table 12.14. The reduction in staple strength on the 5% and 10% micron premium selection index can easily be overcome by including CV of diameter as a selection criterion in the index to counter this effect.

Table 12.14 Selection differentials resulting from selecting the top 3% of ewes according to the soft rolling skin criterion or an index of fleece weight and fibre diameter with either a 5% or 10% micron premium. Source: Daily et al. (1997).

Soft rolling skin 5% index 10% index

Clean fleece weight (kg) +0.8 +1.7 +1.2

Fibre diameter (m) -2.0 -1.5 -3.0

CV of diameter (%) +0.3 +0.2 +1.4

Staple length (mm) -3.4 -2.0 -6.7

Staple strength (N/ktex) 0.0 -5.7 -6.7

The potential value of using skin-based selection criteria for within-flock selection is inconclusive. In some instances, there may be definite economic advantages in terms of both cost reductions and enhanced genetic progress from using skin-based assessments. But equally, the same approach may not deliver the same outcome when applied to different scenarios of breeding objective and Merino strain.

The SRS merino breeding ‘system’One limitation of the studies outlined in the previous section is that skin-based assessment has only been considered within the context of within-flock selection. Advocates of the soft rolling skin approach promote more than just a change in the selection criteria. A high priority is given to mate allocation (i.e. which rams to use with which ewes) and the use of breeding stock that have themselves been produced under an established soft rolling skin system (i.e. ram source).

In 1996, the SA Research and Development Institute in collaboration with the University of Adelaide established three demonstration flocks, of which each represents a major industry view on the way in which a Merino breeding program should be conducted. The three breeding programs were as follows:

Professional Classer Assessment Flock (PCA) – Selection mainly based on traditional classer assessment of wool quantity and quality, and on the ability of the sheep to thrive and reproduce, with some occasional and non-systematic use of objective measurements

Measurement and Performance Recording Flock (MPR) – Selection mainly based on performance records, but with some attention to visually assessed wool and body faults

Elite Wool Flock (EWF) – Selection based on skin assessment, objective measurement and mate allocation.

The broad aim in all three flocks was to increase the profitability of SA Merino sheep through genetic improvements in wool quantity, wool quality, reproductive rate and the saleability of surplus sheep.

The selection and mating decisions to be made for each flock, including choice of outside sires to be used, was the responsibility of a group comprising advocates of the breeding system associated with the flock. These groups consisted of practicing ram breeders, woolgrowers, sheep classers and sheep consultants. While technical persons working for research or teaching organisations could be consulted at any time by any group, such persons were not involved in the selection and mating decisions. In all other aspects of management, the three flocks were treated identically. By undertaking all three strategies side-by-side, the relative advantages and disadvantages of each can be compared. A randomly-selected control flock (CON) was also been established and managed identically, to provide a reference point against which genetic change in the other flocks can be measured. The experiment was completed in 2006 and the genetic changes in the different lines for the main production traits are indicated in Figures 12.3 from 1997 to 2004.


Figure12.3. Genetic trends of clean fleece weight, fibre diameter, live body weight at 16 months of age, staple strength, staple length and coefficient of variation of fibre diameter

of the Control (Con), Measured and performance recorded (MPR), Professional Classer Assessed (PCA) and Elite Wool Flocks (EWF) from 1997 to 2004. All records have been

adjusted for lamb age and birth-rearing status. Source: Greef (2006).

Genetic Trend - Clean Fleece Weight

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

1997 1998 1999 2000 2001 2002 2003 2004Drop

CFW

EB

V (k

g)

CON

MPR

PCA

EW F

Genetic Trend - Fibre Diameter

-2.5

-2

-1.5

-1

-0.5

0

0.5

1997 1998 1999 2000 2001 2002 2003 2004Drop

FD E

BV

(u) CON

MPRPCAEWF

Genetic Trend - 16 mo Live Weight

-4

-2

0

2

4

6

1997 1998 1999 2000 2001 2002 2003 2004Drop

LWt E

BV

(kg) CON

MPRPCAEWF

Genetic Trend - Staple Strength

-5

-3

-1

1

3

1997 1998 1999 2000 2001 2002 2003 2004Drop

SS E

BV

(N/K

tex)

CONMPRPCAEWF

Genetic Trend - Staple Length

-6

-4

-2

0

2

1997 1998 1999 2000 2001 2002 2003 2004Drop

SL16

EB

V CONMPRPCAEW F

Genetic Trend - Coefficient of Variation of Fibre Diameter

-2

-1.5

-1

-0.5

0

0.5

1

1997 1998 1999 2000 2001 2002 2003 2004Drop

CV

FD E

BV

(%)

