Evaluation of Gonadotropin-Releasing Hormone and Insemination Time Using the CO-Synch Protocol in Beef Cows

Professional Animal Scientist, Jun 2005 by Walker, R S, Burns, P D, Whittier, J C, Sides, G E, Zalesky, D D

Abstract

The objectives of this study were to evaluate optimal timing for timed AI (TAI) using a CO-Synch protocol with or without gonadotropin-releasing hormone (GnRH) at TAI and calf removal. Multiparous composite and Hereford suckling beef cows (n = 226) from two separate calving seasons (March and April or May and June) were synchronized for AL Early- (ECC) and late-calving cows (LCC) were randomly assigned to one of four treatment groups for a 2 × 2 factorial arrangement of treatments. All cows were injected with 100 µg of GnRH on a 0 followed by 25 mg of prostaglandin F^sub 2α^ (PGF^sub 2α^) on d 7. All calves were removed at unie of PGF^sub 2α^ injection and returned to nurse at TAI. One-half of the cows were inseminated at 48 h post-PGF^sub 2α^ with (48-TAI-G) or without (48-TAI) GnRH and the other one-half were inseminated at 72 h post-PGF^sub 2α^ with (72-TAI-G) or without (72-TAI) GnRH. Pregnancy rates to TAI were not affected (P>0.10) by season; therefore, data from both ECC and LCC were combined. Body condition and postpartum interval did not influence fertility (P>0.10); however, TAI pregnancy rates increased (P

(Key Words: Estrous Synchronization, Gonadotropin-Releasing Hormone, Timed AI, Beef Cows.)

Introduction

Estrous synchronization has become a powerful tool in managing the breeding season, but can become labor intensive when utilizing estrous detection for AI (Hixon et al, 2001). However, timed AI (TAI) utilized as an alternative method can produce pregnancy rates comparable with synchronization protocols that require detection of estrus (Geary and Whittier, 1998; Hixon et al., 2001; Perry et al., 2002). To utilize TAI, gonadotropin-releasing hormone (GnRH) was incorporated with prostaglandin F^sub 2α^ (PGF^sub 2α^) to coordinate follicular growth and luteal regression to synchronize ovulation (Select Synch; Geary et al., 2000). Administering GnRH induces a luteinizing hormone (LH) surge that stimulates luteinization or ovulation of dominant follicles (Twagiramungu et al., 1992; Pursley et al., 1995). A modified version of the Ovsynch protocol, CO-Synch (Geary and Whittier, 1998), incorporates a second injection of GnRH at 48 h TAI after PGF^sub 2α^ to induce ovulation. Pregnancy rates of remaining cows not inseminated by 72 h with a Select Synch protocol and TAI at 72 h were not improved when GnRH or no GnRH was administered at insemination (Geary et al., 200Ib). However, pregnancy rates from TAI were acceptable when GnRH was administered at 54 and 72 h post-PGF^sub 2α^ in beef cows (Twagiramungu et al., 1995a; Perry et al., 2002). Average interval to standing estrus is estimated to be 70 h after PGF^sub 2α^ injection (Geary et al., 2000). Although GnRH may or may not be needed at TAI, it may guard against reduced TAI pregnancy rates.

Calf removal has become an important part of breeding season management for synchronization programs and has increased in the US over the years. Geary et al. (200Ia) suggested that 48-h calf removal with the CO-Synch protocol induces fertile ovulation in cyclic and non-cyclic cows. Therefore, the objectives of this study were to determine whether pregnancy rates improved by delaying TAI to 72 h after PG in a CO-Synch protocol and to determine whether the addition of a second injection of GnRH at 72-h TAI would increase fertility by targeting the window of estrous synchrony with calf removal.

