The measurement of, and factors affecting, the strength of attachment of wool to the skin of sheep

Abstract

To measure wool loosening before and after treatment with defleecing agents a 2 kg spring balance was used to measure the force required to pluck wool staples from sheep. The results are expressed as g force per fibre or as g force per unit of staple linear density (kTex), where 1 kTex = area staple cross section (mm2) x wool specific gravity (1 -3). The linear density of each staple plucked can be determined by a gravimetric method or more simply and quickly by using a pair of hand calipers that have been previously calibrated. Where fibre diameter is known an estimate can be made of the number of fibres present. The average force required to pluck a fibre from the follicle (depilation force) is about 15 to 25% of that required to break the fibre. The optimum reduction in fibre strength for purposes of harvesting weakened wool lies somewhere between the depilation and breaking forces. There are breed differences in depilation force that cannot be entirely explained by differences in fibre diameter, SIP ratio, fibre density or follicle depth, although a high degree of follicle bending in fine-wool Merinos (mean score 5.6 vs 3.6 for other breeds) may cause the higher than expected values observed for depilation force in fine-wool Merinos. The depilation force was found to vary within animals from day to day as well as between animals. It increased (ca. 30%) following an increase in intake from 600 to 1200 g day-I and decreased rapidly (ca. 40%) for up to 3-5 hours after death. Oral dosing of four sheep with cyclophosphamide (CPA) at 12 mg kg-1 had no effect on the rate of wool loosening for the first four days, but there was an exponential reduction to 40% of the control values over the next six days. Injecting six sheep with a micronized preparation of dexamethasone-21 -isonicotinate (Voren �) caused an immediate loosening of the wool but at a slower rate, the maximum extent (25%of the control values) not being achieved until after 40 days, lnjecting 27 mg of L-thyroxine eight days after dosing had no effect on this response or the rate of wool regrowth. Voren and CPA given together to four sheep acted synergistically, resulting in depilation forces about 20% of the control values 30 days later.