Mineral and Amino Acid Composition of Wool from New Zealand ...

Aust. J. Agric. Res., 1994, 45, 391-401

Mineral and Amino Acid Composition of Wool from New Zealand Merino Sheep Differing in Susceptibility to Yellowing F. J. Aitken,A D. J. C ~ t t l e , T. ~ ?C. ~ ~ e i d * ,and ~ B. R.

WilkinsonA

Department of Wool Science, Lincoln University, Canterbury, New Zealand. Department of Wool and Animal Science, University of New South Wales, P.O. Box 1, Kensington, NSW 2033. To whom correspondence should be addressed. A

Abstract The susceptibility of wool to yellowing was determined on samples from fleeces of 36 Merino wethers by a laboratory procedure involving incubation at 40°C for 5 days. The sheep, although originating from several different flocks, had been grazing together for 6 years. These susceptibilities were compared with the concentrations of suint and wax in the fleece, with fibre diameter, and with mineral concentrations in suint or yolk from the greasy wool. While the susceptibility of the fleeces was correlated with suint content ( r = 0.852), the highest correlations were with the potassium concentrations in either suint before ( r = 0.947) or yolk after ( r = 0.938) incubation. There were significant differences in the concentration of potassium in the wool after washing, between the four most resistant and the four most susceptible fleeces, and in the effect of incubation on these potassium concentrations. No differences were detected in concentrations of other minerals in the clean wool nor in the relative proportions of amino acids.

Keywords: Wool colour; propensity; suint minerals; potassium concentration.

Introduction Yellow discolourations are a common cause of concern in wool processing. About 13%, 2%, and 0.1% of wool sale lots in Australia are subjectively assessed as having light, medium and heavy non-scourable colour respectively. In New Zealand, mean measured base colour (Y-Z) of wool sold at auction in 1987-88 was 4.2, ranging from -1.5 to 14.0 (Maddever 1989). These discolourations may occur during growth, storage, or processing and may limit the end use of the wool. Yellow discolouration of raw wool is classified as scourable or non-scourable, but there is no clear distinction between these two categories (Wilkinson 1982). The yellow discolouration of wool may vary from a diffuse yellow associated with the yolk fraction, to a band of bright yellow bound to the fibre or a band of fleece rot which may or may not be yellow (Henderson 1968). In individual fleeces the expression of yellow discolouration is dependent on propensity to yellow and on environmental and fleece conditions enabling expression of this characteristic. Propensity to yellow varies between fleeces. Only after fleeces are challenged will the inherent propensity be expressed. Thus

F. J. Aitken et al.

measurement of colour prior to challenge provides little or no indication of propensity of fleeces to yellow. A predictive test for propensity to yellow, based on incubating samples of greasy fleece at 40°C and high humidity for 5 days, was described by Wilkinson (1981). This test is useful for indicating relative resistance or susceptibility to yellow discolouration faults (Wilkinson 1981) or liability to fleece rot (Raadsma and Wilkinson 1990), allowing identification and ranking of fleeces according to relative resistance to yellowing. The biological relationship between propensity to yellow and liability to fleece rot is not understood. Development of the two conditions requires similar environmental challenge. The phenotypic correlations between measures of the colour of greasy wool and fleece rot were the strongest and most consistent of the characteristics studied by James et al. (1987). Genetic correlations observed ranged from 0.19 to 0.55. Greasy wool colour and propensity to yellow are good indicators of liability to fleece rot, and either would be useful as an indirect selection criteria of fleece rot (Raadsma and Wilkinson 1990). While differences between fleeces which are resistant or susceptible to fleece rot or yellowing are not well understood, they do appear to be related to the amount and properties of suint and yolk. It is typical for susceptible fleeces to have a low wax/suint ratio (Chipalkatti et al. 1965; David and Lead 1982; Lipson et al. 1982; Wilkinson and Aitken 1985). One feature often associated with yellow banded fleeces is high yield, reflecting low amounts of protective grease layers on wool fibres. This arises not only from removal of suint from the fleece by rain, but also from scouring of grease, for which suint may act as a detergent (Hay and Mills 1982a; James et al. 1984). While several greasy fleece characteristics and fibre measurements have been related to susceptibility to yellow (Wilkinson 1981), none has proved as successful as the predictive test. Studies reported in this paper were aimed at determining some chemical differences between resistant and susceptible fleeces as identified by the predictive test. Such information would aid in understanding the basis of wool yellowing and assist in development of more rapid tests for propensity to yellow or for liability to fleece rot.

