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GENETIC PARAMETERS FOR FLEECE WEIGHT AND FIBRE DIAMETER

PROFILE TRAITS IN SOUTH AFRICAN ANGORA GOATS

 

 M.A. Snyman,1  C. Visser2 & E. van Marle-Köster2

 

1Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg (EC) 5900, South Africa

2Department of Animal & Wildlife Sciences, University of Pretoria, South Africa

E-mail: Gretha Snyman

 


INTRODUCTION

Reducing fibre diameter of mohair has been a focal point both in research and within the Angora goat stud breeding industry over the past two decades (Snyman, 2002). Fibre diameter is the single most important price-determining variable of mohair and is of critical importance in determining the textile qualities and final manufacturing applications of mohair (Qi et al., 1994). There is substantial variation in fibre diameter over the fleece of individual Angora goats, with mohair from the neck usually stronger than that from the rest of the fleece (Venter, 1959; Taddeo et el., 2000; Snyman & Strauss, 2004). Fibre diameter also varies considerably within a staple, as well as along the length of individual fibres (Venter, 1959; Grobbelaar & Landman, 1984). For instance, in staples with an average diameter of 32 µm, fibre diameter of individual fibres may vary from 12 µm to 52 µm (Van der Westhuizen et al., 2004).

 

The normal spinning performance and properties of a yarn of specified twist and thickness (linear density) could be accurately predicted from knowledge of fibre properties (Lamb, 1997). From a technical perspective, considerable research into the effects of diameter distribution in processing and fabric properties has been undertaken for wool (De Groot, 1992; Lamb, 1992; Naylor et al., 1995; Dolling et al., 1992). Increasing coefficient of variation of fibre diameter reduces yarn evenness and increases the number of ends down in spinning. For skin comfort the number of coarser fibres in the diameter distribution increases with increasing coefficient of variation of fibre diameter (CV) and hence the resultant fabric is more likely to be uncomfortable when worn next to the skin. Increasing mean fibre diameter also has a similar effect on both spinning and skin comfort. Although the mechanisms associated with the effect of CV on spinning and skin comfort are quite different, they are of similar magnitude relative to the effect of mean fibre diameter (Lamb, 1992). The implications of this, for typical Merino wools, are that a 5 unit increase in CV will have a similar effect on spinning and fabric skin comfort as a one micron increase in mean fibre diameter. For example, wool with a mean fibre diameter of 20 µm and a CV of 25 % and another wool with a mean fibre diameter of 21 µm and a CV of 20 % would have a similar spinning performance and a similar potential fabric skin comfort (assuming other characteristics to be the same) (Lamb, 1997; Naylor, 1997). Much less is known regarding the effect of CV of fibre diameter on the processing properties of mohair.

The availability of two new techniques, the Laserscan and the OFDA, enables measurement of the full fibre diameter profile, including the width of the distribution, and not just the mean fibre diameter (Naylor, 1997). The width of the distribution is measured in terms of the coefficient of variation of fibre diameter (Van der Westhuizen, 1981; Kritzinger, 1992; Hunter, 1993). Optical Fibre Diameter Analyser (OFDA) technology developed in the early 1990s provided an objective method for accurate and rapid determination of fibre diameter and was shown to be effective for measuring staples, tops and cores in Angora goats (Qi et al., 1994). OFDA100 technology was developed in 1989, introduced to and tested in South Africa during 1992 and has since been applied commercially (Baxter et al., 1992). OFDA2000 technology was since developed to be a portable instrument capable of real-time measurements of the fibre properties of greasy fibre with minimal sample preparation as an aid to selection or clip classing (Brims et al., 1999) and is currently routinely used for mohair fleece measurements in South Africa. OFDA2000 provides the same accuracy, but faster measurement than the previously used OFDA100 and correlations between the measurements of the two instruments are high (AWI Project EC 397 Final Report, 2004).  The full diameter profile can be measured objectively and accurately, leading to the necessity to estimate genetic parameters for these newly measured traits.

 

Most studies on Angora fleece traits in the past included fleece weight, fibre diameter, standard deviation of fibre diameter and sometimes kemp score. Heritabilities and correlations have been reported for South African (Snyman & Olivier, 1996; Snyman & Olivier, 1999), Australian (Gifford et al., 1991), Argentinean (Taddeo et al., 1998) and New Zealand (Nicoll et al., 1989) Angora goats. In these studies not all fleece traits were included and the estimates obtained varied based on different model structures, sample sizes and measuring techniques. 

