Submitted to: Journal of Animal Science Supplement
Publication Type: Abstract Only
Publication Acceptance Date: 4/1/2003
Publication Date: 6/1/2003
Citation: HALL, J.B., DEJARNETTE, J.M., WHITTIER, J.C., GEARY, T.W. TIMING OF INSEMINATION AND GNRH ON PREGNANCY RATES IN BEEF COWS IN A MODIFIED CO-SYNCH ESTROUS SYNCHRONIZATION SYSTEM. JOURNAL OF ANIMAL SCIENCE SUPPLEMENT ABSTRACT #206. 2003. v. 81(Suppl. 1). p. 51. Interpretive Summary:
Technical Abstract: The objective of this study was to examine the effect of timing of fixed time AI (TAI) in the CO-Synch protocol on pregnancy rates in cows not displaying estrus by 48 h after prostaglandin. Postpartum beef cows (n = 825) from four herds were synchronized with an injection of GnRH (100 ug, Cystorelin®) on d 0 followed by PGF (25 mg, Lutalyse®) on d 7. Estrus detection was performed twice daily from d 6 to d 9. Cows detected in estrus (HD) were bred by AI 12 h after estrus. At 48 h after PGF, all cows not detected in estrus were randomly assigned by parity and days postpartum (74.5 +/- 0.5 d) to TAI at 48 h (TAI48) or 64 h (TAI64). All TAI48 and TAI64 cows received an injection of GnRH (100 ug) at AI. Body condition score (BCS; 5.2 +/- 0.02) was recorded at initiation of synchronization. Pregnancy was determined by ultrasonography between d 45 and d 60 post AI. Pregnancy rates to TAI were different across herds (herd x TAI, P < 0.05). In three herds, pregnancy rates were similar (P > 0.3) between TAI48 and TAI64 cows; whereas, in one herd pregnancy rate was greater (P < 0.002) in TAI64 than TAI48 cows. Across all herds, pregnancy rates were greater (P < 0.001) in cows bred after HD (66.8%; 129/193) compared to cows bred TAI48 (40.7%; 129/317) or TAI64 (41.3%; 130/315). Overall pregnancy rate for all AI times was 47.0% (388/825). The percentage of cows bred after HD was influenced by herd (P < 0.01) with a range of 5.3 to 26.6%. Pregnancy rate to timing of AI was not affected (P > 0.10) by BCS or days postpartum. We conclude that, in the CO-Synch protocol, TAI with GnRH at 48 h or 64 h after PGF results in acceptable AI pregnancy rates for cows not detected in estrus. Because TAI at 64 h would allow more cows to be inseminated following a detected estrus, heat detection + TAI64 may improve overall AI pregnancy rates.