Materials and Met hods Animals Thirty-six sheep were selected from the Central Otago Merino wether trial (Cottle and Wilkinson 1989) by stratified random sampling based on a visual score of the appearance of the greasy wool from 1 to 10. Mid-side patch samples were collected from each of these sheep 1 month before shearing. The samples represented approximately 1 year's wool growth. The sheep in the Central Otago Merino wether trial originated from 30 flocks. At the time of sampling, all animals were 7 years old and had grazed together for 6 years. They thus represented animals with a range of fleeces which had been subjected to the same environmental challenge.

Climate The sheep used in this trial were grazed in the dry environment of Central Otago (mean annual rainfall, 250-280 mm). They were sampled 68 days after a 17-day period over which there had been approximately 80 mm of rain or snow.

Suint Minerals and Propensity of Wool to Yellow

Wool Preparation Fibre diameter, yield, wax and suint content were determined on full length wool samples. For all other tests, butt ends of staples of greasy wool were prepared by cutting tips off at the weathering line and removing vegetable matter by hand. This procedure removed the dirty tips and the vegetable contamination. The samples were blended before subsampling for analysis.

Incubation Procedure In all incubations, the water used was saturated with thymol t o prevent growth of fungi. The samples incubated for the colour test and for the analysis of incubated yolk were prepared and incubated together.

Incubation Method The predictive test used was a modification of that of Wilkinson (1981) and Raadsma and Wilkinson (1990). Briefly, 3.0 g of wool was incubated in 25x75 mm flat-bottomed glass tubes with 1 . 4 mL thymol saturated water at 40°C, 100% relative humidity for 5 days. A further 1 . 4 mL thymol saturated water was added on the third day. After incubation, colour was extracted from the samples by soaking them in 12.0 mL 63% acetone for 2 h. The acetone was then extracted by squeezing the wool in a modified syringe. pH was measured on these extracts immediately after extraction. The extracts were then sealed, left to stand for 1-3 days to clear, filtered and the absorbance read at 430 nm. Under these conditions, increasing susceptibility is indicated by increasing absorbance.

Fibre Diameter Fibre diameter was measured on scoured, blended wool samples by Airflow (IWTO-28-82).

Chemical Assays Fresh suint Samples of greasy wool (2.0 g) were soaked in thymol saturated water (8 mL) for 30 min and pH measured on the extract (4.7 mL). The extraction was repeated and potassium, sodium, magnesium, calcium, copper, and zinc concentrations were measured on the combined extracts by atomic absorption spectrophotometry or flame photometry. Phosphate was determined on combined extracts by the stannous chloride-hydrazine method (Lawrence 1974).

Yolk extracted from incubated samples After incubating in tubes as above, wool samples (3.0 g) were extracted by soaking in 12 mL thymol saturated water for 4 h. pH was measured on the yolk thus extracted. After adjusting the volume to 10 mL, potassium, sodium and calcium were determined by flame photometry or atomic absorption spectrophotometry.

