2001 Fulloon et al. - Livestock Library

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Proc. Assoc. Advmt. Anim. Breed. Genet. Vol 14

513

COMPARISON OF SUBJECTIVE ASSESSMENT AND OBJECTIVE MEASUREMENT FOR THE CLASSING OF FINE WOOL MERINOS

O.A. Fulloon¹, A.A. Swan², L.R. Piper² and J. van der Werf¹

1University of New England, Armidale, NSW 2351

2CSIRO Livestock Industries, Armidale, NSW 2350

SUMMARY

The reliability and profitability of subjective assessment for the classing of Fine Wool Merinos was compared with objective measurement in the form of selection indices using CSIRO Fine Wool Project (FWP) data. Estimated heritability for classer grade using subjective assessment was 0.12.

Classer grade was more strongly correlated with style, colour and body weight than correlations with objectively measured fleece traits. The repeatability between years of classer grade (0.34) was lower than that of greasy fleece weight (0.64) and mean fibre diameter (0.76). Average fleece value was estimated on a per bloodline basis and used to compare selection efficiency of a 12% index selection with classer grade.

Keywords: Profitability, subjective assessment, Merino sheep.

INTRODUCTION

Sheep classing in stud and commercial Merino flocks has traditionally been carried out using subjective assessment. However, since the late 1960’s Merino studs have increasingly combined objective measurement with traditional methods in their annual sheep classing. While this trend is gaining momentum throughout the industry, subjective assessment is still the most common classing method. Objective measurement has the potential to be integrated with existing classing methods to provide greater efficiency in determining grades of Merinos for stud and flock enterprises. This study compares the efficiency of classing based on subjective assessment with index selection in a flock of fine wool Merinos.

MATERIALS AND METHODS

The data used were derived from CSIRO Livestock Industries’ Fine Wool Project flock (FWP), described by Swan et al. (2000). This flock was run on CSIRO research farms from 1990 to 2000, comprising nine fine wool and two medium wool bloodlines. Two hundred breeding ewes per bloodline were mated each year. The bloodline principals or their nominees were invited annually to class the female progeny from their own bloodline. At the time of classing, these animals were approximately 18 months of age, prior to their first mating. A portion of the classed animals were re- classed at 30, 42 and 54 months of age. Numbers of animals classed at each age are shown in Table 1.

The classing involved allocating each sheep to one of three grades: “top”, “middle”, or “cull”.

Classers were provided with greasy fleece weight, mean fibre diameter and body weight records on each sheep, collected at their first shearing at 10 months of age. Use of these measurements was at the classers discretion. For animals assigned to the top and cull grades, “positive” and “negative”

comments on visually assessed wool quality and conformation traits were also recorded. This method is essentially the same as the classing system used in central test sire evaluation schemes in Australia.

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The classers were allowed a small percentage of “double culls” which they deemed to be unsuited to remain in the flock and which were physically culled. There was little variation in the proportion of double culls across bloodlines, and they have been ignored for the purpose of these analyses. All other animals were available as replacements irrespective of their classer grade.

Table 1. Number of animals classed and sires represented at each of the four classing ages

Age (months)

18 30 42 54

Animals 3725 2947 2679 1908

Sires 370 333 308 245

The data were analysed using ASREML (Gilmour et al. 2001). Classer grade at 18 months was analysed as a trait, coded as 1 = top, 2 = middle, and 3 = cull, fitting an animal model allowing the estimation of heritability. Multivariate analyses were also conducted to estimate genetic and phenotypic correlations between classer grade and production traits measured at 21 months. These traits included greasy fleece weight (gfw), clean fleece weight (cfw), yield (yield), mean fibre diameter (mfd), coefficient of variation of fibre diameter (cvfd), standard deviation of fibre diameter (sdfd), staple strength (ss), staple length (sl), style (sty), colour (col) and body weight (bwt). Style was recorded on a 1 to 7 scale where 1 corresponds to the best style grade (Choice). Colour was recorded on a 1 to 5 scale, where 1 = white and 5 = yellow. The repeatability for classer grade was estimated in a model which included classer (bloodline) as a fixed effect. The resulting estimate is therefore a pooled within classer repeatability. Repeatabilities for production traits were estimated using the same fixed effect model.

