Incidence of ovulation to GnRH at onset of 5-d CO-Synch + CIDR and impact on reproductive responses
The objective of this study was to determine how response to GnRH at the onset of the 5-d CO-Synch + CIDR protocol (5dCO) affected estrous response, follicular dynamics, and pregnancy success to timed-AI (TAI) in beef cows. Suckled primiparous (n = 95) and multiparous (n = 264) beef cows at 4 locations (1; n = 126, 2; n = 121, 3; n = 73, 4; n = 39) were enrolled in the 5dCO that consisted of GnRH (GnRH-1) and CIDR insertion on d -8, CIDR removal and two, 25-mg doses of PGF2α given concurrently on d -3, and TAI on d 0 concurrent with GnRH (GnRH-2). Estrus was detected twice daily from d -3 to 0. Estrous cyclicity (70.6%) was determined at locations 1 and 2 via assessment of circulating progesterone concentrations. Ovarian ultrasound was conducted on d -8, -3, 0, and 2. Ovulation to GnRH-1 was defined by the disappearance of a dominant follicle observed on d -8 and development of a new corpus luteum on d -3. Follicle diameter at GnRH-2 (d 0) was assessed and ovulation confirmed on d 2 via ultrasonography. Pregnancy to TAI was determined approximately 30 d after TAI via ultrasonography. Cows were classified as having ovulated (OV; n = 196) or not ovulated (NoOV; n = 163) to GnRH-1. Statistical analyses were conducted using the MIXED and GLIMMIX procedures of SAS with location included as a random variable. Response to GnRH-1 (54.6%) was not influenced by parity (multiparous versus primiparous) or estrous cyclicity status. Estrus prior to TAI was greater (P < 0.05) in NoOV (47.6%) than OV (40.8%) cows. In cows that displayed estrus, interval from CIDR removal to estrus tended to be greater (P = 0.07) in OV (64.4 ± 0.9 h) than NoOV (60.6 ± 1.0 h), and was greater (P < 0.01) in multiparous (64.4 ± 0.8 h) than primiparous (58.3 ± 1.4 h) cows. Ovulation to GnRH-1 did not impact follicle diameter at GnRH-2. Pregnancy rate to TAI was greater (P< 0.05) in NoOV (65.0%) than OV (51.5%), primiparous (68.4%) than multiparous (53.8%) cows, and those cows that did (63.9%) than did not (52.7%) exhibit estrous. In summary, ovulation in response to GnRH-1 at the onset of the 5dCO protocol reduced estrous response and TAI pregnancy rates in suckled beef cows.
Timed-AI, Beef Cow, 5-day CO-Synch + CIDR, GnRH