Clean wool Samples from the four most resistant and four most susceptible fleeces, as defined by the predictive test, were selected for analysis of mineral and amino acid contents in the clean wool before and after incubation. Greasy wool was washed by soaking and rinsing three times in each of, commercial hexane, commercial absolute ethanol and distilled water. After drying, subsamples of these were analysed for minerals by atomic absorption spectrophotometry, flame photometry, or colorimetry following acid digestion. Additional subsamples were subjected to acid hydrolysis and 17 common amino acids determined by reversed phase H.P.L.C. (Cohen and Strydom 1988). The amino acids determined by this procedure were alanine, arginine, aspartic acid, cyst (e)ine, glutamic acid, glycine, histidine,

F. J. Aitken et al.

isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tyrosine and valine.

Yield, wax and m i n t content Wax content was measured by soxhlet extraction in distilled Shell X4 commercial hexane and evaporated to dryness. After air drying the wool, suint was extracted by soaking the wool for 5 min in distilled water, rinsing with 200 mL distilled water, filtering and evaporating the filtrate to dryness. After further thorough'washing in tap water to remove dirt, and rinsing in two changes of distilled water, wool was dried at below 60°C and weighed at 16% regain to determine yield.

Statistical Analysis Data were analysed by regression analysis using the Minitab computer package. The degree of colour developed during incubation, as measured by the absorbance (at 430 nm) of the acetone extract after incubation, was used to indicate the degree of resistance or susceptibility t o yellowing. Data on the mineral and amino acid content of the four resistant and four susceptible fleeces were analysed using a two sample t test.

Results The animals sampled in this study had originated from several different properties. They had been grazing as one group for 6 years before sampling. The data presented here therefore represents a wide range of animals grazing in the same environment. No attempt has been made to account for differences between originating flocks. Mean, standard error and range of absorbance values are given in Table 1. Mean values, their standard errors and correlations with absorbance are given for visual colour score, fleece fractions and fibre diameter in Table 1, for minerals and pH extracted in fresh suint before incubation in Table 2, and in yolk after incubation in Table 3. There was a considerable range in absorbance values, indicating a range in susceptibility to yellowing. The high correlation between absorbance and visual appraisal score ( r = 0.858) suggests some challenge and colour development had occurred before sampling. Table 1. Absorbance in the predictive colour test, and visual score, wool fractions and fibre diameter and their relationships with colour in the predictive colour test (n = 36) Suint and wax contents are expressed on the basis of clean, dry wool

~bsorbance~ Visual score (1-10) Suint (%) Wax (%) Wax/suint Yield (%) Fibre diameter (pm)

Mean

s.e.m.

Range

correlationA

0.223 2-58 5.45 27-94 7.25 73.10 23.82

0.030 0.274 0-584 1.48 0.788 1.15 0.443

0-04-0-63 1-6 1.43-15.42 11.99-54.59 1.61-20.41 55.40-86.53 19.4-29-7

0.858 0.852 -0.009 -0- 716 -0.199 0.690

A Correlations with absorbance (at 430 nm) of acetone extract of wool incubated in the predictive colour test. Absorbance (at 430 nm) of acetone extract of wool incubated in the predictive colour test.


There was a high correlation between absorbance at 430 nm and suint content (Table 1). Although the correlation coefficient between waxlsuint ratio and absorbance was moderate ( r = -0.716), the relationship was markedly curved. The relationship between the logarithms of absorbance and of suint/wax ratio was a straight line with a higher correlation (r = -0- 898) than that for the untransformed data. Correlations of absorbance with wax content and yield were both low, while that with fibre diameter was moderate. Concentrations of all minerals analysed in fresh suint (Table 2) or in yolk after incubation (Table 3) were all positively correlated with absorbance. Potassium, the mineral in highest concentration in both suint and yolk, showed the greatest range of concentrations (50-fold) and the highest correlation with absorbance. In fresh suint , the only other mineral accounting for more than 66% of the variance in absorbance was phosphorus. In incubated yolk, the two pH measurements were both highly correlated with absorbance, with the pH of the acetone extract, being more highly correlated. Both these pH measurements of yolk after incubation (Table 3), were more highly correlated with absorbance than was pH of fresh suint before incubation (Table 2). Table 2.

