Pituitary and Ovarian Function in Postpartum Beef Cows. III.Induction of Estrus, Ovulation and Luteal Function with Intermittent Small-Dose Injections of GnRH1

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Abstract
The effects of intermittent small-dose injections of gonadotropin-releasing hormone (GnRH) and/or exogenous estradiol-17.beta. (E2) on estrus, ovulation and luteal function were examined. Fifty-five anestrous postpartum beef cows were allotted at random to 1 of 5 treatment groups (11 cows/group). The groups were designated: suckled control (SC, no GnRH or E2); GnRH; E2; GnRH + E2; and weaned control (WC, no GnRH or E2, calves weaned on day 20 .+-. 2 postpartum). E2 implants and/or injections of GnRH (500 ng/injection every 2 h) were given for 4 days starting on day 20 .+-. 2 postpartum. The intermittent injections of GnRH induced pulses of luteinizing hormone (LH) (2.8-3.3 ng/ml) and FSH (57.3-65.4 ng/ml) that were greater (P < 0.01) in magnitude than basal concentrations of LH and FSH in the SC and E2 groups. Cows in the GnRH and GnRH + E2 groups had more (P < 0.01) pulses of LH (1.4-1.9/h) and FSH (2.0-2.4/4 h) than did cows in the E2 and SC groups. Cows in the WC group also had more (P < 0.01) pulses of LH and FSH than did cows in the SC group. The exogenous E2 reduced (P < 0.05) the frequency of LH pulses but had no effect (P > 0.10) on the frequency of FSH pulses. The magnitude of the LH and FSH pulses was similar (P > 0.10) between the 5 treatment groups Serum E2 concentrations were increased (P < 0.01) in cows treated with GnRH and/or exogenous E2. Also, WC cows had higher (P < 0.01) E2 concentrations than SC cows. Cows in the GnRH, GnRH + E2 and WC groups had shorter (P < 0.01) postpartum intervals than cows in the SC and E2 groups since more (P < 0.01) of these cows had exhibited estrus and ovulated by 8 days posttreatment. Cows in the E2 group had postpartum intervals that were 10 days longer than cows in the SC group, indicating that reduction of pulsatile LH release can prolong the postpartum interval. All cows that ovulated in response to either the GnRH injections or weaning continued to cycle. However, a large percentage of GnRH and WC cows had short (< 17 days) 1st estrous cycles. Exogenous E2 appeared to overcome this problem since almost all of the cows in the GnRH + E2 group normal length 1st estrous cycles. Suckling evidently prolongs the postpartum interval of the beef cow by reducing the frequency of pulsatile GnRH release from the hypothalamus.

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