Effect of continuous infusion of a GNRH agonist (Buserelin™) on ovarian hormone secretion and estrous cycle length in cows

Abstract

The importance of the ovarian steroid hormones estradiol and progesterone in the control of luteolysis in domestic ruminants is well established. However, there is a lack of studies specifically investigating the effect of stimulating “hysiological” changes in endogenous estradiol or progesterone secretion on subsequent luteolysis. In this study we have stimulated endogenous ovarian hormone secretion by infusion of the GnRH analogue, Buserelin™, and have assessed the effect of these changes on the timing of luteolysis. Concentrations of estradiol and progesterone were monitored in plasma samples collected from 6 mature, cyclic, lactating, Friesian cows during a control cycle and during a cycle in which Buserelin™ was infused via osmotic minipump (8.6 µg/h) for 28 days starting on Day 2 of the cycle. Buserelin™ infusion had little effect on progesterone secretion but did result in a marked stimulation of estradiol secretion from Days 6 to 10 of the cycle (treated cycle 4.3±0.2 pg/mL; control cycle 1.8±0.3 pg/mL; P< 0.001). In addition, there was a significant advancement in the timing of luteolysis during the Buserelin™ -infused cycle (Day 19.3±0.3 compared with Day 21.3±0.4; P<0.01). In this study, we have found that infusion of buserelin caused both a significant stimulation of estradiol secretion from the first follicle and a significant advancement in the timing of luteolysis. We hypothesise that the increased secretion of estradiol may have been involved in causing this advancement of luteolysis.