Concentrations of minerals and pH of fresh suint from wools with a range of resistance and susceptibility to yellowing ( n = 36) Mineral concentrations are expressed in terms of greasy wool extracted

Mean

s.e.m.

Range

correlationA

A Correlations with absorbance (at 430 nm) of acetone extract of wool incubated in the predictive colour test.

Table 3.

Concentrations of minerals and pH of yolk extracted after incubation from wools differing in resistance and susceptibility to yellowing (n = 36) Mineral concentrations are expressed in terms of greasy wool incubated

K ( g kg-') Na (mg kg-') Ca (mg kg-') PHW pH,,

Mean

s.e.m.

Range

5.73 353.0 59.28 7.78 8.15

1-11 52.5 8.68 0.047 0.064

0.10-22.73 38.0-1149.0 7-0-239.0 7-35-8-55 7-60-9.15

correlationA 0.938 0.643 0 776 0.802 0.937

-

Correlations with absorbance (at 430 nm) of acetone extract of wool incubated in the predictive colour test.

A

Analysis of the data before incubation indicated that suint content accounted for 71.8% of the variance in absorbance, and inclusion of the wax content increased this significantly (R2 = 77-5%). Wax content alone accounted for none

F. J. Aitken et al.

of the variance in absorbance. The best predictor variable for absorbance was potassium concentration in the suint (R2 = 89.5%). Inclusion of suint magnesium concentration (R2 = 92 2%), log of wax/suint ratio (R2 = 93 - 0%), and suint phosphorus (R2 = 94 - 4%) each improved the prediction significantly. The best predictor of the log transformed absorbance was log(wax/suint ratio) (R2 = 80.1%). Inclusion of suint potassium (R2 = 89.1%) and magnesium (R2 = 90 - 1%) each improved the prediction significantly. Of the variates measured after incubation, potassium concentration of yolk was the best predictor of absorbance (R2 = 8707%). Inclusion of pH of the acetone extract of the yolk (R2 = 93.3%) increased this prediction significantly. Yolk sodium (R2 = 94.3%) and calcium (R2 = 94 9%) concentrations each had small but significant effects on this prediction. Thus the two variates with greatest ability t o predict absorbance were concentration of potassium either in suint before incubation, or in yolk after incubation: Absorbance = 0 -0439

+ 0 O295*[Kfs]

R~ = 89.5%

Absorbance = 0.0766

+ 0.0256' [Kiy]

R~ = 87.7%

Absorbance is the absorbance at 430 nm of the 63% acetone extract of the incubated wool in the predictive colour test; [Kfs] is the potassium concentration in suint before incubation (g kg-' of greasy wool); [Ki,] is the potassium concentration in yolk after incubation (g kg-' of greasy wool). Inclusion of magnesium concentration of suint in the regression for suint and the pH of the acetone extract of incubated yolk in the regression for yolk significantly improved the prediction of absorbance in both cases: Absorbance = 0 - 0405

Absorbance = -1.75

+ 0.0393' [Kfs]- 0.OO26*[Mgfs]

+ 0.0138' [Kiy]+ 0 - 232*pHac

R~ = 92 2%

R~ = 93 - 3%

[Mgfs ] is the magnesium concentration of suint before incubation (mg kg-' of greasy wool) ; pHac is the pH of the yolk extracted in acetone after incubation. The other significant variates had relatively small effects on these relationships. Mean concentrations of minerals in the clean wool and the standard errors of the differences between the means for resistant and susceptible fleeces before and after incubation are given in Table 4. The concentration of potassium in the clean fibre of the resistant fleeces tested was much lower than in fibre from the


susceptible fleeces, both before and after incubation. In addition, whereas only 25% of the potassium was lost from the susceptible fleeces on incubation, all that in the resistant fleeces was lost. There were no differences between resistant and susceptible fleeces in the concentrations of any other minerals in the fibre, nor in the effect of incubation on those concentrations. Table 4. Mineral content before and after incubation of the four most resistant and four most susceptible fleeces to yellowing: mean and standard deviation Mineral concentrations are expressed in terms of clean wool analysed Before incubation ~ e s . ~ ~ u s c . ~ Sig.

