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Department of Animal Science and Production, University of Western Australia, Nedlands, Western Australia 6009, Australia
(Received 13 March 1978)
The activity of the adrenal gland is believed to be governed by the secretion of corticotrophin (ACTH) in a positive stimulus/negative feedback equilibrium. There is increasing evidence that in man, the secretion of corticosteroids never actually reaches a steady-state condition and that the circadian rhythm displayed by these hormones in the circulation is therefore the result of a number of secretory episodes over a 24 h period (Hellman, Nakada, Curti, Weitzman, Kream, Roffwarg, Ellman, Fukushima & Gallagher, 1970; Weitzman, Fukushima, Nogeire, Roffwarg, Gallagher & Hellman, 1971). Data presented by McNatty, Cashmore & Young (1972) also raise the possibility that a similar pattern of hormone release may exist in the sheep. However, McNatty et al. (1972) collected samples relatively infrequently and it is hard to define peaks in cortisol concentration. With more
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Three Merino ewes, adapted for about 3 weeks to their environment, were bled at 10 min intervals through a jugular venous cannula. Radioimmunoassay of plasma samples for cortisol revealed marked diurnal variations with peak levels just after midnight and lowest values in the afternoon. This rhythm appeared to result from a changing amplitude associated with a distinct ultradian rhythm (frequency 0·8–1·2 cycles/h) in the plasma level of cortisol. Calculation of the daily rate of secretion of cortisol from the hormone profiles gave a mean value of 8·49 mg. Arguments are put forward in favour of this method for obtaining the true rate of secretion of cortisol.
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SUMMARY
Lactation was induced in non-pregnant, intact ewes. The injection of 60 mg progesterone plus 240 μg oestradiol benzoate every third day for 60 days increased udder size. Subsequent daily injection of 10 mg dexamethasone trimethylacetate or 5 mg oestradiol benzoate plus 12·5 mg progesterone for 6 days led to further udder development and initiated secretion of fluid similar in appearance and composition to normal ovine milk.
Both treatments were equally effective in initiating milk secretion and subsequent production (approximately 0·50 kg/day) was similar to that of ewes lactating after normal pregnancy [0·59 ± 0·06 (s.e.m.) kg/day]. Glands of ewes receiving no treatment other than milking after oestrogen and progesterone priming remained small in size and produced only trivial volumes of secretion when milked out daily for 14 days.
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SUMMARY
Subcutaneous injections of oestradiol benzoate (OB) were given daily to lactating ewes for 7 days beginning 20 (early lactation) or 46 (late lactation) days after parturition.
Injections of 50 and 500 μg OB enhanced milk yield in ewes during late lactation and resulted in significant increases in the proportion and daily production of milk components. During early lactation, injections of 500 μg OB failed to enhance milk yield but significant increases in the fat content of milk were recorded.
The administration of 5000 μg OB resulted in a marked decline in milk yield. In milk from these ewes there was a fall in lactose content with a concomitant rise in the number of leucocytes and proportion of vacuolated macrophages suggesting that mammary involution had begun.
