178 Optimization of the time of insemination and presynchronization treatment in beef heifers submitted to fixed-time artificial insemination

Progesterone (P₄) administration followed by a 48-h withdrawal period before initiation of a 6-day CO-Synch enhances ovulatory response to GnRH-1 and fertility to fixed-time AI (FTAI) in beef heifers. However, proper timing of insemination and the need for P₄ withdrawal have not been adequately evaluated. Thus, the objectives were to evaluate the effect of (1) time of AI after presynchronization and (2) presynchronization with or without P₄ withdrawal on expression of oestrus and fertility to FTAI for first service in yearling heifers. In Exp. 1, 225 Bos taurus beef heifers were randomly assigned to receive a 6+6 CO-Synch with FTAI at 60 or 72 h. Briefly, a once-used intravaginal P₄ device (CIDR, 1.38 g of P₄, Zoetis) was administered on Day −17 and removed on Day −11 concurrent with administration of 500 µg of cloprostenol sodium (PGF, Parnell). On Day −9, heifers were administered 100 µg of gonadorelin acetate (GnRH, Parnell) and a new P₄ device was inserted. On Day −3, the P₄ device was removed, 1000 µg of PGF administered, and an oestrus detection patch fitted. Fixed-time AI was performed 60 or 72 h after P₄ device removal (Day 0) using conventional semen, concurrent with GnRH administration. Pregnancy was determined at 31 and 96 days. In Exp. 2, 166 B. taurus beef heifers were randomly assigned to receive a presynchronization treatment with (6 + 6 CO-Synch) or without (6&6 Synch or 8&6 Synch) P₄ withdrawal before initiation of a 6-day CO-Synch. Heifers received a new P₄ device on Day −17 (8&6 Synch and 6+6 CO-Synch) or Day −15 (6&6 Synch), concurrent with 500 µg of PGF. On Day −11, the P₄ device was removed from heifers in the 6+6 CO-Synch group. On Day −9, all heifers received GnRH, and heifers in the 6+6 CO-Synch group had a once used P₄ device inserted. On Day −3, P₄ devices were removed, 1000 µg of PGF administered, and an oestrus detection patch fitted. Fixed-time AI, using conventional semen, and GnRH administration was performed 60 h after P₄ device removal. Pregnancy status was determined at 33 days. Data were analysed using generalized linear mixed models and results are depicted in Table 1. In Exp. 1, the percentage of heifers displaying oestrus tended (P = 0.08) to be greater when AI was performed at 72 h. Conversely, there was no difference in P/AI or pregnancy loss between heifers receiving AI at 60 or 72 h. In Exp. 2, there was no effect (P > 0.2) of treatment on either expression of oestrus or P/AI. In conclusion, preliminary results indicate neither time of AI nor presynchronization with or without P₄ withdrawal before GnRH-1 of CO-Synch affected fertility in yearling beef heifers.