EFFECT OF STAGE OF LACTATION AND OF DAILY INJECTIONS OF GRF AND TRF ON SERUM CORTISOL IN LACTATING SOWS

Two experiments were conducted with 51 primiparous sows to determine the effects of stage of lactation and treatment with growth hormone-releasing factor (GRF) and thyrotropin-releasing factor (TRF) on serum cortisol concentrations and pulse profiles. Animals were injected with either saline, GRF (20 μg kg−1 in both experiments), TRF (1 μg kg−1 and 9 μg kg−1 in exps. 1 and 2, respectively) or GRF-TRF (administered in combination at the levels mentioned above). Peptides were given subcutaneously twice daily between days 5 and 25 postpartum. In exp. 1, mean cortisol levels were higher on day 5 than on subsequent sampling days due to higher peak amplitudes. This result is discussed with respect to stress. Mean serum cortisol concentrations did not vary significantly between days 5, 15 and 25 of lactation in exp. 2. No evidence of a morning peak and late afternoon low was observed in cortisol secretion of sows during lactation. Finally, none of the hypothalamic factor treatments altered the basal serum concentrations or the peak amplitudes of cortisol. However, in exp. 2, the frequency of cortisol peaks was lower for the GRF- and TRF-treated sows than for the control and GRF-TRF sows. Key words: Cortisol, GRF, TRF, lactation, sows