Abstract
This experiment was conducted to determine the extent to which susceptibility to cortisol-induced follicle shutdown is influenced by fleece phentotype. Twenty Finewool (10 sheep low fibre diameter, low coefficient of fibre diameter-LL and 10 low fibre diameter, high coefficient of variation of fibre diameter-LH) and twenty Strongwool (10 low fibre diameter, low coefficient of variation of fibre diameter-HL and 10 high fibre diameter and high coefficient of variation of fibre diameter-HH) sheep of 9 months of age were individually penned in an animal house and were injected intramuscularly with an aqueous suspension of hydrocortisone acetate at a rate of 1.42 mg/kg body weight for a period of two weeks. Fibre diameter was measured from clipped tattooed patch wool samples. Follicle activity was measured by histological changes in skin biopsies taken weekly. Blood samples were collected at two-week intervals and plasma cortisol measured. Increased plasma cortisol concentration significantly (p<0.05) reduced clean wool production and mean fibre diameter dropped to its lowest level four weeks after commencement and two weeks after the cessation of cortisol injection. Elevation of plasma cortisol concentration significantly (p<0.0001) increased the percentage of inactive follicles two weeks after injection started. High fibre diameter groups (Strongwool sheep; i.e. HL+HH) had significantly (p<0.0001) higher percentage of follicle shutdown than low fibre diameter groups (Finewool sheep; i.e. LL+LH). Average percentage of shutdown follicles for Finewool (LL+LH) and Strongwool (HL+HH) Merino sheep was 9.8±0.9 and 13.5±0.9 respectively. Shutdown of primary follicles was more pronounced in Finewool than Strongwool sheep. There was no significant effect of coefficient of variation of fibre diameter on propensity to follicle shutdown induced by exogenous cortisol. It is concluded that elevation in plasma cortisol concentration is inhibitory to the normal activity of follicles in Strong-wool sheep but that variation in fibre diameter has little or no effect.
Highlights
This experiment was conducted to determine the extent to which susceptibility to cortisol-induced follicle shutdown is influenced by fleece phentotype
Twenty Finewool (10 sheep low fibre diameter, low coefficient of fibre diameter-LL and 10 low fibre diameter, high coefficient of variation of fibre diameter-LH) and twenty Strongwool (10 low fibre diameter, low coefficient of variation of fibre diameter-HL and 10 high fibre diameter and high coefficient of variation of fibre diameter-HH) sheep of 9 months of age were individually penned in an animal house and were injected intramuscularly with an aqueous suspension of hydrocortisone acetate at a rate of 1.42 mg/kg body weight for a period of two weeks
There was no significant interaction between week and group indicating that all 4 groups had a similar pattern of elevation in plasma cortisol concentration
Summary
This experiment was conducted to determine the extent to which susceptibility to cortisol-induced follicle shutdown is influenced by fleece phentotype. Ansari-Renani and follicle inactivity and fibre shedding have been induced Hynd (2001) reported that daily intramuscular injection of 5 under conditions of ACTH treatment Successive increases in plasma cortisol level up to 30 ng/ml between primary and secondary follicles, and inhibited wool growth in sheep on a restricted plane of considerable variation was evident between individual secondary follicles This raises the question ‘why do follicle types differ in susceptibility to shutdown’? To test the hypothesis that sheep follicle types differ in susceptibility to cortisol-induced shutdown, 2 groups of Finewool (LL = Low fibre diameter, low coefficient of variation of fibre diameter and LH = Low fibre diameter, high coefficient of variation of fibre diameter) and 2 groups of Strongwool (HL = High fibre diameter, low coefficient of variation of fibre diameter and HH = High fibre diameter, high coefficient of variation of fibre diameter) Merino sheep were selected. Wool growth rate and percentage of follicle shutdown were measured and compared between the fleece phenotype groups
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