Responsiveness of Merino wethers with different staple strengths to adrenocortical stimulation

Abstract

Animal Production in Australia 1998 Vol. 22 RESPONSIVENESS OF MERINO WETHERS WITH DIFFERENT STAPLE STRENGTHS TO ADRENOCORTICAL STIMULATION A.C. SCHLINKAB, N.R. ADAMSAB, J.M. LEAAB, J.R. BRIEGELAB and A.J.M. RITCHIE A B C C CSIRO Division of Animal Production, Private Bag, PO Wembley, WA 6014 CRC for Premium Quality Wool, Fitzroy, Vic 3065 Agriculture Western Australia, GSARI, PO Box 757, Katanning, WA 6317 Thwaites (1972) demonstrated that cortisol administration to sheep on a low plane of nutrition resulted in a depression in fibre diameter and fibre shedding which is associated with tender wool. Schlink and Dollin (1995) showed that declining staple strength was significantly related to an increasing incidence of shed fibres at the point of break (POB). In this experiment we have examined whether sheep with different staple strength phenotypes respond similarly to cortisol administration. Four groups of Merino wethers with either low or high staple strength (average of 3 shearings, 20.6 and 30.5 N/ktex, respectively) were allocated as follows: (1) low staple strength, placebo (n=11); (2) low staple strength, cortisol (n=11); (3) high staple strength, placebo (n=10); (4) high staple strength, cortisol (n=10). Sheep were offered a maintenance ration of 88% hammermilled oaten hay, 10% lupins and 2% mineral mix for 20 weeks. At week 12 the sheep were injected intra-muscularly daily for 12 days with a placebo or 120 mg cortisol. Blood samples were collected for plasma glucose determinations. Liveweights were recorded weekly, and dyebands placed in the wool at regular intervals. Sheep were shorn approximately 4 months after completion of the experiment. Mid-side wool samples were collected for staple strength determination and the POB examined for incidence of shed fibres and fibre diameter. Dyebands were used to calculated the time of the POB. Results are shown in Table 1. Table 1. Ef fects of intramuscular cor tisol administration on the staple strength of wool fro m sheep with low or high staple strength Low Staple Strength Cortisol Staple strength (N/ktex) A FD at POB (mm) Shed fibres (%) FD shed fibres (mm) B POB (days) A High Staple Strength Nil 45.3 18.0 1.4 17.5 33 B Probability Phenotype 0.049 0.999 0.954 0.997 0.583 Cortisol 0.001 0.142 0.001 0.007 0.047 Nil 39.3 17.6 0.9 17.2 28 120 mg/day 28.4 17.2 7.1 14.0 19 120 mg/day 33.3 16.7 6.5 13.7 23 Fibre diameter (FD) of broken fibres at the point of break; Days from start of injections Liveweight was maintained for the 20 weeks of pen feeding. Staple strength averaged 33.9 and 39.3 N/ktex for low and high staple strength phenotypes, respectively and cortisol treatment reduced staple strength from 42.3 to 30.9 N/ktex. After 12 days of cortisol administration plasma glucose was elevated to 102 mg/dl compared with 66 mg/dl for the placebo groups (P<0.001) with no significant differences between the staple strength phenotypes. Likewise cortisol treatment did not significantly differentiate between phenotypes in average diameter of broken fibres at the POB, proportion of shed fibres, average diameter of shed fibres or staple POB. Cortisol did not affect the average diameter of broken fibres at the POB, but significantly increased the proportion of shed fibres at the POB. Shed fibres had a significantly lower average diameter at the POB than continuous fibres. Average diameter of continuous fibres at the POB was not affected by cortisol administration, contrary to the findings of Thwaites (1972). However, cortisol increased the level of fibre shedding and shed fibres were the finer. This suggests a critical minimum wool follicle bulb size before the follicle bulb is responsive to cortisol stimulation. It would follow from Thwaitess (1972) observation that a low plane of nutrition is required to produce a staple strength response to cortisol treatment. We conclude that the phenotype differences in staple strength did not result from differential responsiveness to stress as represented by cortisol injections. This work was partly funded by the International Wool Secretariat. SCHLINK, A.C. and DOLLIN, A.E. (1995). Wool Technol. Sheep Breed. 43, 268-84. THWAITES, C.J. (1972). Anim. Prod. 15, 39-46. 420