REPRODUCTIVE-PERFORMANCE OF LACTATING DAIRY-COWS FOLLOWING ESTRUS SYNCHRONIZATION REGIMENS WITH PGF2-ALPHA AND PROGESTERONE

The objective of this study was to evaluate the reproductive performan ce of lactating cows in seasonal dairy herds after estrus synchronizat ion with PGF(2 alpha) (PG) with or without supplementation with proges terone (P4). In Trial 1, synchronized cows (S1; n = 521) were compared with untreated control cows (C; n = 518) in 5 herds. Estrus of cows i n the S1 group was synchronized with 2 treatments of PG (Lutalyse) 13 d apart. The breeding season started 2 d after the second PG. Cows wer e first bred by AI for 7 wk and then herd sires were used. Compared wi th C cows, estrus synchronization in the treated cows reduced the conc eption rate to first AI(61.1 vs 70.5%; P < 0.01) and the intervals fro m start of the breeding season to conception for cows conceiving to AI (11.0 vs 14.6 d; P < 0.05) or to both AI and natural mating (16.5 vs 18.4 d; P < 0.05). There was no effect on conception rate to second AI (68.8%), on pregnancy rate by Day 24 (72.3%) or Day 49 (86.3%) of the breeding season, or on the percentage of cows not pregnant at end of the breeding season (5.0%). In Trial 2, effects of P4 supplementation before the second PG on reproductive performance were evaluated in 4 h erds. Estrus of each cyclic cow was synchronized with PG as in Trial 1 . Half of the cows in each herd were treated with an intravaginal P4 d evice (CIDR) for 5 d before the second PG (S2+P4, n = 608), whereas th e remaining half received no CIDR treatment (S2, n = 593). Compared wi th S2 cows, P4 treatment increased the estrous response rate to the se cond PG (89.6 vs 82.9%; P < 0.01), the conception rate to first Al (65 .1 vs 59.7%; P = 0.07), the pregnancy rate by Day 6 of the breeding se ason (59.3 vs 49.0%; P < 0.001), and reduced the intervals from start of the breeding season to conception for cows conceiving to AI (8.6 vs 10.4 d; P < 0.10) or to both AI and natural mating (12.7 vs 16.4 d; P < 0.01). Treatment with a used CIDR from Days 16 to 21 after start of breeding to resynchronize returns to service had no effect on concept ion rate to first or second AI but may decrease the conception rate to second AI in cows previously treated with CIDR. In conclusion, estrus synchronization with the double PG system can reduce fertility, while P4 supplementation for 5 d before the second PG can improve estrous r esponse and overall reproductive performance. Stage of the estrous cyc le at the time of the second PG can affect fertility following synchro nization. (C) 1997 by Elsevier Science Inc.