Materials and Methods

Animal Management. Suckling, multiparous composite and Hereford beef cows fn = 226; postpartum interval (PPI) = 67 ± 19.3 d; body condition score (BCS; 1- to 9- point scale where 1 = emaciated and 9 = obese) = 5 ± 0.4] were synchronized for TAI with the CO-Synch protocol and calf removal. Cows from the herd were separated by calving season 2 mo apart (March and April or May and June) for an ongoing calving season study. Cows from both calving seasons were treated within the same year and were analyzed equally. Early-calving cows (ECC; n = 97, PPI = 67 ± 16.2 d, BCS = 5.2 ± 0.4) and late-calving cows (LCC; n = 129, PPI = 67 ± 21.3 d, BCS = 4.9 ± 0.4) were given 100 µg (i.m.) of GnRH (Fertagyl; Intervet., Inc., Millsboro, DE) on d O and 25 mg (i.m.) of PGF^sub 2α^ (In-Synch; Pro Labs, St. Joseph, MO) on d 7. Calves from all cows were removed at PGF^sub 2α^ injection and returned to their dams the day of TAI. In a 2 × 2 factoral arrangement, animals were randomly assigned to one of four treatment groups: 1) 48-h TAI POSt-PGF^sub 2α^ without GnRH (48-TAI), 2) 48-h TAI POSt-PGF^sub 2α^ with GnRH (48-TAI-G), 3) 72-h TAI post-PGF^sub 2α^, without GnRH (72-TAI), and 4) 72-h TAI POSt-PGF^sub 2α^ with GnRH (72-TAI-G). Cows from both calving seasons were inseminated by one of three experienced AI technicians. Clean-up bulls were turned out 14 d after 72-h TAI and left with all cows for 60 d. All cows were diagnosed for pregnancy via transrectal ultrasonography 33 d following 72-TAI.

Statistical Analysis. Preliminary analysis revealed no significant differences in TAI pregnancy rates between treatments (48- vs 72-h TAI and GnRH vs no GnRH at TAI), treatment interaction, or AI technician for both calving seasons (ECC and LCC). Therefore, data from both ECC and LCC were pooled, and AI technician was not included in the final model. Effect of treatment, treatment interaction, sire, BCS, and PPI on TAI pregnancy rates were analyzed using PROC GENMOD in SAS® (SAS Inst, Inc., Gary, NC) with differences in TAI pregnancy rates between treatments, treatment interactions, and sire analyzed using least squares means. Significance was determined using Chi-square at P

Results and Discussion

Treatment interaction, BCS, and PPI among treatments on TAI pregnancy rates were not different (P>0.10), and TAI pregnancy rates for ECC (43.3%) and LCC (43.4%) were similar (P>0.10). Final pregnancy rates (pregnancy rates from TAI and natural service) for cows in the early- and late-calving seasons were 90.7 and 83.7%, respectively. The proportion of pregnant cows to TAI was less (P

Cows receiving a second injection of GnRH at 48 or 72 h post-PGF^sub 2α^ had greater pregnancy rates to TAI (50.8%; P85% ovulated in response to a second injection of GnRH 48 h after the Select Synch protocol (Thompson et al., 1999).

Pregnancy rates to TAI were greater (P0.10) in TAI pregnancy rates for cows in 72-TAI vs 48-TAI-G and 48-TAI groups; cows in 48-TAI group had the least fertility overall (Table 1). The addition of a second injection of GnRH might have induced ovulation in cows not responding to PGF^sub 2α^ because of low progesterone levels (regressing or developing corpus Iutea) or cows that were anestrous. The first injection of GnRH either luteinized smaller growing follicles or induced ovulation of a dominant follicle present on the ovary (Schmitt et al., 1996). If ovulation of a dominant follicle occurred as a result of the first injection of GnRH, development of new growing follicles in cows not responding to FGF^sub 2α^ were induced to ovulate when a second injection of GnRH was administered to cows at 48 and 72 h post-PGF^sub 2α^, possibly the reason for increased fertility observed with cows receiving GnRH vs no GnRH. Pregnancy rates to TAI were different for 72-TAI-G (62.7%) vs 72-TAI (43.6%), 48-TAI-G (39.3%). The 48-TAI (25.5%) group had fewer cows conceiving to AI. These results suggest that delaying TAI to 72 h post-PGF^sub 2α^, injection might improve TAI pregnancy rates (Geary et al., 2001b; Perry et al., 2002), and incorporation of a second injection of GnRH at TAI may increase the percentage of cows ovulating just after TAI (Wood et al., 2001).