Effect of cortisol acetate on wool quality in sheep selected for divergent staple strength

Abstract

These studies utilised cortisol treatment to clarify the role of stress in reducing staple strength (SS). The first study established the impact of the duration of exposure and nutritional status on SS, wool parameters, and epidermal growth factor (EGF) receptors. Sheep (n = 42) were fed at 0.75 or 1.5 times maintenance for 62 days and then administered with 120 mg cortisol acetate/day for 0, 3, or 12 days. SS was reduced significantly (P < 0.05) only in the group fed below maintenance and treated with cortisol for 12 days. In the sheep fed below maintenance, the mean SS was 31.6, 32.7, and 21.5 N/ktex for groups treated for 0, 3, and 12 days of cortisol, respectively. Cortisol administration in these sheep also increased the rate of fibre shedding (P < 0.001) but did not affect their mean fibre diameter. The shed fibres had a finer diameter than those that were not (P < 0.001). The concentration of EGF receptors was not affected by cortisol administration, although submaintenance feeding (P < 0.05) significantly reduced EGF receptor concentration. The second experiment examined whether some sheep were predisposed to reduce SS in response to cortisol. Sheep (n = 42) of low and high prior SS were fed to maintain liveweight for 84 days and then administered with 0 or 120 mg cortisol/day for 12 days. Cortisol administration significantly reduced SS (P < 0.001) and wool growth (P < 0.001), and increased the rate of fibre shedding (P < 0.001) but did not affect fibre diameter. SS history did not affect the response to cortisol administration. We conclude that cortisol reduced SS by increasing fibre shedding, without decreasing fibre diameter. Treatment was only effective in sheep fed at or below maintenance, and required between 3 and 12 days of exposure to cortisol. This is longer than would be observed in many stressful situations in the field.