 

The additional fibre properties that are obtained from the OFDA2000 analysis, such as coefficient of variation, spinning fineness and comfort factor, have a direct impact on processing performance and skin comfort of the final mohair product (Lamb, 1992). Inclusion of these quality traits as selection criteria is, however, dependent on their heritabilities and genetic correlations with other traits. To date only Allain & Roguet (2006) has reported genetic parameters for OFDA-measured traits in mohair, including coefficient of variation of fibre diameter, kemp and medullated fibres. The objective of this study was therefore to estimate heritabilities and genetic correlations for mohair traits in South African Angora goats.

 

MATERIALS AND METHODS

Data for this study were obtained from 11 different Angora goat herds, consisting of phenotypic records for kids born during the 2000 to 2006 kidding seasons. Data were recorded on the second (8 to 12 months) and third (16 to 18 months) shearings. Rams were mostly sheared at eight to 12 months, while the ewes were mostly sheared at 16 to 18 months. Only one shearing record was included per animal. 

 

Greasy fleece weight (measured to the nearest 0.1 kg) was determined just after shearing. Individual midrib samples were taken from each kid for determination of fibre diameter. From 2000 to 2003, fibre diameter of mohair samples was determined with the OFDA100 at the Wool Testing Bureau in Port Elizabeth. Owing to the availability of OFDA2000 technology, micron testing of mohair samples collected since 2004 was done using an OFDA2000 instrument.  A single sub-sample (prepared from three different locks) was analysed for each animal to obtain fibre properties with the OFDA2000 (Snyman & Strauss, 2004). 

 

The final data set comprised of 6221 records which included the following traits: greasy fleece weight (FW; kg), fibre diameter (FD; µm), coefficient of variation of fibre diameter (CVFD; %), standard deviation of fibre diameter (SDFD; µm), comfort factor (CF; %), spinning/effective fineness (SF; µm) and standard deviation of fibre diameter along the length of the staple (SDA; µm). Standard deviation of fibre diameter is a measure of the variation in fibre diameter within the sample. The coefficient of variation is the standard deviation expressed as a percentage of the average micron (Baxter et al., 1992) and the comfort factor is the percentage of fibres below 30 micron.

 

The original raw data was edited and animals with missing information on birth date, sex, age of the dam or dam identification, were omitted.  All the available pedigree information (7119 records) was included in the analyses. For the analyses of fleece weight (FW) there were 302 sires and 3602 dams with progeny, while there were only 27 sires and 510 dams with progeny for the OFDA2000 fibre traits in the data set. A total of 1073 animals in the pedigree file had unknown sires, but all dams were known.

 

General Linear Model (GLM) procedures of the SAS computer package (SAS, 2004) were used to identify fixed effects which contributed significantly to variation. The fixed effects finally included in the models for all traits were herd-year of birth (HY), sex (male or female), birth status of the kid (single or twin), age of dam at kidding (2 to 12 years of age) and a covariate for age of the kid at shearing (in days). 

 

Variance components were estimated using the ASREML program of Gilmour et al. (2002). A single trait animal model was fitted for all traits. Direct additive and maternal additive genetic effects, with or without a covariance between them, and maternal permanent environmental effects were tested in different combinations to determine the most effective model for each trait. Subsequently, multi-trait analyses were done to estimate covariance components and correlations among the traits, using the most suitable model for each trait, as determined under single trait analyses.

 

RESULTS AND DISCUSSION

A description of the dataset, including number of records analysed, as well as the mean and coefficient of variation for each trait, is given in Table 1.

 

Table 1. Description of the data set for estimation of genetic parameters for fleece traits of Angora goats

Trait

Mean

CV (%)

Number of records

Total in data set

Rams second shearing

Ewes second shearing

Rams third shearing

Ewes third shearing

Greasy fleece weight (kg)

1.44

20.23

6211

1627

1328

535

2721

Fibre diameter (µm)

27.97

8.40

6041

1617

1326

532

2566

Coefficient of variation of fibre diameter (%)

25.43

11.98

898

393

-

374

131

Standard deviation of fibre diameter (µm)

7.29

13.53

898

393

-

374

131

Comfort factor (%)

64.30

20.66

898

393

-

374

131

Standard deviation of fibre diameter along the length of the staple (µm)

1.33

37.54

898

393

-

374

131

 

Two shearing age periods were included (8 to 12 months and 16 to 18 months) with an average fleece weight of 1.44 kg. In a previous study (Snyman & Olivier, 1999) on data from the South African Angora Goat Performance Testing Scheme, the average fleece weights reported varied between 1.97 kg at 10 months of age to 2.33 kg at 16 months of age.  The fibre diameter of 27.97 mm in this study was lower than that reported by Snyman & Olivier (1999) for South African Angoras at 10 (28.81 µm) and 16 (32.62 µm) months of age and by Allain & Roguet (2003) for French Angoras measured at 18 months of age (30.4 mm).  The CVFD, however, corresponded closely to that reported in the French Angora goat study (25.43 % vs. 25.30 %).

