|Hall, John - VIRGINIA TECH|
|Dejarnette, J - SELECT SIRES INC|
|Whittier, J - COLORADO STATE UNIV|
Submitted to: Western Section of Animal Science Proceedings
Publication Type: Proceedings
Publication Acceptance Date: April 1, 2003
Publication Date: June 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. WESTERN SECTION OF ANIMAL SCIENCE PROCEEDINGS. 2003. v. 54. p. 257-260. Interpretive Summary: In the CO-Synch protocol, fixed-time AI (TAI) with GnRH at 48 h or 64 h after prostaglandin results in acceptable AI pregnancy rates for cows not detected in estrus. Pregnancy rates to AI were greater in cows bred after detected estrus than TAI. Because TAI at 64 h would allow more cows to be inseminated following a detected estrus, estrous detection + TAI at 64 h after PG may improve overall AI pregnancy rates if estrous detection efficiency is high. We interpret these data to indicate some flexibility for producers in selecting the optimum TAI for cows based on estrous response from PG to 48 h later. If producers realize a high percentage of cows in estrus between PG and 48 h later, TAI at 48 h may be more appropriate. If producers do not detect > 10% of cows in estrus between PG and 48 h later, TAI at 60 h would probably result in higher pregnancy rates.
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.