Fading with light and greying with age in the fleece of black Australian Polwarth sheep

Abstract

In order to measure the variation of colour related to some external and internal factors, 136 samples of wools were collected in 1979 and 1980 in a flock of black Polwarth sheep kept in the South of Victoria State, in Australia, 38" latitude South in open air all year long. The genetic formula of these animals at Agouti, Brown and Extension loci was probably aa, B+B+, E+E+. The animals were female or wethers, they were rugged for control or non rugged with a coat opaque to visible and U.V. radiations and cut off at 2.5, 3.5 months or one year of wool growth. The colour measurements were made with a colour atlas based on the D.I.N. System. Except for very rare cases, the growing wool (at the base of the staple) was in the grey scale (which goes from pure white to jet black, without a hue component). With age one could see a greying or silvering of the coloured fleece. This is due to an increase in the number of white fibers and seems independant from light exposure. Due to exposure to solar radiation there was a fading of the staple to the range ot browns. This means the addition of a spectral colour component : the hue, orange in this case. The fading first affects the tip on the staple, about 2 cm after one year of exposure, and is less intense on the average with lighter greys. In a second step the middle staple is affected, after at least 3.5 months. The middle staple fading is less intense but affects a longer length : about 4 cm after one year of exposure. The brown shades after fading overlap the shades genetically induced by mutants at the Brown locus B. There were some rare cases of spontaneous browning from the base of the staple not due to the light of sun but in these animals the hairy parts remained jet black or dark grey. Greying and fading were mainly restricted to the fleece and result probably from physical properties of the wool fibers deriving from their greater fineness compared to the coarse hair of hairy parts. In the fleece there was a range of individual variations inside our Polwarth strain for fading and greying which may be explained by genetical differences. Due to the fading with light the yarns of common mixed coloured wool from spinning mills is in a brown scale : the traditionnal « frieze » (« bure » in French). By rugging the sheep, the discolouration of black and grey fleeces due to light, is easily prevented however it apparently does not have any effect on the progressive greying of fleeces which was seen in some animals.