AGE CHANGES IN WOOL TRAITS OF MERINO SHEEP IN WESTERN NSW
S. Hatcher, K.D. Atkins, and K.J. Thornberry
NSW Department of Primary Industries, Orange Agricultural Institute, Orange, NSW 2800 SUMMARY
Repeatability and the effect of increasing age (from 2 to 5 years) on wool production and quality were determined for a mixed bloodline flock of superfine, fine and medium wool sheep in western NSW. For those traits which have previous estimates published in the literature the repeatability estimates in this study were similar with the exception of fibre diameter which was higher than previous estimates. This indicates it may be possible to select for fibre diameter in fine wool at an earlier age than in broader flocks. This study identified a general trend of wool production increasing to a maximum at 3 years of age and then reaching a plateau while wool quality traits tended to decrease with age. Partitioning of the interaction variance indicates that in general variation in the age trends was the result of bloodline differences rather than micron group.
Keywords: Merino, wool production, wool quality, repeatability, age changes INTRODUCTION
Wool production and wool quality both vary from one period to another if they are measured at different times in the life of a Merino sheep. It is important for Merino breeders to be able to identify and select animals for superiority at an early age as this enables them to cull the less productive animals and retain the superior sheep. Most estimates of repeatability for Merino wool production have been derived from medium (Beattie 1961;Young et al. 1960) or strong wool flocks (Morley 1951), with few estimates for fine wool Merinos. Mullaney et al. (1970) estimated the repeatability of a range of wool production and quality traits from a medium and three fine wool Merino flocks and found that the estimates for the fine wool Merinos tended to be slightly higher than those published previously for broader Merino strains. The higher repeatability estimates for fine wool flocks indicate that selection may be able to be made at an earlier age in finer flocks.
The change in wool production and quality traits with age will allow decisions on the optimal age structure of flocks to be determined (Brown et al. 1966; Mullaney et al. 1969). It is well known that wool production and wool quality characteristics can alter substantially with increasing age of sheep (Corbett 1979). Previous studies predominantly using medium to strong wool Merino strains, identified a general trend of wool production increasing to 3 to 4 years of age and then declining (Brown et al. 1968; Brown et al. 1966). The rate of change of wool production following the peak level identified in each of these studies differed, from a plateau to a slow or rapid decline. Although variable results have been observed with respect to wool quality traits, the general trend is for wool quality traits to gradually deteriorate with age (Mullaney et al. 1969; Swan and Purvis 2000). The Condobolin Fine Wool Flock provides an ideal basis to investigate the repeatability of wool production and quality traits and age trends, as it represents the performance of 5 drops of 11 Merino bloodlines each run at Condobolin for 4 years.
MATERIALS AND METHODS
Between 1992 and 1996, the majority of wethers from the CSIRO Fine Wool Project bloodlines (Swan et al. 1993) at Armidale, were transferred to the Condobolin Agricultural Research and Advisory Station, following their hogget shearing at 10 months of age. Condobolin is located in the centre of the NSW cereal belt with an average rainfall of 427 mm which is non-seasonal and highly variable. Prior to shearing in August of each subsequent year, a midside sample was taken from each wether and a number of subjective assessments of wool quality made (handle, dust, crimp definition, staple size, density and colour). Assessments of these traits were made on a 1 to 5 scale with 1 being desirable and 5 undesirable. The midside samples were each tested to determine the percentage yield (CSY - %), average fibre diameter (FD - µm), standard deviation of fibre diameter (FDSD - µm), coefficient of variation in fibre diameter (FDCV - %), fibre curvature (CURVE - °/mm), staple length (SL - mm), staple strength (SS - N/ktex), clean colour (C - Y-Z) and resistance to compression (RTOC - kpa). At shearing each wether’s fleece was weighed to determine greasy fleece weight (GFW – kg) and the CSY used to calculate clean fleece weight (CFW - kg). In addition each fleece was assigned a style grade (STYLE - MF1 - MF7) and assessed for greasy colour (AGC) and assessed tenderness (AT). Off shears body weights (BWT - kg) were recorded for each wether.
