Animal Reproduction (AR)
Animal Reproduction (AR)
Original Article

Effects of short-term feeding of MGA combined with GnRH and PGF2α on reproduction of beef cows

S.Z. El-Zarkouny, D.J. Kesler

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Short-term feeding of melengesterol acetate (MGA) was used in two trials to improve synchrony of ovulation, increase pregnancy rates to first service TAI (experiment 1), and shorten the interval from PGF2α injection to estrus in beef cows (experiment 2). In experiment 1 a total of 192 beef cows were assigned to either the Co-Synch (n = 95) or the MGA: 7-11Synch + TAI (n = 97) protocols. MGA: 7-11Synch + TAI cows were fed MGA (0.5 mg/h/day) for 7 days and administered PGF2α on the last day of MGA feeding. On day -7, four days later, cows in both groups (Co-Synch and MGA: 7-11Synch + TAI) were administered GnRH followed with PGF2α on day 0, then GnRH was administered 48 h later and concurrently inseminated. Overall pregnancy rate was 97/192 (51%). Number of cows with functional CL at day 0 was greater (P > 0.001) in the MGA: 7- 11Synch + TAI group (94/97) than the Co-Synch group (76/95) suggesting that the MGA: 7-11Synch + TAI protocol successfully induced ovulations and formation of functional CL or luteinized follicles capable of responding to PGF2α. In experiment 2 purebred Angus cows (n = 109) and purebred Simmental cows (n = 63) were included in Select Synch (n = 84) or MGA: 7- 11Synch (n = 88) protocols. Cows in the MGA: 7-11Synch group were fed MGA (0.5 mg/h/day) for 7 days and PGF2α was administered on the last day of MGA feeding (day -11). On day -7, four days later, all cows (Select Synch and MGA: 7-11Synch) were administered GnRH followed with PGF2α on day 0. Within -24 to 96 h of PGF2α administration, 128/171 (75%) cows exhibited estrus and were inseminated. First service conception and pregnancy rates during the first synchronization period (day -1 to day 4) were not different (P > 0.10) between the two groups. Overall conception and pregnancy rates were 99/128 (77%) and 99/171 (58%), respectively. Cows in the Select Synch group had a shorter (P < 0.05) interval to estrus (54.5 ± 22.2 h) than cows in the MGA: 7-11Synch protocol (61.2 ± 14.9 h). The MGA: 7-11Synch protocol resulted in a higher degree of estrus synchrony (49/56, 88%) in a 24 h peak response period (48 to 72 h) after PGF2α injection compared to Select Synch cows (46/67, 69%). Tighter synchrony of estrus suggests that MGA: 7- 11Synch offers the potential for fixed-time AI programs.


beef cows, Co-Synch, MGA: 7-11Synch, Select Synch
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