Submitted to: Professional Animal Scientist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/20/2005
Publication Date: 11/1/2005
Citation: Walker, R.S., Enns, R.M., Geary, T.W., Wamsley, N.W., Mortimer, R.G., Lashell, B.A., Zalesky, D.D. 2005. Evaluation gonadotropin-releasing hormone at fixed-time ai in beef heifers synchronized using a modified co-synch plus cidr protocol. Professional Animal Scientist 21:449-454 Interpretive Summary: Inseminating beef heifers for timed artificial insemination (TAI) at 54 h using a modified CO-Synch protocol with a CIDR insert produced acceptable pregnancy rates to timed AI for all locations (mean = 50.7%). In the current study, synchronization with GnRH+CIDR+PG induced cyclicity and ovulation in prepubertal heifers. While TAI pregnancy rates were not improved for heifers receiving GnRH at 54 h TAI in two locations, this injection was beneficial for heifers at the third location. These results warrant further studies to determine factors leading to the cause of pregnancy rate variations observed in response to GnRH administration at TAI.
Technical Abstract: The objectives of this study were to determine if a second injection of GnRH at timed AI (TAI) increases the percentage of induced ovulations and improves pregnancy rates in beef heifers synchronized with the CO-Synch plus CIDR protocol. Nulliparous crossbred beef heifers (n = 375, BW = 362.7 kg, body condition score, BCS = 5.6) at three locations (Colorado [CO; n = 39], Wyoming [WY; n = 125] and South Dakota [SD; n = 211]) were stratified by BW, within BCS and location, and randomly allotted to one of two treatments. All heifers received 100 µg of GnRH with a CIDR insert on d 0, followed by CIDR removal and 25 mg of PGF2' (PG) on d 7. At 54 hours post PG, heifers were artificially inseminated and either received (TRMT) or did not receive (CON) a second injection of GnRH. Blood samples were collected at d -10 and 0 to determine estrous cyclicity status from CO and WY heifers. Ultrasonography was used to classify diameter of the dominant follicle at TAI and percentage of heifers ovulating within 40 h after TAI at the CO and WY locations. Estrous cyclicity rates were greater (P < 0.01) for heifers at CO (97.4%) than WY (46.4%). No location x treatment interaction occurred and TAI pregnancy rates were similar between treatment groups and for cycling and prepubertal heifers at CO and WY. However, TAI pregnancy rates were greater (P < 0.05) for heifers in the TRMT (54.2%) vs CON group (40.4%) at SD. Body weight did not alter TAI pregnancy rates for either treatment group, but at the SD location pregnancy rates to timed AI tended to decrease (P = 0.08) for heifers with body weights greater than 409.1 kg (39%) compared to those with body weights less than 409.1 kg (53.2%). The percentage of CO and WY heifers ovulating a follicle within 40 h after GnRH were similar and ovulation rates tended to be greater (P = 0.10) for TRMT heifers (81.3 and 73.9%) vs CON heifers (62.5 and 66.7%), respectively. Timed AI pregnancy rates were higher (P < 0.05) for CO and WY heifers ovulating by 40 h after TAI (50 and 61.8%) vs heifers that had not ovulated by 40 h after TAI (16.7 and 16.7%). We conclude that synchronizing beef heifers with a modified CO-Synch plus CIDR protocol induces ovulation in cycling and prepubertal heifers and produces acceptable pregnancy rates at 54 h TAI. The value of incorporating a second injection of GnRH at timed AI remains questionable.