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1116
Requirement of GnRH administration at the onset of the 5 day CO-Synch + CIDR protocol in suckled beef cows
The objective of this experiment was to evaluate the requirement of GnRH administration at controlled internal drug release (CIDR) insertion in the 5-d CO-Synch + CIDR protocol (5dCO). Postpartum, suckled beef cows (n = 2159) from 11 herds at 5 universities were assigned by age, BCS, and days postpartum to receive either: 1) standard 5dCO hormone administration including 100 µl of GnRH at CIDR insert and 2 concurrent 25-mg doses of PGF2α (PG) at CIDR removal (G2PG), 2) no GnRH at CIDR insert and 2 concurrent, 25-mg doses of PG at CIDR removal (NoG2PG), or 3) no GnRH at CIDR insert and a single, 25-mg dose of PG at CIDR removal (NoG1PG). Estrous response between PG and timed-AI (TAI) was determined using estrous detection aids. All cows were TAI 72 h after CIDR removal, concurrent with administration of 100 µl of GnRH. Estrous cyclicity prior to synchronization was determined using a combination of 2 blood samples collected 10 d apart and estrus detection aids administered approximately 24 d prior to CIDR insert. Transrectal ultrasonography was used on a subset of cows at both CIDR insert and removal to record all ovarian structures as well as to detect pregnancy 31 to 42 d after TAI. Data were analyzed using the MIXED and GLIMMIX procedures of SAS for continuous and binary response variables, respectively. Herd nested within university was included as a random effect. Number of total follicles and size of the largest 2 follicles at CIDR insertion were not different (P ≥ 0.34). However, the largest follicle at CIDR removal was greater (P = 0.02) in NoG2PG than G2PG and NoG1PG (13.2, 11.5, and 12.1 ± 0.5 mm, respectively). Though estrus response was not different (P = 0.99) prior to TAI, detection aid activation was more advanced (P = 0.01) in NoG1PG than G2PG and NoG2PG. However, pregnancy to TAI did not differ (P = 0.66) among G2PG (55.4 %), NoG2PG (52.8 %), and NoG1PG (50.5 %) treatments. Cows exhibiting estrus prior to TAI had greater (P < 0.001) TAI pregnancy rates (58.1 %) than those not exhibiting estrus (39.3 %) and cows determined to be cyclic at synchronization initiation had greater (P< 0.001) TAI pregnancy rates (53.6 %) than non-cyclic cows (37.1 %). In conclusion, TAI pregnancy rates were not negatively affected by removal of initial GnRH in the 5-d CO-Synch + CIDR protocol.
Keywords:
5-d CO-Synch, GnRH, Synchronization