181 Expression of estrus and fertility of different presynchronization strategies for fixed-time artificial insemination in suckled beef cows

Progesterone (P₄) administration followed by a short (48 h) P₄ withdrawal period enhances ovulatory response to GnRH-1 of CO-Synch and fertility to fixed-time AI (FTAI) in beef cows. However, P₄ withdrawal requires additional handling and thus increases cost. The objective of the present study was to evaluate the effect of presynchronization strategies, with or without P₄ withdrawal, on expression of oestrus and fertility to FTAI in beef cows. A total of 919 Angus and Angus-cross suckled beef cows in 12 breeding groups at 6 locations were utilised. At study onset, cows had an average age of 5.2 ± 2.9 years, body condition score of 5.6 ± 0.9 (scale 1 to 9), and postpartum interval of 62.0 ± 21.4 days. Cows, within breeding group, 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. Cows received a 1.38-g intravaginal P₄ device (CIDR, Zoetis) and PGF (500 µg of cloprostenol sodium, Parnell) on Day −17 (8&6 Synch and 6+6 CO-Synch) or Day −15 (6&6 Synch). On Day −11, the P₄ device was removed only from cows in the 6+6 CO-Synch group. On Day −9, all cows received GnRH (100 µg of gonadorelin acetate, Parnell) and cows in the 6+6 CO-Synch group had the previously used P₄ device reinserted. On Day −3, all cows had the P₄ device removed, PGF (1000 µg cloprostenol sodium) administered, and an oestrus detection patch fitted to assess expression of oestrus. Fixed-time AI was performed 72 h after P₄ device removal (Day 0), concurrent with GnRH administration. Pregnancy status was determined using ultrasonography at 33 and 91 days after FTAI. Data were analysed using generalized linear mixed models (SAS 9.4) and results are reported in Table 1. The percentage of cows with a corpus luteum on Day −17 was 42.9% and did not differ between groups (P = 0.46). Expression of oestrus was greater for cows in the 6&6 Synch and 8&6 Synch groups than for cows in the 6+6 CO-Synch (P = 0.01). Onset of oestrus was observed later for cows in the 6+6 CO-Synch group than cows in the 6&6 and 8&6 Synch groups (P = 0.0007). Pregnancies per AI (P/AI) at 33 days and 91 days, and pregnancy loss did not differ (P > 0.10) between groups. In conclusion, presynchronization strategies with or without short P₄ withdrawal before GnRH-1 of CO-Synch resulted in similar fertility despite differences in expression of oestrus. Thus, presynchronization strategies that do not require P₄ withdrawal, such as the 6&6 or 8&6 Synch, provide a more user-friendly alternative.