PS2-28. Effect of New versus Previously Used CIDR Inserts on Follicular Dynamics in Postpartum Beef Cows Synchronized with a 7-day Co-synch + CIDR Protocol for First-service Timed AI.

The objective of this study was to compare the effects of a new versus previously used (7-day) CIDR insert on follicular dynamics in Angus beef cows using a 7-day Co-Synch + CIDR protocol for first-service timed AI. Postpartum Angus cows (n = 100) were synchronized with GnRH (0.1 mg gonadorelin i.m.) and CIDR insertion (1.38 g progesterone) on d0, followed by CIDR removal and PGF2α (0.5 mg cloprostenol i.m.) on d7, and timed AI with GnRH (0.1 mg gonadorelin i.m.) at 66 h post-removal. Cows were randomized on d 0 to receive a new CIDR (New; n = 52) or a previously used CIDR (Used; n = 48). Transrectal ultrasonography was performed on days 0, 7, 10, and 17 to measure follicle and corpus luteum (CL) diameters. Ovulation to d0 GnRH was determined by the presence of a new CL on d5, which previously had a large follicle on d0; whereas d10 ovulation was determined by the presence of a new CL on d17 that previously had a large follicle on d10. Data were analyzed using ANOVA in SAS (P ≤ 0.05 for significance). Ovulation on d0 (OV) was also included in the model and evaluated with a treatment (New vs. Used [TRT]) interaction (TRT*OV). The dominant follicle diameter on d0 was similar between New (11.25 ± 0.45 mm) and Used (11.66 ± 0.60 mm; P = 0.58), with ovulation rates of 58% (30/52) vs. 67% (32/48; P = 0.35). The large follicle diameters were also similar on d7 (CIDR removal; 10.85 ± 0.45 vs. 10.96 ± 0.41 mm; P = 0.87) and d10 (12.62 ± 0.41 vs. 12.52 ± 0.41 mm; P = 0.86). Ovulation post-d 10 was 90.4% (47/52) for New vs. 89.6% (43/48) for Used (P = 0.89). The CL diameter on d17 (7 days post-timed AI) was similar (New: 20.21 ± 0.60 mm; Used: 20.47 ± 0.60 mm). Pregnancy per AI was 69% vs. 50% for New vs. Used (P = 0.05). Cows were subdivided by d0 ovulation response: New_OV (n = 30), Used_OV (n = 32), New_notOV (n = 22), Used_notOV (n = 15). Dominant follicle diameter was similar among subgroups on d7 (New_OV 11.15 ± 0.68; Used_OV 11.27 ± 0.54; New_notOV 10.43 ± 0.51; Used_notOV 10.31 ± 0.55; TRT=0.99; OV = 0.19; TRT×OV=0.86) and d10 (New_OV 12.6 ± 0.61; Used_OV 12.7 ± 0.49; New_notOV 12.6 ± 0.54; Used_notOV 12.2 ± 0.75; TRT=0.76; OV = 0.71; TRT×OV=0.72). Ovulation after TAI was New_OVa 93% (28/30), Used_OVa 100% (30/30), New_notOVab 86% (19/22), and Used_notOVb 69% (11/16) (TRT=0.36; OV < 0.01; TRT×OV=0.04). In summary, regardless of initial GnRH-induced ovulation, used CIDRs resulted in similar follicular dynamics and CL development in postpartum beef cows under a 7-day Co-Synch+CIDR protocol. However, reduced fertility was observed in the present study. Larger trials needed to confirm these findings regarding ovulatory response and new versus used CIDRs.