Luteinizing hormone response to different doses of synthetic gonadotropin-releasing hormone during early and late lactation in primiparous sows

Abstract

The object of this investigation was to study the luteinizing hormone (LH) response to different doses of synthetic exogenous gonadotropin-releasing hormone (GnRH) during early (days 7–15) and late (days 20–28) lactation in primiparous sows. Five crossbred primiparous lactating sows were used in this study. The lactation period was 35 days. Five doses of GnRH, 0, 2.5, 5.0, 10.0 and 20.0μg were administered via a jugular vein catheter every second day in a latin square design during both the early and late lactation period. Daily (9.00 a.m.) blood samples were taken for oestradiol–17β and progesterone analysis. Frequent blood samples were taken before and after GnRH treatment for LH analysis. The total LH response was measured from post-treatment samples as the area under the curve above the base level, obtained from pre-treatment samples. No elevation of plasma oestradiol–17β or plasma progesterone occurred during lactation. The LH response increased with increasing doses of GnRH during early as well as late lactation. The total LH response during late lactation was a little greater than during early lactation, but the difference was not significant ( P=0.055). There was no effect of the time sequence of treatment within phase of lactation.