Effects of GnRH administration in the prepubertal heifer and post-partum cow

Abstract

Postpartum cows received 1, 2.5 or 5 µg GnRH intravenously every 2 h for 48 h, as pulsed injections or continuous infusion. Of 6 cows, 4 ovulated after pulsed GnRH infusion; 0, 5 and 3 of 5 cows ovulated after continuous infusion of 1.0, 2.5 and 5.0 µg GnRH resp. Preovulatory LH surges occurred in 17 of 33 cows, and 20 cows showed luteal activity. These cows had higher oestradiol-17? and lower FSH concentrations during the pretreatment period than cows not showing a preovulatory LH surge. 23 four-month old beef heifers received 0.25, 0.5 or 2.5 µg GnRH per h for up to 120 h by continuous intravenous infusion. A further 12 received 5.0 µg GnRH per h for 120 h by subcutaneous infusion with or without progesterone pretreatment. Only 9 of 35 heifers showed luteal function, and the pituitary response was related to oestradiol concentration. A pulsatile pattern of LH release continued during and after constant GnRH infusion. It is concluded that the pituitary/ovarian response to GnRH treatment is governed by follicular state before treatment.