Effect of interval between induction of ovulation and artificial insemination (Al) and supplemental progesterone for resynchronization on fertility of dairy cows subjected to a 5-d timed Al program

摘要

Objectives were to investigate 2 intervals from induction of ovulation to artificial insemination (AI) and the effect of supplemental progesterone for resynchronization on fertility of lactating dairy cows subjected to a 5-d timed Al program. In experiment 1, 1,227 Holstein cows had their estrous cycles presynchronized with 2 injections of PGF_(2α) at 46 and 60 d in milk (DIM). The timed AI protocols were initiated with GnRH at 72 DIM, followed by 2 injections of PGF_(2α) at 77 and 78 DIM and a second injection of GnRH at either 56 (OVS56) or 72 h (COS72) after the first PGF_(2α) of the timed AI protocols. All cows were time-inseminated at 72 h after the first PGF_(2α) injection. Pregnancy was diagnosed on d 32 and 60 after AI. In experiment 2, 675 nonpregnant Holstein cows had their estrous cycles resynchronized starting at 34 d after the first AL Cows received the OVS56 with (RCIDR) or without (RCON) supplemental progesterone, as an intravaginal insert, from the first GnRH to the first PGF_(2α). Pregnancy diagnoses were performed on d 32 and 60 after AI. During experiment 2, subsets of cows had their ovaries scanned by ultrasonography at the first GnRH, the first PGF_(2α), and second GnRH injections of the protocol. Blood was sampled on the day of Al and 7 d later, and concentrations of progesterone were determined in plasma. Cows were considered to have a synchronized ovulation if they had progesterone <1 and >2.26 ng/ mL on the day of Al and 7 d later, respectively, and if no ovulation was detected between the first PGF_(2α) and second GnRH injections during resynchronization. In experiment 1, the proportion of cows detected in estrus at AI was greater for COS72 than OVS56 (40.6 vs. 32.4%). Pregnancy per AI (P/AI) did not differ between OVS56 (46.4%) and COS72 (45.5%). In experiment 2, cows supplemented with progesterone had greater P/AI compared with unsupplemented cows (51.3 vs. 43.1%). Premature ovulation tended to be greater for RCON than RCIDR cows (7.5 vs. 3.6%), although synchronization of the estrous cycle after timed AI was similar between treatments. Timing of induction of ovulation with GnRH relative to insemination did not affect P/ AI of dairy cows enrolled in a 5-d timed AI program. Furthermore, during resynchronization starting on d 34 after the first AI, supplementation with progesterone improved P/AI in cows subjected to the 5-d timed AI protocol.