Submitted to: Western Section of Animal Science Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/1/2001
Publication Date: 6/1/2001
Citation: GEARY, T.W., WHITTIER, J.C., MORTIMER, R.G., YOUNG, J.W., SALVERSON, R.R. SYNCHRONIZATION OF ESTRUS IN BEEF COWS USING GNRH AND PGF WITH ESTRUS AI ORTIMED AI 72 HR AFTER PGF WITH OR WITHOUT A SECOND GNRH INJECTION. WESTERN SECTION OF ANIMAL SCIENCE PROCEEDINGS. 2001. v. 52. p. 369-371. Interpretive Summary: There are several options available for timing of AI following synchronization of estrus with the Select Synch protocol. These options range from breeding only those cows that are detected in estrus (Select Synch), to breeding only with a timed insemination with a second injection of GnRH at 48 h following PGF (CO-Synch), to breeding by estrus and then time inseminating non-responding cows at a specific time. These data suggest that reasonable success can be achieved by breeding by estrus until 72 h after PGF and time inseminating cows not detected in estrus at 72 h. However, producers should also consider using estrous response up to 72 h to influence that decision. If the 72-h estrous response is high, producers will have bred the majority of cows by estrus and can expect reasonable success with timed AI. If the 72-h estrous response is low, it might be wise to continue to detect estrus and use AI for an additional 2 d rather than use timed AI at 72 h. In either case, addition of a second injection of GnRH at 72 h did not improve pregnancy rates in the current study.
Technical Abstract: This study was designed to evaluate the pregnancy response of cows that were artificially inseminated following synchronization of estrus with the Select Synch protocol and bred following estrus up to 72 h after PGF or time inseminated at 72 h with or without a second GnRH injection. Mature beef cows (n=994) from two herds received GnRH (100 mcg) 1 wk before an injection of PGF (25 mg) to synchronize estrus. Cows (n=257) in herd 1 (mean body condition score, BCS = 5.5, and postpartum interval to AI, PPI = 69 d) were synchronized and bred during the 1997-breeding season, and cows (n=737) in herd 2 (BCS = 4.9, PPI = 77 d) were synchronized and bred during the 1999-breeding season. Cows observed in estrus received AI approximately 12 h later (EAI). Cows not detected in estrus by 72 h after PGF were divided into two groups and time inseminated at 72 h with (TAI+GnRH) or without (TAI) a second injection of GnRH (100 mcg). The 72-h estrous response was 51% and 27% for cows during 1997 and 1999, respectively (P<0.01). Pooled across years, pregnancy rate of EAI cows (60%) was greater (P<0.01) than pregnancy rate of TAI+GnRH (33%) or TAI (27%) cows. During 1997, pregnancy rates were not different (P>0.10) between EAI (61%), TAI+GnRH (54%), and TAI (55%) cows. During 1999, pregnancy rates were higher (P<0.01) for EAI (59%) cows but did not differ (P>0.10) between TAI+GnRH (30%) and TAI (23%) cows. We conclude that incorporating a timed AI at 72 h after PGF for cows synchronized with the Select Synch protocol may be feasible if the 72-h estrous response is high, but caution against timed AI at 72 h in herds when the 72-h estrous response is low. Also, addition of a second injection of GnRH at 72 h may be unnecessary.