A

After incubation Res.

Susc.

Sig.

Effect of incubation Res. Susc.

Mean of four most resistant fleeces. Mean of four most susceptible fleeces.

There were no significant differences in the proportions of amino acids in the fibre between the four most resistant and the four most susceptible fleeces, nor in the effects of incubation on the proportions of amino acids analysed. Discussion

Warm temperatures and high humidity promote a scourable diffuse yellow colour in wool, and with further wetting, promote unscourable canary yellowing. Colour develops while on the sheep or during storage, in the presence of alkaline suint and aided by moisture, heat and impaired ventilation (Hoare and Stewart 1971). Under these conditions, the grease coating on the fibres is degraded. This degraded wax with suint and bacterial products form yolk. A hypothesis is that in susceptible fleeces, wool fibres absorb potentially yellow products from water-soluble components of yolk. During prolonged rainfall suint acts as a detergent to aid removal of the wax from around wool fibres. At the same time the chemical composition of wax is also altered (Hay and Mills 1 9 8 2 ~ ) .These changes and those in pH of yolk during wetting (Hay and Mills 1 9 8 2 ~ James ; et al. 1984) or incubation (Tables 2 and 3) may alter the structure of the fibres, allowing potential yellowing compounds into the fibres. Without these conditions, fleeces with a high propensity to develop yellow discolourations may appear white, but can still develop yellow colour during storage and processing. The incubation procedure used in this

F. J. Aitken et al.

trial provides an environmental challenge allowing expression of the propensity to yellow. High potential to yellow is a characteristic feature of susceptible wools and is in part genetically determined (Wilkinson and Aitken 1985; Raadsma and Wilkinson 1990). Selecting optimum shearing time can limit exposure of full fleece to high humidity and temperature before shearing. This will reduce the yellow discolouration evident in the fleece. However, susceptible fleeces may still become yellow in storage, during processing or in a laboratory test such as that used in this study. There is evidence that wool yellowing and fleece rot may be related. Propensity to yellow is a good indicator of liability to fleece rot (Raadsma and Wilkinson 1990) and greasy wool colour is correlated with fleece rot (James et al. 1987). Similar fleece and environmental conditions give rise to the two conditions. Relationships with high suint content has been noted for yellowing (David and Lead 1982; Wilkinson and Aitken 1985), and for fleece rot (Lipson 1978). James et al. (1987) reported a weak relationship between suint content and fleece rot. Watts et al. (1981), who reported no relationship between suint content and fleece rot, attributed that finding to the effect of recent heavy rains. As suint is by definition water soluble, wetting of the fleece would change the relationship between suint content and susceptibility to fleece rot or yellowing (Raadsma and Thornberry 1988). Suint consists largely of cations (principally potassium), anions of non-volatile organic acids and other compounds including cholesterol (James et al. 1984). Such a mixture may act as a detergent for grease on wool and aid its removal by rainfall (Hay and Mills 1982). In addition, as suint is highly hygroscopic (Lipson et al. 1982), high levels would encourage retention of moisture within the fleece. The speed of drying or retention of water within fleeces can be important indicators of susceptibility t o fleece rot (Evans and McGuirk 1983; Raadsma 1989). Fleeces resistant to fleece rot or yellowing have been variously described as higher (Hayman 1953; James et al. 1987), or similar (Lipson 1978; Lipson et al. 1982; Evans and McGuirk 1983; Wilkinson and Aitken 1985) in wax content than those which are more susceptible. In this study, there was no correlation between wax content and resistance or susceptibility to yellowing (Table 1). Wax removed from the fibre is not replaced (Hay and I'vlills 19823). The reported differences in the association between wax content and resistance or susceptibility to yellowing or fleece rot could thus reflect removal of wax by rainfall in the presence of differing levels of suint observed. Data reported by Wilkinson and Aitken (1985) were obtained from sheep shorn, washed and then housed for 9 months. Their findings therefore reflect the genetic potential of the sheep unaffected by rainfall. As the sheep used in this study had not been exposed t o rainfall for 68 days, the effect of rainfall on the wax and suint content of the wool would be minimal. The hypothesis outlined above relates integrity of the wax layers to resistance to yellowing and fleece rot. The level of suint or the relative amounts of wax and suint, as reflected in the waxlsuint ratio, are important in determining resistance to yellowing or fleece rot. In this study, relative susceptibility to yellowing was correlated with suint content (positive), and waxlsuint ratio (negative). In several other studies (Chipalkatti et al. 1965; Lipson 1978; David and Lead 1982; Lipson