 

The most effective model for all traits was the model that included only direct additive genetic effects. Snyman & Olivier (1996) also fitted several models for fleece weight and fibre diameter, and also concluded that the model including only the direct additive genetic effects was the most appropriate for parameter estimation. The heritability estimates for the traits are summarised in Table 2. 

 

The estimated heritability values vary from as low as 0.14±0.08 for SDA to as high as 0.63±0.10 for CF, and falls within the general range of values cited in literature (Nicoll, 1985; Pattie et al., 1990; Gifford et al., 1991; Sumner & Bigham, 1993, Taddeo et al., 1998).  The heritability estimated for fleece weight was 0.24±0.03, which is similar to estimates of 0.22±0.04 and 0.19±0.04 reported by Snyman & Olivier (1996, 1999) for South African Angora goats and 0.25±0.04 for French Angora goats (Allain & Roguet, 2006), but lower than estimates reported for Australian Angora goats of 0.45±0.23 (Gifford et al., 1991). 

 

Table 2. Heritabilities (± s.e.) of mohair traits in Angora goats 

Trait

Heritability

Greasy fleece weight

0.24±0.03

Fibre diameter

0.45±0.03

Coefficient of variation of fibre diameter

0.37±0.10

Standard deviation of fibre diameter

0.32±0.11

Comfort factor

0.63±0.10

Standard deviation of fibre diameter along the length of the staple

0.14±0.08

 

The heritability estimated for fibre diameter (0.45±0.03) was higher than previous estimates of 0.30 reported by Snyman & Olivier (1999) for South African Angora goats and 0.33 by Taddeo et al. (1998) for Argentinean Angora goats. The latter studies were performed before OFDA technology became commonly available.  A similar trend was observed where Allain & Roguet (2003) estimated a heritability of 0.32 for fibre diameter of French Angora goats, but found a much higher estimate of 0.51 three years later when using OFDA measures (Allain & Roguet, 2006).   

 

CVFD and SDFD both had moderate heritabilities of 0.37±0.10 and 0.32±0.11 respectively, while CF (0.63±0.10) and SF (0.61±0.10) had high heritability estimates. The heritability for coefficient of variation for fibre diameter reported by Allain & Rouget (2006) from OFDA data for French Angoras was lower (0.29) than found in the present study (0.37±0.10), while Nicoll et al. (1989) also reported a lower heritability value (0.21) for SDFD. SDA had the lowest heritability of all traits analysed (0.14±0.08), which could be expected, as it is directly influenced by any variation in nutritional status throughout the fibre growth period.

 

The genetic and phenotypic correlations among the traits are summarised in Tables 3 to 6. The standard errors for most of the estimated genetic correlations were relatively high, indicating that these results could only be taken as an indication of the relationship among the traits. Further analyses will be done when more data become available. The ideal goat will be the one who produces a high fleece weight with a low fibre diameter, low CVFD, low SDFD, low SDA and high CF. However, the nature of the genetic correlations among these traits does not allow favourable genetic progress in the desired direction for each trait.

 

The low positive genetic correlation (0.08±0.22) between FW and FD does not correspond with literature estimates. Medium to strong positive correlations of between 0.35±0.04 (Allain & Roguet, 2003) and 0.55±0.07 (Snyman & Olivier, 1996) have been reported earlier. Sumner & Bigham (1993), however, reported genetic correlations between these two traits ranging from 0.14 to 0.98 in previous literature, reflecting the large variation in reported estimates. The genetic relationship between FW and FD is, however, positive and fleece weight should be monitored in a selection program aimed at decreasing fibre diameter, as is also discussed by Snyman (2002). The positive genetic relationship between FW and FD also contributes to the negative genetic correlations between FW and CVFD (-0.17±0.27) and between FW and CF (-0.46±0.18). Thus, the higher the fleece weight, the lower the CF (less fibres below 30 µm) and lower the CVFD.

