EFFECTS OF TEMPORARY CALF REMOVAL AND OSMOTIC PUMP DELIVERY OF GONADOTROPIN-RELEASING HORMONE ON SYNCHRONIZED ESTRUS, CONCEPTION TO A TIMED ARTIFICIAL-INSEMINATION AND GONADOTROPIN-SECRETION IN NORGESTOMET-ESTRADIOL VALERATE-TREATED CATTLE
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The objective of this study was to determine whether a constant, lowdose infusion of gonadotropin-releasing hormone (GnRH) following norgestomet-estradiol valerate estrus synchronization (SMB) and temporary calf removal (CR) would enhance fertility to a timed, 48-h artificial insemination (AI) in cattle. A subcutaneous osmotic pump was employed to deliver approximately 2.5 g GnRH per hour for 6 to 7 d following norgestomet implant removal. In Experiment 1, suckled Brahman crossbred cows (n = 221) were assigned randomly to one of four treatments: 1) SMB (n = 58); 2) SMB+48-h CR (SMB+CR, n = 55); 3) SMB+GnRH (n = 56); and 4) SMB+CR+GnRH (n = 52). Virgin heifers (n = 69), 12 to 14 months of age, were allotted to Treatments 1 and 3. The CR (cows) and GnRH (cows and heifers) treatments began at SMB implant removal and all females were inseminated 48 h later. Experiment 2 assessed luteinizing hormone (LH) secretion following SMB implant removal and(or) osmotic pump insertion in 44 cyclic cows (n = 11/treatment group) treated as in Experiment 1. Cows received SMB treatment either early (Days 3 to 5) or late (Days 11 to 15) in the estrous cycle (n = 22/period). Conception rate at timed AI for SMB+CR was superior (50.9%; P<0.025) to all other treatments (SMB, 27.5%; SMB+GnRH, 26.9%; SMB+CR+GnRH, 26.9%). Gonadotropin-releasing hormone attenuated the positive effects of CR by reducing (P<0.02) cumulative pregnancy rates on Day 24 in SMB+CR+GnRH cows. Conception at timed AI in virgin SMB heifers (52.9%) was higher (P<0.025) than SMB+GnRH heifers (25.7%), and it remained higher (P<0.05) on Day 24 (85.3% vs 60.0%). Gonadotropin-releasing hormone stimulated a premature, preovulatory-like surge of LH which peaked 6.80.6 h following pump insertion. As employed here, GnRH reduced fertility by disrupting the normal sequence of endogenous endocrine events in progestogen-primed females. 1989.