et al. 1982; Wilkinson and Aitken 1985), either suint contents were lower, or waxlsuint ratios higher, in resistant than in susceptible fleeces. Large differences were observed between resistant and susceptible fleeces in potassium concentration in suint, in yolk and in the wool fibre. That all the potassium in resistant fleeces was lost from the fibre during incubation (Table 4) suggests that it was only loosely bound to the fibre. Loss of potassium in susceptible fleeces was less, both in absolute and relative terms (25%), than in resistant fleeces (100%). This suggests that potassium may be bound more tightly to the fibre in susceptible than in resistant fleeces. Whether this is a fundamental difference between resistant and susceptible fleeces is not known, but it could reflect differences in associated anions and in their binding to the fibre. Other authors (Hoare and Stewart 1971) have suggested that all wools have high levels of potassium. As noted above, susceptible fleeces with high levels of suint are likely to lose more suint in a shower of rain than do fleeces of resistant sheep, thus reducing differences between resist ant and susceptible fleeces. Across different strains of Australian Merino, liability to fleece rot increased with increasing fibre diameter (Raadsma et al. 1989). In this trial there was a significant correlation between susceptibility to yellowing and fibre diameter. Sheep were selected for this trial across a range of observed differences in the appearance of the greasy wool. The relationship shown between fibre diameter and susceptibility to yellowing may have been influenced by the selection procedure used. Changes in skin or fleece pH have been implicated in fleece rot or yellowing (Lipson 1978; Lipson et al. 1982; Wilkinson 1982). Other workers (Evans and McGuirk 1983) detected no relationship between pH and fleece rot. Correlations between pH and susceptibility to yellowing were detected in this trial, both in suint before (Table 2) and in yolk after (Table 3) incubation. The correlations of absorbance with yolk pH after incubation were higher than those with suint pH before incubation. Differences before incubation presumably reflect a degree of colour development which had occurred in these fleeces before sampling, even in conditions not particularly conducive to discolouration. In spite of the dry environment, animals were selected for this trial across a range of visually detected differences in the appearance of the greasy wool. This suggests that some challenge had occurred before shearing in early spring (September). Incubation increases colour development, and increases the correlation between absorbance and pH. This suggests that the pH changes observed in this paper resulted from changes occurring during colour development, rather than being causes of yellowing. The highest correlations with susceptibility and the greatest range of values between resistant and susceptible fleeces in this trial lay in the potassium concentrations in either the suint or yolk. Highly significant differences were also detected in the potassium concentrations in clean wool (Table 4). Differences between resistant and susceptible fleeces in the proportions of amino acids in the fibre protein were negligible in comparison to differences in the mineral concentrations. Characteristics other than amino acid composition appear to account for differences in resistance or susceptibility to yellowing. Because of these large differences in potassium concentrations, and the high correlations with susceptibility to yellowing, it is tempting to ascribe to them

