12 INITIAL GONADOTROPIN-RELEASING HORMONE TREATMENT INCREASED PREGNANCY PER TIMED INSEMINATION ONLY IN ACYCLIC BEEF HEIFERS SUBJECTED TO A 5-DAY CO-SYNCH PROTOCOL

Abstract

We have shown that inclusion of initial gonadotropin-releasing hormone (GnRH) in a 5-day Co-synch protocol is unnecessary to achieve acceptable pregnancy rates in dairy heifers (2011 Theriogenology 76, 578). This study determined the efficacy of a modified (without initial GnRH) 5-day Co-synch protocol for timed AI (TAI) and the effect of interval from progesterone device removal to TAI on pregnancy per TAI (P/TAI) in acyclic and cyclic beef heifers. Heifers (n = 1062; 302 to 515 kg of body weight, and 13 to 15 months of age) at 2 locations were assigned to either a 5-day Co-synch protocol with (control) or without (modified) an initial injection of 100 μg of GnRH (Fertagyl; Merck Animal Health, Kenilworth, NJ, USA) at progesterone device (CIDR; Zoetis Animal Health, Florham Park, NJ) insertion (Day 0). All heifers received a single dose of 500 μg of cloprostenol (Estrumate; Merck Animal Health) at CIDR removal (Day 5) and were divided into 2 groups to receive GnRH and simultaneously TAI at either 66 or 72 h (Day 8) after CIDR removal. All heifers were inseminated by one technician with frozen-thawed semen from 1 of 4 commercially available sires. Ovarian ultrasonography was performed on Day 0 to determine cyclicity (presence of corpus luteum) and normalcy of the reproductive track, and on Day 35 (27 days after TAI) to determine pregnancy status. Data were analysed using the PROC GLIMMIX in SAS (SAS Institute Inc., Cary, NC, USA). The CIDR retention rate was 98%; P/TAI in heifers that lost the CIDR was 39%. Pregnancy per TAI [least squares means (n)] did not differ (P > 0.05) among sires [48 (235), 51 (318), 52 (274), and 53% (235)], location [57 (330) v. 53% (732)], timing of TAI [52 (533) v. 50% (529) for 66 and 72 h], or treatment groups [48 (525) v. 53% (537) for control and modified groups]. A total of 226 (21%) heifers were acyclic; P/TAI was lower in acyclic heifers compared with that in those cyclic (P < 0.01; 43 v. 59%). Administration of GnRH at CIDR insertion increased P/TAI in acyclic heifers (P < 0.01; 50 v. 35% for control and modified groups) but did not affect P/TAI in those that were cyclic at initiation of protocol (P > 0.05; 57 v. 62% for control and modified groups). In conclusion, timing of TAI did not affect fertility. However, administration of initial GnRH increased P/TAI by 15 percentiles in acyclic females, but as previously shown in Holstein heifers, initial GnRH was unnecessary to achieve acceptable P/TAI in cyclic beef heifers subjected to a 5-days Co-synch protocol. This study was supported by Alberta Agriculture and Forestry, Intervet Canada Corp., and collaborative beef producers.