Abstract

The objective was to investigate the use of Cue-Mate and porcine LH (pLH) or GnRH with or without presynchronization on pregnancy rate following timed AI (TAI) in Angus heifers (n = 462). Approximately half of the heifers (Control; n = 236) were treated at random stages of the estrous cycle, and the other half (Presynch; n = 226) received two injections of 500 µg cloprostenol (PGF: Estrumate^®; Schering Plough Animal Health, Pointe-Claire, Quebec, Canada) 14 days apart; synchronization treatments were initiated 11 days after the second injection of PGF. On Day 0, heifers received an intravaginal progesterone device (Cue-Mate; Bioniche Animal Health, A/Asia Pty, Armidale, Australia) and were treated with 100 µg GnRH IM (Cystorelin^®; Merial Canada Inc., Victoriaville, Quebec, Canada; n = 233) or 12.5 mg pLH (Lutropin-V^®; Bioniche; n = 229). On Day 7, Cue-Mates were removed and heifers were given PGF IM. GnRH or pLH (same as the first treatment) was given concurrently with TAI on Day 9 (54–56 h after PGF) with frozen–thawed semen of one of 3 sires. Ultrasonographic examinations were performed in a subset of 182 heifers on Days 0 and 7 for CL and follicle development, and in all heifers on Days 41 to 49 for confirmation of pregnancy. Data were compared using CATMOD procedures in the Statistical Analysis System (SAS Institute, Inc., Cary, NC, USA). The proportion of cycling heifers on Day 0 was 90.7% (165/182). Pregnancy rate tended to differ among bulls (46.7, 56.9, and 61.4% for Bulls A, B, and C, respectively; P≤ 0.1). Heifers that ovulated in response to the first GnRH or pLH injection had a higher pregnancy rate than those that did not ovulate (66.3 vs. 51.9%; P ≤ 0.03). In addition, heifers treated with GnRH tended to have a higher ovulatory response to the first treatment and a higher pregnancy rate to TAI than those treated with pLH (60.9 and 60.5% vs. 50.0 and 52.4%, respectively; P ≤ 0.09). Although ovulatory response to the first GnRH or pLH treatment was 46.5 and 63.5% for Control and Presynch groups, respectively (P ≤ 0.02), pregnancy rates did not differ (59.8 vs. 53.1%; P ≥ 0.2). However, there was an interaction between presynchronization treatment and ovulatory response to the first injection of GnRH or pLH on pregnancy rates (P ≤ 0.02). The pregnancy rate was higher in Control heifers that ovulated (77.5%) to the first injection of GnRH or pLH than in Control heifers that did not ovulate (52.2%) or Presynch heifers that did (59.0%) or did not (51.4%) ovulate. In summary, Cue-Mate-treated heifers that ovulated in response to the first GnRH or pLH treatment had higher pregnancy rates to TAI. Although presynchronization with PGF increased the ovulation rate, it did not significantly affect the pregnancy rate.