Effects of Administration of Prostaglandin F2α 7 Days Prior to Initiation of the 7-Day CO-Synch + Cidr Protocol in Beef Heifers on Estrus Response and Pregnancy Rates

To determine the effects of administration of PGF_2α (25 mg of dinoprost tromethamine/mL, Lutalyse; Zoetis Animal Health) 7 days prior to the initiation of the 7-day CO-Synch + CIDR fixed-time AI (TAI) protocol, 985 Bos taurus beef heifers were enrolled at 9 locations from April 1 to July 15. Within location, all heifers were randomly assigned to one of two treatments: 1) CONTROL (n = 496); 100 µg injection of GnRH (Factrel; gonadorelin hydrochloride; Zoetis Animal Health) and a controlled internal drug releasing (EAZI-BREED CIDR; 1.38 g of progesterone; Zoetis Animal Health) insert for 7 d [d -7], administration of 25 mg of PGF_2α (Lutalyse, dinoprost tromethamine; Zoetis Animal Health) at CIDR removal [d 0], followed by a second injection of GnRH and fixed-time AI (TAI) 54 ± 2 h later; or 2) PRESYNCH (n = 489); same as CONTROL but heifers received an additional injection of 25 mg of PGF_2α 7 d prior [d -14] to CIDR insertion. Estrotect (Rockway Inc., Spring Valley, WI) estrus detection patches were applied to all heifers on d -14 and were evaluated for estrual activity on d -7. Similarly, estrus alert patches were placed on all heifers on d 0 and evaluated for estrual activity at the time of TAI. Pregnancy was diagnosed via transrectal ultrasonography between 35 and 55 d after TAI to determine the presence of a viable embryo, thereby assessing AI pregnancy rates The percentage of heifers exhibiting estrus between d -14 and d -7 was greater (P < 0.001) for the PRESYNCH (70.3%) than the CONTROL (40.0%) treatment, whereas the percentage of heifers exhibiting estrus between d 0 and TAI was significantly (P < 0.001) greater for the CONTROL (57.9%) than the PRESYNCH (41.4%) treatment. Estrus response rates differed (P < 0.001) among locations. Pregnancy rates to TAI differed (P = 0.023) among locations, however, did not differ (P =0.739) between CONTROL and PRESYNCH treatments (46.7 vs 45.5%, respectively). Final breeding season pregnancy rates did not differ (P = 0.811) between treatments. Therefore, an injection of PGF_2α 7 days prior to initiation of the 7-day CO-Synch + CIDR protocol failed to improve pregnancy rates to TAI in replacement beef heifers.