Clinical and endocrine responses to pulsatile administration of gonadotropin-releasing hormone (GnRH) treatment during lactation in primiparous sows

Abstract

The object of the investigation was to study the clinical and endocrine responses to pulsatile administration of GnRH by means of an automatic injection pump during the early (Days 8–15) or late lactation (Days 21–28) period in primiparous sows. A total of 22 sows were used in two experiments. In Experiment 1, 10 μg GnRH were administered intravenously by pulsatile injection in eight pairs of sows every 89 min for 7 days during the early (Group IA) or late lactation (Group IB) period. In Experiment 2, 10 μg (Group IIA) or 2.5 μg (Group IIB) GnRH were administered intravenously by pulsatile injection in three pairs of sows every 89 min for 7 days during the late lactation period. All sows responded to GnRH treatment with increased peripheral plasma levels of LH. There was no difference in the LH response between the early and the late lactation period. The LH response on the first day of treatment was greater than that on subsequent treatment days. Laparoscopic examination revealed that the follicular size increased in the majority of sows in both experiments. Concomitantly with increased follicular size, there was a gradual rise in the plasma level of oestradiol-17β. In Experiment 1, three sows in Group IA and four sows in Group IB showed lactational oestrus, but only one out of three lactational oestrous sows in Group IA ovulated while all four lactational oestrous sows in Group IB ovulated. In Experiment 2, two sows in each group ovulated during lactation, due to GNRH treatment. The present results demonstrate that lactational oestrus can be induced by pulsatile injection of GnRH every 89 min for 7 days during the early or late lactation period. The stage of the lactation did not influence the LHR response to GnRH. However, a larger number of the sows showed oestrus and ovulated as a result of GnRH treatment during the late lactation than during the early lactation period.