Simple test for wool yellowing susceptibility

Press, 16 July 1982, Page 16

Simple test for wool yellowing susceptibility

, The susceptibility of woe fleeces to yellow discolours tion can be predicted sue cessfully by a simple proces which could even be carrier out by farmers, according ti Dr B. R. Wilkinson of th< Wool Science Department Lincoln College. Dr Wilkin son told the opening sessioi of the Massey Wool Associa tion of New Zealand annua conference in Christchurcl this week that fleece disco louration was a significanl problem to the wool processing trade and often resulted in a grower's fleece wools being marked down in price. But it was a highly heritable wool factor and now that the Wool Science Department had devised a simple test for susceptibility before yellowing occurred, farmers could select to try to avoid the problem, if they wished. The test is an incubation, which accelerates the condi- , tions which lead to wool yellowing and is so straightforward that Dr Wilkinson suggested that farmers might even be able to use their own hot water cupboards for the procedure. Samples of wool are clipped from the midside of the sheep and incubated for several days in a small glass phial at blood heat and with as high a humidity as possible. The tips are clipped oft the wool fibres. to prevent colour contamination from dirty wool. A mandatory requirement was the inclusion of a small amount of water at the base of the sample. Colour develops at the I base of the samples and is subjectively assessed. At Lincoln a flock of Corriedales has been selected for the last 15 years on the basis of fleece yellowness or whiteness. Samples from this flock have been incubated and the results ranked according to the incubation scores and compared with the historical data of assessed fleece colour at November shearing from 1974 through to 1979. "The division of sheep into resistant and susceptible groups in the flock was confirmed by the incubation techniques and there was a high degree of agreement between the incubation' scores and the historical data,’’. Dr Wilkinspn told the Massey Wool Association. “The incubation technique did not depend on the incidence of fleece yellowness in the field, but rather predicted which fleeces were most likely to discolour under the warm moist conditions typically associated with this fault." he said. "Whether in fact the fleeces did discolour in the field depended on subsequent climatic conditions that prevailed. "Fleece architecture appeared to have only a secondary, rather than a primary role since-the incubation reaction occurred with both short and long wools and was independent 1 of staple configuration.”

He said a number of variations in the technique had established that the fleece pH was not a satisfactory indicator of susceptibility. “Furthermore, it was found that the pH of midside patch samples clipped monthly from May to September were similar in May for resistant and susceptible fleeces but appreciably different in September, with susceptible fleece having a much higher pH. “In other words, the pH of susceptible fleeces was high only when the fleeces were yellow, not while they were still white.” he said. Dr Wilkinson said that there were four commonly recognised kinds of wool yellowing. The first was canary yellow, which does not scour out and commonly affects the belly wool and lower edges of the fleece, particularly cotted wool and clips sown later in the year.

1 The second was fleece ro or yellow banding, which i: horizontal banding across th< staple an'd usually scours out The third is a diffuse over all colour within the fleece called butter-coloured yolk which is generally consid ered to be fully scourable. "Greasy fleeces may alsc discolour during storage anc while it is by no mean; certain that this discolouration is different from the ■ third kind mentioned, the characteristic brownish tinge to the discolouration suggests some difference,” said Dr Wilkinson. The names are descriptive rather than scientific and there is a lack of clear distinction between the four conditions. They may be variations in degree of the same problem. For sheep owners, yellow fleeces pose two problems. The first is that yellow wool is worth less than white wool. The other problem is a husbandry and management one — fleeces with diffuse yellow discolouration are prone to fleece rot and other faults such as bacterial staining, some of which are not scourable.

Blowflies are attracted tc these fleeces and the resulting damage and distress •caused by maggots to flyblown sheep are a very serious problem," he said. “The problem for the buyer is essentially one of deciding to what extent the discolouration is scourable, how the discolouration will limit flexibility of end use and whether the discolourations are related to other forms of wool fault such as tenderness. - “The conditions of high moisture and temperature in the fleece which provoked the discolouration are likely to be detrimental to the fibre. “From all points of view the simple solution is to grow white wool, but this is not a simple matter. “Because the problem does not occur in all years, the ability of the breeder to identify susceptible animals is restricted. "Indeed, if identification of susceptible sheep must wait, until the problem occurs, it is rather too late to avoid it. “An additional problem is that sheep in one locality may not have discoloured fleeces, but could be badly affected in another locality where the climate is different. “This is an acute problem for farmers buying rams from a breeder and transferring them to their own properties. “Furthermore, management practices, such as early.

shearing may 'hide' suscepti bility to yellow discoloura 1 tion, as the problem is usu ally associated with Ion; wool in spring and summer at least in Canterbury. “While fleece colour is ai important selection criterion it is by no means the only one. so that progress toward’ improving the colour of r clip is moderated by the necessity to select for'those fleece characteristics which have most influence or price.” Dr Wilkinson said that among the fleece characteristics which had been associated with yellowing were: wax content, suint content, wax/suint ratio, suint pH, soluble nitrogen, fibre diametre and variability, staple length, handle, greasy fleece weight, fleece variability and fleece architecture. The Lincoln College W 7 ool Science team also investigated some of the relationships between the scouring processes and susceptibility to yellowing in fleeces, using the incubation technique. "Scoured wools gave no colour reaction in the incubator, but tended to putrify quite rapidly, as did raw wools degreased with an organic solvent. "Raw wools washed in

cold water alone gave a much reduced colour reaction but remained fresh. “This supported other work showing that wool grease acts, as a bacteriostat in the fleece, limiting bacterial growth. "An unexpected finding was that wools resistant to discolouration remained sound on prolonged incubation whereas susceptible wools degraded under the same conditions. “The end result of this degradation was the appearance of free cells, suggesting that the intercellular cement had been attacked. "From the manufacturer’s point of view there was something sinister about the limited data we obtained.

“It appears as though the resistant wools were sound under conditions of the laboratory procedures, but susceptible wools were inclined to degrade even though their starting colour was white.

"Some of the laboratory procedures were not dissimilar from the conditions found in scours and dye baths, suggesting perhaps that the fleece conditions in susceptible wools caused a degree of fibre weakness, even though the conditions had not provoked discolouration at the

time of sampling," he said. Dr Wilkinson said the department had succeeded in isolating a compound from fleeces which is in high concentration in susceptible fleeces and either absent or in low concentration in resistant wools. The compound, if left in the laboratory, goes yellow after a few days.

"This material is the subject of continuing work, as identification of the pigment may assist us in establishing rather more clearly how it develops," he said.

He said the heritability of susceptibility to discolouration was high at 0.63 to 0.67 and this gave farmers the scope to select away from the problem. To shear early, before the onset of fleece length and weather conditions likely to result in yellowing, “was hardly fair,” said Dr Wilkinson.

Growers could send their fleece samples to Lincoln College for incubation, and several were already doing this.

Then susceptibility to yellowing could become one more flock selection criterion, although Dr Wilkinson admitted there were more important selection criteria.