A 12% micron premium index (assuming a 12% increase in price per kg of clean wool for a one micron reduction in diameter) was constructed, based on genetic and phenotypic parameter estimates from the FWP. The breeding objective traits were hogget measurements of gfw, mfd, cvfd, and bwt.

Index values were calculated for each animal, and used to assign objective top, middle and cull grades in the same proportions applied by the classers.

Results from analyses of New England wool sales between July 1997 and June 2000 were used to calculate fleece value. This was done by calculating a price per kg of clean wool, based on premiums and discounts for mfd, ss, sl, and sty. Fleece values were obtained as price per kg multiplied by cfw.

Average fleece values for classer and index grades within bloodlines were then compared.

RESULTS AND DISCUSSION

Heritability, correlations and repeatability estimates. At the 18 months classing, the estimated heritability of classer grade was 0.12 ± 0.03 and the phenotypic variance 0.69. Estimates of genetic and phenotypic correlations between classer grade and production traits measured at 21 months are shown in Table 2. The correlations between classer grade and fleece traits were low to negligible, except for the genetic correlations involving bwt (-0.27), sty (0.38) and col (0.49). That is, those animals graded as tops by classers tended to be heavier and had higher scores for style and colour.

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Classer grade had little or no association with gfw or mfd, which suggests that little emphasis was placed on these traits during classing.

Table 3 shows that the majority of objectively measured traits have a moderate to high repeatability (0.54 to 0.76), whereas classer grade had a relatively low repeatability at 0.34. This indicates that in these data, objective measurement is more consistent at grading animals across years than subjective assessment.

Figure 1. Fleece value of classer tops and 12% index tops for 21 month fleeces.

Comparison of index and classer in terms of fleece value. From Figure 1, it can be seen that selection on a 12% index consistently resulted in higher average fleece values of tops than classer selection. For some bloodlines the difference in fleece value between selection method is quite small.

Additional work is required to quantify the comparative profitability of index and classer selection.

Table 2. Genetic and phenotypic correlations (±S.E) between classer grade at 18 months and wool production traits recorded at 21 months of age

Trait Genetic Phenotypic gfw 0.01±0.11 -0.06±0.02 cfw -0.09±0.11 -0.09±0.02 mfd 0.07±0.09 -0.01±0.02 cvfd 0.07±0.11 0.10±0.02 sl -0.21±0.10 -0.09±0.02 ss -0.01±0.12 -0.11±0.02 sty 0.38±0.12 0.17±0.02

col 0.49±0.10 0.17±0.02

bwt -0.27±0.09 -0.13±0.02

Table 3. Repeatability (±S.E) across years of major wool production traits and classer grade

Trait Repeatability

gfw 0.64 ± 0.01

cfw 0.59 ± 0.01

mfd 0.76 ± 0.01

cvfd 0.59 ± 0.01

sl 0.54 ± 0.01

ss 0.29 ± 0.01

sty 0.19 ± 0.01

col 0.49 ± 0.01

bwt 0.68 ± 0.01

classer grade 0.34 ± 0.01

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This should take the form of a discounted cash flow analysis which recognises the value of future returns in the current flock and of genetically superior progeny.

Figure 2 shows average higher fleece values for those animals culled by the classer compared to animals culled on the 12% index. This highlights a lower efficiency of classing using subjective assessment.

Figure 2. Fleece value of classer culls and 12% index culls for 21 month fleeces.

CONCLUSION

Merinos selected using objective measurement based indexes produced greater quantities and quality of wool than sheep selected using visual assessment. Consequently, animals selected by index had a higher fleece value than animals selected by visual assessment. Although the classers had access to objective measurements of greasy fleece weight and mean fibre diameter, they appeared not to use them to any significant degree to assist their selection. It was shown that classers tended to place more emphasis on style, colour and body weight. Progress will be more rapid and definite if selection is based on objective measurements, rather than on visual appraisal alone.

REFERENCES

Gilmour, A.R., Cullis, B.R., Welham, S.J and Thompson, R. (2001) “ASREML Ref. Man.” p 239.

Swan, A.A., Purvis, I.W., Piper, L.R., Lamb, P.R and Robinson, G.A. (2000) In “Finewool 2000:

Breeding for customer needs”, p 65. CSIRO and The Woolmark Company.

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