 

Table 3. Genetic and phenotypic correlations (± s.e.) between fleece weight and the fibre diameter quality traits 

Correlation of fleece weight with:

Genetic correlation

Phenotypic correlation

Fibre diameter

0.08±0.22

0.39±0.04

Coefficient of variation of fibre diameter

-0.17±0.27

-0.04±0.04

Standard deviation of fibre diameter

0.35±0.26

0.30±0.03

Comfort factor

-0.46±0.18

-0.46±0.03

Standard deviation of fibre diameter along the length of the staple

0.11±0.43

0.15±0.03

 

Table 4. Genetic and phenotypic correlations (± s.e.) between fibre diameter and the other fibre diameter quality traits 

Correlation of fibre diameter with:

Genetic correlation

Phenotypic correlation

Greasy fleece weight

0.08±0.22

0.39±0.04

Coefficient of variation of fibre diameter

-0.36±0.17

-0.19±0.04

Standard deviation of fibre diameter

0.51±0.15

0.45±0.03

Comfort factor

-0.81±0.04

-0.86±0.01

Standard deviation of fibre diameter along the length of the staple

0.44±0.31

0.23±0.03

 

High genetic correlations were estimated between fibre diameter and the other fibre diameter traits, which could be expected. The negative genetic correlation between FD and CVFD of -0.36±0.17 corresponded to the values of -0.33 (Allain & Roguet, 2003) and -0.44 (Allain & Roguet, 2006) reported previously. Selection for decreased fibre diameter will result in an increased CVFD (unfavourable) and CF (favourable), while SDFD (favourable) and SDA (favourable) will decrease.

 

High positive genetic correlations were estimated between CVFD and SDFD (0.41±0.19), CVFD and CF (0.50±0.16) and CVFD and SDA (0.71±0.45). Decreasing CVFD will lead to a correlated increase in fleece weight and fibre diameter, while a favourable response will be obtained in SDFD and SDA, which should also decrease. CF, however, will also decrease, which will lead to an unfavourable increase in the number of fibres above 30 µm. It has been shown that the fibre diameter distribution has a significant effect on yarn properties and processing performance of wool (Qi et al., 1994; Smith et al., 2006). A decrease in CVFD is associated with more sound wool in Merino sheep and can be used as an indirect selection criterion to improve staple strength in Merinos as well as decreasing susceptibility to fleece rot (Greeff, 2006).

 

Table 5. Genetic and phenotypic correlations (± s.e.) between coefficient of variation of fibre diameter and the other fibre traits 

Correlation of coefficient of variation of fibre diameter with:

Genetic correlation

Phenotypic correlation

Greasy fleece weight

-0.17±0.27

-0.04±0.04

Fibre diameter

-0.36±0.17

-0.19±0.04

Standard deviation of fibre diameter

0.41±0.19

0.71±0.02

Comfort factor

0.50±0.16

0.21±0.04

Standard deviation of fibre diameter along the length of the staple

0.71±0.45

0.15±0.04

 

Table 6. Genetic and phenotypic correlations (± s.e.) between standard deviation of fibre diameter and the other fibre traits 

Correlation of standard deviation of fibre diameter with:

Genetic correlation

Phenotypic correlation

Greasy fleece weight

0.35±0.26

0.30±0.03

Fibre diameter

0.51±0.15

0.45±0.03

Coefficient of variation of fibre diameter

0.41±0.19

0.71±0.02

Comfort factor

-0.55±0.14

-0.48±0.03

Standard deviation of fibre diameter along the length of the staple

-0.30±0.54

0.34±0.03

 

Decreasing SDFD will lead to a correlated decrease in fleece weight, fibre diameter and CVFD. This will be accompanied by a simultaneous unwanted increase in SDA and CF. A negative genetic correlation of -0.26±0.03 was estimated between SDA and CF, implying that an increase in CF, will be accompanied by a favourable decrease in SDA.

 

Smith et al. (2006) concluded that the fibre diameter profile could be a useful tool to improve fleece quality in fine wool sheep breeds. Much less is known regarding the effect of fibre diameter quality traits of mohair on its processing performance. Fleece homogeneity is, however, of economic importance to Angora farmers (Allain & Roguet, 2003) and information on the fibre diameter profile gives opportunity to select for less variability in fleece quality. Lupton & Pfeiffer (1998) stated the efficiency of both quality assessment and selection programs in Angora goats could be improved by including fibre diameter quality traits.

 

CONCLUSION

Since the development of OFDA2000 technology, additional traits for improvement of mohair quality have become available to use as possible selection criteria. From the results it is clear that fibre diameter quality traits are moderately to highly heritable. Future studies should focus on the economical importance of these traits in mohair production and processing, to evaluate their possible inclusion in selection strategies for Angora goats.

 

ACKNOWLEDGEMENTS

The authors want to convey their sincere appreciation to the participating Angora goat breeders for their collaboration in this project and to Mohair South Africa for partial funding of the project.

 

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Published

Grootfontein Agric 10 (1) :14-23