Fleece changes as a sheep matures so the age of a sheep when its fleece
is sampled can have a big impact on the results. Fleece will probably change
most in the first three years of the sheep’s life. A great deal of that
change occurs in the first 12 – 18 months. Keep this in mind when looking at
lamb histograms.
Fleece usually continues to change throughout the life of the sheep, but
at a much slower rate than during the growth from lamb to mature animal.
Adult sheep with a slow rate of fleece change maintain their fleece quality
longer. A desirable histogram from an older sheep is more impressive (and
the sheep more potentially valuable in a breeding program) than the same
histogram in a lamb or young sheep.
As sheep become elderly, the rate of change in their fleeces may increase
again. This may result from poorer nutrition due to change in their status
in the flock or a change in general health, some other, genetic factors or a
combination. Their histograms may show this with an increase in the SD and
CV. As many sheep also continue to become coarser with age, you can expect
the AFD to increase also. Many age grey fibres are coarser than the
pigmented fibres of youth.
In addition, fleece growth reflects differences in nutrition and
environment throughout the year. This causes changes in diameter along the
length of each fibre. At the extreme, the diameter is reduced enough to
result in a break point in the fleece. This may happen because of stress,
lack of protein or in the case of some Shetlands, as a natural shedding
mechanism as summer approaches (the Rise).
