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The concentrations in peripheral plasma of oestradiol-17 p, cortisol and progesterone were determined in 10 sows from 5-6 days prepartum to 4 days post partum. At the same time, the presence and composition of mammary secretion was monitored. Plasma progesterone levels declined from 4906flgl-1 (means.e.m.) 12h prepartum to 2605flgl-1 at farrowing, and to about 1 flg 1- I by day 1 post partum, although the timing of the fall in plasma progesterone level ranged from day 4 prepartum to the day of parturition in individual sows. Farrowing in all sows was associated with a fall in plasma oestradiol-17P levels, from 0'680'07 flgl-' to O'13flgl-' 12h after the start of labour, and with a rise in plasma cortisol level from 14'093'74flgl-1 24h prepartum to 43 66  7 06 flg 1- I when lactation was becoming established. Mammary secretion was obtained from individual sows up to 3 days prepartum: the onset of lactation was assessed visually by evaluation of the colour and viscosity of secretions against a six-point scale ranging from no secretion to precolostral secretion to mature milk. Lactogenesis was estimated also from the concentrations of lactose, immunoglobulins G, Na + and K + in mammary secretions obtained postnatally. The timing of the first expression from the teats was not correlated with the onset of lactation as measured by changes in milk composition. Further more, there was no relationship between circulating progesterone, oestradiol-17 p or cortisol levels and the day on which secretion was first expressed. We conclude that, in the sow, lactogenesis, as indicated by changes in milk composition, coincides postnatally with decreased plasma concentrations of oestradiol-17 p, cortisol and progesterone. Furthermore, observation of the colour and viscosity of mammary secretion, rather than analysis of its constituents, may determine lactogenesis inaccurately in sows owing to the rapid but variable onset of lactation in this species.

Perinatal Changes in Plasma Oestradiol-17b, Cortisol and Progesterone and the Initiation of Lactation in Sows