CONMPRPCAEWF

The genetic trends show that all three selection methods resulted in significant improvements in clean fleece weight and a reduction in fibre diameter relative to the control flock at the start of the experiment. This indicates that progress can be made very fast even in two traits that are genetically unfavourably correlated, by identifying genetically superior rams. The rate of genetic gains declined subsequently but the MPR flock has made the most progress towards the original breeding objective over the experimental period. However, an unfavourable change occurred in live weight which was rectified relatively quickly by using more suitable rams when it became obvious.

The key conclusions from this project was that selection resulted in changes in all the major production traits and that all three selection flocks improved their fleece value which increased the profitability of these flocks relative to that of the unselected control flock, with hogget values for progeny born in 2004 ranging from an advantage of $5.25 per head for the PCA to $10.33 per head for the MPR flock.

Follicle density of the selection flocks also increased as a correlated response. The average follicle density was 53.3, 55.7, 58.5 and 60.1 follicles per mm2 for the Control, Measured Performance, Professional Classer and Elite Wool flock respectively. This shows that follicle density of the selection flocks increased significantly relative to that of the control flock. There were also small improvements in subjectively assessed traits such as wool colour, handle and staple structure.


An experiment such as this provides an objective and unbiased context within which to compare the relative advantages and disadvantages of the various Merino breeding programs, including those utilising skin-based selection criteria. In particular, it provides the industry with an opportunity to ‘see and feel’ the outcomes generated under each system, within one environment and with identical husbandry and grazing regimes.

Readings The following readings are available on CD:

1. Atkins, K.D. 1997, ‘Genetic improvement of wool production’, in The Genetics of sheep, (eds. L. Piper and A. Ruvinsky), CAB International, pp. 471-504.

2. Greeff, J.C., Lewer, R.P., Ponzoni, R.W. and Purvis, I. 1995, ‘Staple Strength: Progress Towards Elucidating its Place in Merino Breeding’, Proceedings Association of Advancement of Animal Breeding and Genetics, vol. 11, pp. 595-601.

3. Greeff, J.C., Ritchie, A.J.M. and Lewis, R.M. 1997, ‘Lessons from the staple strength resource flocks’, Proceedings Association of Advancement of Animal Breeding and Genetics, vol. 12, pp. 714-718.

4. Greeff, J.C. 1999, ‘Relationship between staple strength and coefficient of variation of fibre diameter within and between flocks’, Proceedings Association of Advancement of Animal Breeding and Genetics, vol. 13, pp. 54-57.

5. Greeff, J.C. 2001, ‘Can along and between fibre diameter variation make a contribution in Merino Breeding programs?’ Proceedings Association of Advancement of Animal Breeding and Genetics, vol. 14, pp. 293-296.

6. Lewer, R.P. and Li, Y. 1994, ‘Some aspects of selection for staple strength’, Wool Technology and Sheep Breeding, vol. 42, pp. 103-111.

7. Ramsay, Anne M.M., Grimson, R.J., Smith, D.H., Jaensch, Kaylene S., Ingham, Veronica M., and James, P.J. 2004, Merino demonstration flocks: Background and update with 2002 drop hogget results. In: South Australian Merino Selection Demonstration Flocks Newsletter number 8, pp 4-30. South Australian Research and Development Institute, Adelaide, South Australia.

Suggested readingBrien, F., Young, J.M. 2006 ‘Cost-benefit analysis of selection demonstration flocks’ in Proceedings of the 8th World Congress on Genetics Applied to Livestock Production. August, 13 to 18, 2006 Belo Horizonte, Brazil.

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SummaryAlthough skin based selection adds additional information to breeding programs, the high cost of laboratory skin based traits, such as follicle density, S/P ratio does not make it cost effective to improve wool traits. Classer assessed visual traits can make a small contribution, especially for traits such as fleece colour, fleece rot, dark fibres, etc. Undesirable animals for these traits should be culled, as well as animals with other visually abnormal traits. But using objective measurement for traits such as fleece weight, fibre diameter, coefficient of variation of fibre diameter and fibre curvature, would result in faster genetic gains than subjectively assessed traits. The SRS/Elite wool selection system has so far not proved to be a better selection method than objectively measured selection. Staple strength is an important trait in early stage processing and tender wools are discounted. It is more important at the fine end of the clip and therefore deserves to be included in a breeding program. Staple strength is a moderately heritable trait, and is highly negatively correlated with coefficient of variation of fibre diameter. Thus selecting on coefficient of variation of fibre diameter will result in a correlated improvement in staple strength. Selecting strongly for reduced fibre diameter will result in a decrease in staple strength. This negative effect can be overcome by including breeding for staple strength in the breeding objective.

