The concentration of lactose in the mammary secretion from individual glands of two sows increased significantly (P < 0·01) between 0 and 24 h after parturition. In six sows studied during the perinatal period there was a negative correlation (r = -0 .80; P < 0·02) at parturition between the concentration of progesterone in the blood and the concentration of lactose in the mammary secretion. Furthermore, the increase in concentration of lactose in the mammary secretion after parturition was related to the timing of the decline of plasma progesterone to low levels. The results indicate that the initiation of lactation occurs within 24 h of parturition in most sows, and the results are consistent with the hypothesis that progesterone withdrawal acts as the 'trigger'. Neither the changes in corticosteroid binding globulin nor the changes in total corticosteroids were temporally related to the initiation of lactation. However, a circadian rhythm was observed for total corticosteroids in the blood of three out of nine lactating and pregnant sows, whereas no circadian rhythm was observed in progesterone of the four pregnant sows. The results are discussed in relation to the disease complex mastitis-metritis-agalactia.

The effect of progesterone and prostaglandin administration on the timing of farrowing was studied in three groups of 25 sows each .. Progesterone treatment (100 mg/day) on days 112, 113 and 114 of gestation (group I) significantly prolonged the gestation length to 116 A±0·4 (mean±s.e.) days compared to the control sows (group III; 115·5 ±O· 2; P < 0 ·05). Administration of prostaglandin (200 I1g Cloprostanol intramuscularly) on day 115 of gestation following progesterone treatment (group II) resulted in a gestation length of 116 ·0 ±0·1 days, with the sows farrowing 25 ·4± 1 ·0 h after the prostaglandin injection. 80 % of the sows farrowed between 0800 and 1700 h of day 116 of gestation.Plasma progesterone levels were maintained by the exogenous progesterone during treatment. At farrowing, higher levels of progesterone were observed in groups I and II compared to controls. Prostaglandin treatment did not significantly alter withdrawal of progesterone· in progesterone treated sows, suggesting that the actions of exogenous prostaglandin is primarily on the myometrium and the cervix.Hormonal treatment in late pregnancy did not have any adverse effects on piglet viability and growth rate, or subsequent reproductive performances of sows. Lactation was initiated normally, and the concentrations of lactose, protein, fat, IgG, Na +, Ca2+ and K + in colostrum and milk were similar in all groups during the first 5 days of lactation.

Although it has been demonstrated in the rat that the local ovarian IGF-l concentration is elevated after growth hormone administration in vivo , the influence of growth hormone treatment and elevated serum IGF-I concentrations on ovulation rate has not been determined.Therefore, an experiment was undertaken to test the hypothesis that exogenous growth hormone would increase the ovulation rate in post-pubertal gilts.MATERIALS AND METHODS Thirty-two prepubertal gilts tYorkshire x Landrace) were selected at 78.0 + 1.5 kg body weightand 138.0 + 3.5 dofage, andtransported from the finishing to the breeding barn on a daily basis. The gilts were exposed to a mature boar for 30 min d I and observed for signs of estrus.Fourteen days after the onset of puberty the gilts were assigned to one of two treatment groups involving the daily injection (i.m.) of either porcine growth hormone (1.0 mg ml--r of 1.7 IU mg -' pGH in carbonate-buffered saline, pH 9.4, administered at 9O p.g kg-' bodyweight; GH, n :20) or vehicle buffer (CT, n = l2). The growth hormone dose employed was similar to that considered to give an optimal improvement in growth rate (Etherton et al. 1987) and which, from a pilot study conducted in our laboratory, also resulted in a marked increase in serum IGF-I concentrations (Kirkwood, unpublished data

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