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Cashmere growth cycle and skin histology

AUTHOR: Barry McDonald,

Queensland Department of Primary Industries, Animal Research Institute, Yeerongpilly, Qld. 4105

The fleece of the cashmere-bearing goat contains two different fibre types:

(i) Cashmere fibres

(ii) Hair fibres

Cashmere fibres grow in secondary follicles in the skin (Plate 5). They have a mean diameter of around 15 um, mean maximum length of about 42mm and are non-medullated (of solid appearance) as shown in Plate 1.

Plate 1. Photomicrograph of portions of non­medullated fibres showing brush end cashmere (upper left), brush end hair (lower centre) and mid-shaft region of cashmere (all others).

Hair fibres grow in primary follicles in the skin (Plate 5). They have a mean diameter approaching 80 um, mean maximum length of around 63mm and are medullated (hollow appearance) as shown in Plate 2.

Medullation is not always continuous along the length of a hair fibre as illustrated by the absence of medullation prior to hair brush end formation in Plate 1.

Plate 2. Photomircrograph of portions of hair fibres showing black central medulla and normal root bulb (lower left), medulla mid-shaft region (upper and lower left and lower right), non­medullated shaft above brush end (upper right) and cashmere fibres (all others).

In the newborn kid goat the density of fibre follicles in the skin is around 15 per mm2 and the mean ratio of secondary to primary follicles (S/P ratio) is generally less than 1:1 as shown in Plate 3.

Plate 3. Photomicrograph of a section of skin taken from a newborn kid goat showing groups of three primary follicles containing hollow hairs with few developing solid secondary follicles, all surrounded by a thin layer of clear connective tissue.

The initiation and development of both secondary and primary follicles between birth and age 28 days of age is quite marked. Follicle density at this age is also influenced by an increase in skin area. At age one month, the follicle density is about 37 per mm2 and the S/P ratio is approximately 5:1 (Plate 4).

Plate 4. Photomicrograph of a section of skin taken from a 28 day old kid goat showing groups of three primary follicles containing hollow hairs with secondary follicles nearby and containing solid cashmere fibres, all surrounded by clear connective tissue.

As the goat grows two changes occur in the skin. Firstly, the are of skin increases markedly and therefore the density of follicles decreases and secondly, there is some further initiation and development of secondary follicles. The adult goat has a skin follicle density approaching a mean of 18 per mm2 and a mean S/P ratio of approximately 7:1 (Plate 5).

Plate 5. Photomicrograph of a section of skin taken from an adult goat showing groups of three primary follicles containing large hollow (medullated) hairs. Associated with the hair follicles are many small densely packed secondary follicles containing solid cashmere fibres. The clearer region between the follicle groups is connective tissue.

The fleece of cashmere-bearing goats undergoes a seasonal cycle of changes which are generally controlled by day-length. The cycle consists of a period of fibre growth, then fibre regression followed by a follicle resting phase during which no fibre is grown. One (three-phase) cycle is normally completed in 12 months.

During fibre regression, the root bulb of the fibre forms a brush end as shown in Plate 1. At this stage the fibre is held in the follicle by the enlarged brush end and does not undergo growth since it has withdrawn from the growing region of the follicle. Brush end formation occurs prior to fibre shedding when the fibre is lost from the follicle. When large numbers of adjacent fibres are shed from their follicles, fleece-casting occurs and is seen as regions of fleece separating from the skin.

The timing of cyclical phases is different for cashmere and hair fibres. The growing phase of hair occurs earlier than that of cashmere and hair fibres as a group undergo a longer regressive period than cashmere. There is some overlap of the cycles of different fibres so when the majority of cashmere fibres are shed from their follicles, the goat is protected by new, growing hair fibres.

Considering the annual cashmere cycle, a range in the timing of events has been observed. These differences may be due to latitude or the development of different growth cycles in feral goats as we know them or the effect of pregnancy and lactation. Wethers, maiden does and some kids have been seen to grow cashmere in summer.

There are reports of cashmere growth cycles which are complete by May. Other work has shown that the majority of cashmere is grown between April and August. It appears that many does grow little cashmere during lactation resulting in the majority of growth occurring in autumn and early winter.

The matter of cashmere fibre shedding and fleece-casting is less complex than cashmere growth. In the latitudes between and including 27 degrees and 33 degrees (Brisbane to Sydney), cashmere shedding occurs between the end of July and October. The maximum ratio of cashmere to hair fibres (C/H ratio) in the fleece occurs just prior to the fibre shedding phase. Often the C/H ratio of the fleece does not achieve the theoretical maximum, as indicated by the S/P ratio of the skin. This is due to the early (prior to July) loss of some cashmere fibres which have formed brush ends.

It is worth noting that fibre shedding and fleece-casting is sometimes seen to occur following periods of stress for the animal. These stresses may include disease, transport, kidding for the doe and joining for the buck. This spontaneous loss of fleece is generally not related to the seasonal cycle which has been described.

An understanding of cyclical changes in the fleece of cashmere-bearing goats and factors which effect the cycle has several important practical consequences.

1. Maximum cashmere yield is achieved when the growth cycle is complete but cashmere shedding has not commenced.

2. The C/H ratio declines rapidly after fibre shedding has commenced, resulting in large cashmere losses.

3. Any attempts to stimulate cashmere production during the regressive and resting phase of the cycle will not directly increase cashmere yield.

4. Management practises designed to increase production during the active cashmere growth phase, should be implemented.

5. The density of secondary fibre follicles and the size of the adult goat will influence the per head yield of cashmere.

These conclusions give rise to the following recommendations:

1. Where possible, shear goats before the end of July, but manage animals to minimise the impact of cold stress.

2. The flock should be inspected carefully in late November and those goats showing cashmere growth should be preferentially managed (for parasites, health and nutrition). Inspection again in April will indicate whether the cashmere growth phase is complete and preferential management may be ceased.

3. The balance of the flock not assessed as growing cashmere in November and previously lactating does, should be preferentially managed from April to July to maximise cashmere growth and subsequent yields.

4. Avoid stresses during the cashmere growth phase.

5. Fleece sampling for objective measurement should be performed at the same time each year, preferably in July.

6. Breeding goats (particularly bucks) should be selected with a view to increasing secondary follicle density and cashmere-bearing skin surface area of the resultant offspring.

© 1985 ACGA