The wethers represent 11 bloodlines, which could be classified as medium wool (2), fine (3) and superfine (6). Approximately 1,800 individual animals were involved in this analysis. All animals have an identified sire and were measured for 4 consecutive shearings. ASREML (Gilmour et al.
1999) was used to estimate variance components under a general linear mixed model by residual maximum likelihood. A univariate analysis was undertaken for each trait, which involved fitting a model with the fixed effects of drop, year of measurement and bloodline together with significant interactions. Random effects were estimated for sire (σ2s) animals within sire (σ2b) and error of within animals (σ2w). Repeatability (r2) was estimated from the variance components as (σ2s + σ2b)/(σ2s + σ2b + σ2w). Variance components from models including the bloodline and micron group (genetic group based on bloodline) interactions were compared to determine the relative importance of bloodline x age versus micron group x age interactions.
RESULTS AND DISCUSSION
FD (0.76) was the most repeatable trait of those measured on the wethers at Condobolin (Table 1).
The next most repeatable traits were BWT (0.64), RTOC and GFW (0.62) followed by SL, CFW (0.61) and CURVE (0.56). With the exception of C (0.18), the remaining measured traits had repeatabilities of at least 0.35. In general, the assessed traits were all of lower repeatability than the measured traits. The only deviation from this trend was the slightly higher repeatability for AGC than for C (0.19 vs 0.18). In terms of wool production (both greasy and clean), yield and bodyweight, the repeatability estimates in this study are comparable to those reported in the literature which tended to range between 0.5 and 0.8 (Beattie 1961; Morley 1951; Mortimer 1987; Turner and Young 1969;
Young et al. 1960). However the repeatability of fibre diameter from the Condobolin wethers tended to be higher than previous estimates for medium type wools which tended to be between 0.5 and 0.6 (Turner and Young 1969; Young et al. 1960) but similar to the 0.68 reported by both Mullaney et al.
(1970) for fine wool sheep in western Victoria and Sherlock et al. (2003) for ultra-fine Merinos in
New Zealand. This indicates that selection for fibre diameter in fine wool bloodlines may be made at an earlier age than in medium and broader strains of Merinos
Table 1. Repeatability (r2) and age changes (mean ± se) in assessed and measured traits for a mixed bloodline flock
r2 Age (years) Age
2 3 4 5 trend
Assessed traits
Handle 0.33 ± 0.01 2.63 ± 0.03a 2.79 ± 0.02a 2.89 ± 0.03b 3.00 ± 0.03c Kharsh Dust 0.05 ± 0.01 2.24 ± 0.02a 2.77 ± 0.02b 2.67 ± 0.02b 2.88 ± 0.02c Kdust Crimp 0.25 ± 0.01 2.70 ± 0.02a 2.52 ± 0.02b 2.58 ± 0.02c 2.52 ± 0.03d Lcrimp Staple size 0.10 ± 0.01 2.52 ± 0.02a 2.77 ± 0.02b 2.92 ± 0.02c 2.95 ± 0.02c Ksize Density 0.24 ± 0.01 2.63 ± 0.02a 2.79 ± 0.02b 2.72 ± 0.02c 2.81 ± 0.02 b no trend Colour 0.27 ± 0.01 2.85 ± 0.03a 2.41 ± 0.02a 2.32 ± 0.03b 2.26 ± 0.03c Lcolour Style 0.07 ± 0.01 4.64 ± 0.02a 4.71 ± 0.02b 4.75 ± 0.02b 4.87 ± 0.03c Lstyle AGC 0.19 ± 0.01 0.38 ± 0.01a 0.18 ± 0.01b 0.17 ± 0.01c 0.21 ± 0.18d no trend AT 0.11 ± 0.01 0.39 ± 0.02a 0.45 ± 0.02b 0.26 ± 0.02c 0.24 ± 0.03c no trend Measured traits
