The colour of raw wool is an important attribute affecting processing performance. Since 1984 raw wool sold in New Zealand has been tested for colour prior to sale. In spite of this and the financial penalties for increased yellowness, there has been little change in the colour of New Zealand wool, at least for a major component - full length Crossbred wool. The propensity of wool to develop yellow discolouration is estimated by measuring the colour produced by an environmental challenge in laboratory. This laboratory challenge is designed to mimic the conditions under which yellow discolouration develops on the sheep prior to shearing. A predictive test for this property has been available for some time. The test relies on the estimation of the colour developed in the wax/suint layer on the outside of the fibre. An additional test that measures the colour in the wool fibre has been developed more recently. Both tests increase the mean colour of wool samples and also increased the range, thus increasing the ability to discriminate between fleeces on the basis of their propensity to develop yellow discolouration. The relationship between results of the two tests was highly variable. In the Merino and Corriedale samples there was a wide range in propensity to yellow, indicating selection on that basis would be effective. However, all the wool samples from Crossbreds were highly susceptible to yellowing and the repeatability of the measurements between sampling occasions was poor. Thus the ability to select resistant Crossbred sheep appears to be considerably less than for Merinos and Corriedales. In Merino fleeces, the propensity to discolour was correlated with fibre diameter (mean and standard deviation), indicating that selection for decreased diameter may also reduce yellowness. There was no evidence of similar relationships in the Corriedale and Crossbred samples examined. In flocks of Perendale ewes selected for and against loose wool bulk, the propensity to yellow was moderately correlated with loose wool bulk. This correlation became greater as the environmental challenge increased. This suggests that selection for increasing loose wool bulk may also tend to increase colour at shearing and that care may be needed when dyeing high bulk wools under extreme conditions.
