The 9-d CIDR-PG Protocol: Evaluation of Synchrony of Estrus, Endocrine Parameters, Ovarian Dynamics, and Pregnancy Rates to AI in Comparison to the 7-d CO-Synch + CIDR Protocol

Response of mature beef cows following a recently developed long-term estrus synchronization protocol was characterized in reference to a short-term protocol commonly used in commercial production. Mature beef cows (n = 480) across four locations were assigned based on age and days postpartum to either the 9-d CIDR-PG protocol [25 mg prostaglandin F_2α (PG) and insertion of a 1.38 g progesterone insert (CIDR) on Day -28; 25 mg PG and CIDR removal on Day -19; and 25 mg PG on Day -3] or the 7-d CO-Synch + CIDR protocol [100 μg gonadotropin-releasing hormone (GnRH) and CIDR insertion on Day -10; 25 mg PG and CIDR removal on Day -3]. On Day -3, estrus detection aids (Estrotect^®) were applied, and a subset of cows (n = 203) were fitted with radiotelemetric, pressure-sensitive devices (HeatWatch^®) to characterize timing of estrus onset. Split-time artificial insemination (AI) was performed on Day 0 or 1 based on timing of estrus expression, with cows failing to express estrus by Day 1 administered 100 μg GnRH at AI. In Locations 1, 2, and 3, blood samples were collected for radioimmunoassay on Day -38 and Day -28 to determine pretreatment estrous cyclicity status based on serum progesterone concentrations. Additional blood samples were collected at PG administration and at AI for determination of serum estradiol concentrations. Transrectal ovarian ultrasound was performed at PG administration and AI for a subset of cows (n = 154) to assess ovarian follicle size. Response following the 9-d CIDR-PG protocol differed (P < 0.05) from the 7-d CO-Synch + CIDR protocol with respect to mean interval from PG to estrus onset (71.2 ± 1.3 h versus 56.5 ± 1.6 h), variance in interval to estrus, ovulatory follicle size at AI (13.7 ± 0.2 mm versus 14.6 ± 0.2), and serum estradiol concentrations at AI (6.9 ± 0.3 pg/ml versus 6.0 ± 0.3 pg/ml). Similar pregnancy rates to AI were obtained among cows following the 7-d CO-Synch + CIDR (66%; 163/247) and 9-d CIDR-PG (70%; 163/233) treatments. In summary, when compared among mature beef cows to the 7-d CO-Synch + CIDR protocol, the 9-d CIDR-PG protocol resulted in later timing and enhanced synchrony of estrus expression, greater serum estradiol concentrations and smaller follicle size at AI, and similar pregnancy rates following AI.