Submitted to: Western Section of Animal Science Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 5/15/2004
Publication Date: 6/15/2004
Citation: Walker, R.S., Enns, R.M., Geary, T.W., Wamsley, N.W., Downing, E.R., Mortimer, R.G., Lashell, B.A., Zalesky, D.D. 2004. Fertility in beef heifers synchronized using a modified co-synch plus cidr protocol with or without gnrh at timed ai. Western Section of Animal Science Proceedings 55:3-6. Interpretive Summary: Currently, there is no consistent TAI synchronization protocol that exists for controlling ovulation in beef heifers. In the current study, pregnancy rates were not improved for heifers receiving an additional GnRH injection at TAI at two locations, but were improved at the third location. Producers may be able to achieve acceptable pregnancy rates in beef heifers using a synchronization protocol that utilizes 54 h TAI, without estrous detection, with a CIDR insert plus GnRH at CIDR insertion and PG. Administering the second GnRH injection at timed AI may not improve pregnancy rates to AI, but guard against low pregnancy rates.
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) from three locations (Colorado [CO], Wyoming [WY] and South Dakota [SD]) were stratified by BW within BCS and randomly allotted to one of two treatments. All heifers received 100 µg of GnRH with a CIDR insert on day 0, followed by CIDR removal and 25 mg of PGF2' on day 7. At 54 hours post PGF2', heifers in the control (CON) and treatment (TRMT) groups were mass mated and heifers in the TRMT group were given a second injection of GnRH at that time. Blood samples were collected in heifers at d -10 and 0 to determine cyclicity status at CO and WY. Ultrasonography was used to determine percentage of heifers ovulating 40 h after TAI at the CO and WY locations. Cyclicity rates were higher (P < 0.01) for heifers at CO (97.4 %) vs WY (46.4 %). Pregnancy rates were similar (P > 0.10) between treatment groups and for cycling and non-cycling heifers at CO and WY; however, pregnancy rates were higher (P < 0.05) for heifers in the TRMT (54.2 %) vs CON group (40.4 %) at SD. Body weight did not affect pregnancy rates for either treatment group across all locations (P > 0.10); however, pregnancy rates tended to decrease (P = 0.08) for heifers with body weights greater than 409.1 kg (39 %) vs heifers with body weights less than 409.1 kg (53.2 %) at SD. The percentage of heifers ovulating were similar (P > 0.10) between CO and WY and ovulation rates tended to be higher (P = 0.10) for heifers in the TRMT (81.3 and 73.9 %) vs CON (62.5 and 66.7 %) groups at CO and WY. We conclude that synchronizing beef heifers with a modified CO-Synch plus CIDR protocol induces ovulation in cycling and non-cycling heifers and produces acceptable pregnancy rates at 54 h TAI. The value of incorporating a second injection of GnRH at timed AI remains questionable.