F. J . Aitken et al.

some causative relationship with yellowing. That a considerable amount of potassium was lost from fleeces during incubation suggests that it was not primarily responsible for colour development. As discussed above, the role of potassium may be in forming, with associated anions, detergent that acts to remove the wax layer from the fibre when washed in rain. Many anions associated with potassium in fleeces are those of organic acids, presumably derived from wool wax. Such salts of weak acids have high pH. This may be sufficient to give incubated yolk the high pH observed here (up to mean of 8-83) and may damage the fibre, allowing colour forming compounds to enter. With the ranges of potassium concentrations observed (Tables 2 and 3), either before or after incubation, and the close relationships between potassium and predictive colour, it may be possible to use potassium concentration as an indicator of potential to yellowness. It may also be possible at shearing to separate out wools with varying degrees of susceptibility to yellowing.

Acknowledgments One author (TCR) was supported on a New Zealand Wool Board Post Doctoral Fellowship during preparation of the manuscript. The authors thank Dr J. R. Sedcole, Centre for Con~putingand Biornetrics, Lincoln University, for assistance with the statistical analysis, Peter Isherwood, Department of Animal Science, Lincoln University for mineral analysis; and Andrew Watson, Textile Chemistry Section, Wool Research Organisation of New Zealand (Inc.) for amino acid analyses. Katie Bridges, Department of Wool Science, Lincoln University, provided valuable technical assistance.

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Lawrence, R. (1974). Assay of serum inorganic phosphate without deproteinization. Automated and manual micromethods. Ann. Clin. Biochem. 11(6), 234-7. Lipson, M. (1978). The significance of certain fleece properties in susceptibility of sheep to fleece rot. Wool Technol. Sheep Breed. 25(3), 27-32. Lipson, M., Hilton, R. A., Watts, J. E., and Merritt, G. C. (1982). Factors influencing fleece rot in sheep. Aust. J. Exp. Agric. Anim. Husb. 22, 168-72. Maddever, D. C. (1989). Vector space analysis of the New Zealand Wool Board's auction data. M. Appl. Sc. Thesis. Lincoln University. Raadsma, H. W. (1989). Fleece rot and body strike in Merino sheep. 111. Significance of the fleece moisture following experimental induction of fleece rot. Aust. J. Agric. Res. 40, 897-912. Raadsma, H. W., Gilmour, A. R., and Paxton, W. J. (1989). Fleece rot and body strike in Merino sheep. 11. Phenotypic and genetic variation in liability to fleece rot following experimental induction. Aust. J. Agric. Res. 40, 207-20. Raadsma, H. W., and Thornberry, K. J. (1988). Relationship between wax, suint and fleece rot: effect of sample preparation, time of sampling and fleece rot induction. Aust. J. Exp. Agric. 28, 29-36. Raadsma, H. W., and Wilkinson, B. R. (1990). Fleece rot and body strike in Merino sheep. IV. Experimental evaluation of traits related to greasy wool colour for indirect selection against fleece rot. Aust. J. Agric. Res. 41, 139-53. Watts, J. E., Merritt, G. C., Lunney, H. W. M., Bennett, N. W., and Dennis, J. A. (1981). Observations on the fibre diameter variation of sheep in relation to fleece-rot and body strike susceptibility. Aust. Vet. J. 57, 372-6. Wilkinson, B. R. (1981). Studies on wool yellowing. Part I. Prediction of susceptibility to yellow discolouration in greasy fleeces. Wool Technol. Sheep Breed. 24(4), 169-74, Wilkinson, B. R. (1982). Yellowing in wool. Wool. 7(4), 9-12. Wilkinson, B. R., and Aitken, F. J. (1985). Resistance and susceptibility to fleece yellowing and relationships with scoured colour. Proc. N. 2. Soc. Anim. Prod. 45, 209-211.

Manuscript received 13 July 1992, accepted 2 September 1993