Style is important because more stylish wools attract higher premiums especially at the finer end of the clip. Fortunately the objectively measured fleece traits such as fleece weight, fibre diameter, staple strength and coefficient of variation has a favourable genetic relationship with style. Thus selecting on these objectively measured traits will also result in an improvement in style. These traits are also more accurately measured and therefore will result in faster genetic gains than using the subjectively assessed style traits.

ReferencesBarton, S.A., Purvis, I.W. and Brewer, H.G. 2001, ‘Are wool follicle characteristics associated with

wool quality and production in hogget and adult sheep?’, Proceedings of the Association for the Advancement of Animal Breeding and Genetics, vol. 14, pp. 289-291.

Brien, F. and Young, J.M. 2006, ‘Cost-benefit analysis of selection demonstration flocks’ Proceedings of the 8th World Congress on Genetics Applied to Livestock Production. August, 13 to 18, 2006 Belo Horizonte, Brazil. CD ROM communication 05-02.

Brien, F. 2006. Merino Selection Demonstration Flocks. Final Newletters 10 July 2006. SARDI, Turretfield.

Brown, D.L., Ball, A., Mortimer, R. and Oppenheimer, M. 2002a, ‘ Incorporating subjectively assessed sheep and wool traits into genetic evaluations for Merino sheep I. Phenotypic variation and heritabilities’. Wool Technology and Sheep Breeding, vol. 50, pp. 373-378.

Brown, D.L., Ball, A., Mortimer, R. and Oppenheimer, M. 2002b, ‘Incorporating subjectively assessed sheep and wool traits into genetic evaluations for Merino sheep 2. Phenotypic and genetic correlations’, Wool Technology and Sheep Breeding, vol. 50, pp. 378-382.

Brown, D., Crook, B.J. and Purvis, I.W. 1999, ‘Genotype and environmental differences in fibre diameter profile characteristics and their relationship with staple strength in Merino sheep’, Proceedings of the Association for the Advancement of Animal Breeding and Genetics , vol. 13, pp. 274.-275

Daily, H.G., Ponzoni, R.W., Hynd, P.I., Grimson, R.J., Jaensch, K.S. and Pitchford, W.S. 1997, ‘An evaluation of ‘Soft Rolling Skin’ as a selection criterion for the genetic improvement of South Australian Merino sheep’, Proceedings of the Association for the Advancement of Animal Breeding and Genetics, vol. 12, pp. 516-519.

Denney, G.D. 1990, Phenotypic variance of fibre diameter along wool staples and its relationship with other raw wool characters. Australian Journal Experimental Agriculture, vol. 30, pp. 463-467.

Gifford, D.R., Ponzoni, R.W., Ancell, P.M.C., Hynd, P.I., Walkley, J.R.W. and Grimson, R.J. 1995, ‘Genetic studies on wool quality and skin characters of the Merino’ Wool Technology and Sheep Breeding, vol. 43, pp. 24-29.

Greeff, J.C. 2001, ‘Can along and between fibre diameter variation make a contribution in Merino breeding programs?’, in Proceedings of the Australian Association of Animal Breeding and Genetics, vol. 14, pp. 293-296.


Greeff, J.C. 1997, ‘The Genetics of Staple Strength’, in Access to the Experts: Producing the Wool that the Market Demands, (eds. B.J. Crook and D.R. Lindsay), CRC for Premium Quality Wool, vol. 1, pp. 22-26.

Greeff, J.C., Lewer, R.P., Ponzoni, R.W. and Purvis, I.W. 1995, ‘Staple strength: Progress towards elucidating its place in Merino breeding’, Proceedings of the Australian Association of Animal Breeding and Genetics, vol. 11, pp. 595-601.

Greeff, J.C., Ritchie, A.J.M. and Lewis, R.M. 1997, ‘Lessons from the staple strength resource flocks’, Proceedings of the Association for the Advancement of Animal Breeding and Genetics, vol. 12, pp. 714-718.

Gregory, I.P. 1982, Genetic studies of South Australian Merino sheep IV. ‘Genetic, phenotypic and environmental correlations between various wool and body traits.’ Australian Journal of Agricultural Research, vol. 33, pp. 363-373.