CSY 0.58 ± 0.01 68.59 ± 0.30a 63.58 ± 0.25b 62.89 ± 0.28c 62.91 ± 0.36c Lyield FD 0.76 ± 0.01 17.82 ± 0.07a 18.20 ± 0.06a 18.24 ± 0.06a 18.66 ± 0.08b Kbroad FDSD 0.56 ± 0.01 3.18 ± 0.02a 3.23 ± 0.02b 3.31 ± 0.02c 3.29 ± 0.02c Ksd FDCV 0.47 ± 0.01 17.86 ± 0.11a 17.83 ± 0.09a 18.26 ± 0.10b 17.78 ± 0.13a no trend CURVE 0.56 ± 0.01 125.95 ± 1.38a 134.47 ±0.96b 139.09± 0.66c 136.54± 0.66d Kcurve SL 0.61 ± 0.01 87.33 ± 0.40a 83.39 ± 0.34b 80.89 ± 0.38c 79.88 ± 0.48d Llength SS 0.35 ± 0.01 28.48 ± 0.41a 29.97 ± 0.35b 32.77 ± 0.38c 35.02 ± 0.48d Kstrong C 0.18 ± 0.01 8.42 ± 0.03a 8.96 ± 0.03b 9.23 ± 0.03c 9.38 ± 0.04d Kyellow RTOC 0.62 ± 0.01 9.75 ± 0.07a 10.43 ± 0.06b 10.89 ± 0.06c 11.15 ± 0.08d Kharsh GFW 0.62 ± 0.01 4.28 ± 0.06a 4.96 ± 0.05b 4.97 ± 0.05c 4.97 ± 0.07bc Kthen Q CFW 0.61 ± 0.01 2.98 ± 0.03a 3.16 ± 0.02b 3.16 ± 0.03b 3.15 ± 0.03b Kthen Q BWT 0.64 ± 0.01 44.05 ± 0.47a 52.35 ± 0.39b 56.38 ± 0.43c 59.33 ± 0.57d Kweight
* Different superscripts within rows denote significant differences (P<0.05).
The repeatability estimates for colour and handle for the Condobolin wethers are similar to those reported by Mullaney et al. (1970) despite their use of a 1 to 3 scale. However it is not possible to compare the repeatability estimates for the other wool quality traits both measured and assessed from the Condobolin wethers as no other estimates of these traits have been reported in the literature. The low repeatabilities for the assessed traits were not surprising given the large impact that the environment has on these traits. This suggests that assessed traits be used cautiously as selection criteria in fine wool sheep.
Across the 11 bloodlines in the Condobolin flock there was a general trend of wool production increasing to a maximum at 3 years of age and then reaching a plateau (Table 1). With the exception of density, AGC, AT, and FDCV which showed no clear trend with age, the remaining wool quality traits, both assessed and measured tended to deteriorate with age. Similar trends in wool production and fibre diameter were identified by Swan and Purvis (2000) who analysed the age changes of the ewe progeny from the CSIRO Fine Wool Project which remained in the breeding flock at Armidale.
Furthermore with the exception of FDCV and SS, the age trends identified in this study are in agreement with those reported by Ponzoni and Fenton (2000) for medium and strong Merino strains.
For crimp and FDSD the magnitude of the difference between 2 and 5 years of age of the Condobolin wethers was the same as that reported by Ponzoni and Fenton (2000) (-0.17 and 0.12µm respectively), however for CSY, SL and SS the difference was at least twice as large. The variance partitioning of the bloodline and micron group interactions with age indicates that for the majority of traits the bloodline interaction accounted for a greater percentage of the total interaction variance with percentages ranging from 52% for SS to 99% for colour. However for dust, crimp, staple, BWT, CSY C and CURVE the reverse was true as the micron group interaction accounted for a greater percentage of the interaction variance (51, 78, 86, 69, 58, 58 and 100% respectively). This indicates that the variation with age occurring in the majority of the wool production and wool quality traits are the result of bloodline differences rather than the micron grouping or fibre diameter of the bloodline.
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