摘要 : Most fixed-time insemination protocols utilize an injection of GnRH at the beginning of the protocol to initiate a new follicular wave. However, the ability of GnRH to initiate a new follicular wave is dependent on the stage of th... 展开 Most fixed-time insemination protocols utilize an injection of GnRH at the beginning of the protocol to initiate a new follicular wave. However, the ability of GnRH to initiate a new follicular wave is dependent on the stage of the estrous cycle. We hypothesized that administering PGF_{2 alpha} 3 d before initiating a fixed-time AI protocol would improve synchrony of follicular waves and result in greater pregnancy success. Therefore, our objective was to determine whether inducing luteal regression 3 d before a fixed-time AI protocol would improve control of follicular turnover and pregnancy success to fixed-time AI. Multiparous crossbred cows at 3 locations (n=108, 296, and 97) were randomly assigned to 1 of 2 treatments: (1) PGF_{2 alpha} [25 mg; intramuscularly (i.m.)] on d -9, GnRH (100 micro g; i.m.) and insertion of a controlled internal drug-releasing device (CIDR) on d -6, PGF_{2 alpha} (25 mg; i.m.) and CIDR removal with PGF_{2 alpha} (25 mg; i.m.) at CIDR removal on d 0 (PG-CIDR) or (2) GnRH (100 micro g; i.m.) and insertion of a CIDR on d -5 and CIDR removal with PGF_{2 alpha} (25 mg; i.m.) at CIDR removal and 4 to 6 h after CIDR removal (5-d CIDR). Cows were time-inseminated between 66 and 72 h (PG-CIDR) or 70 to 74 h (5-d CIDR) after CIDR removal, and GnRH was administered at the time of fixed-time AI. At location 1, ovulatory response to the first injection of GnRH was determined by ultrasonography at the time of GnRH and 48 h after GnRH administration. Among cows with follicles >=10 mm in diameter, more (<i>P</i>=0.03) PG-CIDR-treated cows ovulated after the initial GnRH injection (88%, 43/49) compared with the 5-d CIDR-treated cows (68%, 34/50). Pregnancy outcome was not influenced by location (<i>P</i>=0.96), age of the animal (<i>P</i>=1.0), cycling status (<i>P</i>=0.99), BCS (<i>P</i>=1.0), or any 2-way interactions (<i>P</i>>=0.13). However, pregnancy success was influenced by synchronization protocol (<i>P</i>=0.04). Pregnancy outcome was greater (<i>P</i>=0.04) for the PG-CIDR protocol (64%) compared with the 5-d CIDR protocol (55%). In summary, control of follicular turnover was improved by inducing luteal regression 3 d before initiation of a fixed-time AI protocol, and pregnancy success was improved with the PG-CIDR protocol compared with the 5-d protocol. 收起