Heydenrych, H.J., Vosloo, L.P. and Meissenheimer. 1977, Selection response in South African Merino sheep selected either for high clean fleece mass or for a wider S/P ratio. Agroanimalia, vol. 9, pp. 67.

Heydenrych, H.J, du Plessis, J.J. and Cloete, S.W.P. 1984, Increasing the wool production of Merino sheep in the South Western Districts of South Africa by direct and indirect selection. Proceedings 2nd World Congress Sheep and Beef Breeding, Held in Pretoria, South Africa from 16-19 April 1984, pp. 399-412.

Hill, J.A., Hynd, P.I., Ponzoni, R.W., Grimson, R.J., Jaensch, K.S., Kenyon, R.V. and Penno, N.M. 1997a, ‘Skin and follicle characters 1. Heritabilities and correlations among them’ ,Proceedings of the Association for the Advancement of Animal Breeding and Genetics, vol. 12, pp. 520-523.

Hill, J.A., Hynd, P.I., Ponzoni, R.W., Grimson, R.J., Jaensch, K.S., Kenyon, R.V. and Penno, N.M. 1997b, ‘Skin and follicle characters II Correlations with objectively measured and subjectively assessed wool characters’, Proceedings of the Association for the Advancement of Animal Breeding and Genetics, vol. 12, pp. 524-526.

Hill, J.A., Ponzoni, R.W., Hynd, P.I. and Pitchford, W.S. 1998, ‘The inclusion of skin characters in a selection index: how confident are we about their value’ 6th World Congress of Genetics Applied to Livestock Production, vol. 24, pp. 43-46.

Howe, R.R., MacLeod, I.M. and Lewer, R.P. 1991, ’Genetic parameters for some wool tenderness related traits under different nutrition levels in Merino sheep’, Proceedings of the Australian Association of Animal Breeding and Genetics, vol. 9, pp. 347-350.

Hynd, P.I., Ponzoni, R.W., Grimson, R., Jaensch, K.S., Smith, D. and Kenyon, R. 1996, ’Wool follicle and skin characters - their potential to improve wool production and quality in Merino sheep’ Wool Technology and Sheep Breeding, vol. 44, pp. 167-177.

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Glossary of termsCoefficient of variation (%)1

a statistical term for the amount of variation within a set of measurements for a given trait eg fibre diameter. The higher the percentage the more variable the trait

Genetic correlation2 a measure of the direction and strength of the association between breeding values for two traits for the same animal eg. staple strength and clean fleece weight (ranges from -1 to +1)

Handle1 the degree of softness of wool to the touch

Heritability2 the proportion of superiority of parents in a trait (i.e. the proportion of the selection differential) which, on average, is passed on to offspring

Kilotex (ktex)1 the unit of measurement for linear density (or thickness), in grams per metre. The thickness of staples is used in the calculation of staple strength. A staple of 100mm in length with a clean weight of 0.1 g has a pencil ’thickness’ of 1 kilotex. Typical staples range from 1 to 5 ktex

Objective measurement1

the measurement of fibre properties rather than their visual or subjective appraisal

Phenotypic correlation2

a measure of the direction and strength of the association between observed performance (phenotype) in two traits eg. staple strength and clean fleece weight measured on the same animal (ranges from -1 to +1)

Selection differential2 the difference between the mean performance of selected animals and the overall mean of the group of animals from which they were selected

Selection index2 an overall score of genetic merit which combines information on several measured traits, or from different classes of relatives. The emphasis on each trait in the index usually depends on the strength of its association with traits in the breeding objective, and their relative economic value


Staple strength1 the force (Newtons) required to break a staple of given thickness (kilotex)

Style1 a combination of characteristics such as crimp formation, length, colour, condition, density, handle and others that determine the use and value of the wool

Subjective assessment

visual appraisal of traits instead of objective measurement

Tandem selection2 a method of selection for more than one trait. Involves selection of one trait for one or more generations, followed by selection for a second trait for one or more generations, possibly followed by selection on more traits and eventually returning to selection of the first trait and so on

1Taken from Cottle, D.J. (ed.) 1991, Australian Sheep and Wool Handbook, Inkata Press, Melbourne.2Taken from Simm, G. 2000, Genetic Improvement of Cattle and Sheep, Farming Press UK.



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… · Web viewFleece weight and fibre diameter are the two most important wool production traits affecting income. However, secondary wool quality traits such as staple length,