18 THE EFFECTS OF CIDR AND eCG TREATMENTS IN A GnRH-BASED PROTOCOL FOR TIMED AI OR EMBRYO TRANSFER ON PREGNANCY RATES IN LACTATING BEEF COWS

Two experiments were conducted to determine the effects of the addition of a progestin, equine chorionic gonadotropin (eCG), or both, in a GnRH-based protocol for timed AI (TAI) or timed embryo transfer (TET). In both experiments, Angus, Gelbvieh, and Simmental cross-bred cows were randomized by breed and postpartum interval [50 ± 10 days (mean ± SD); range, 27 to 89] into 4 groups in a 2 × 2 factorial design. All injections were given IM. In Experiment 1, 288 cows (89.6% cycling) were given 25 mg dinoprost (PGF; Lutalyse®; Pfizer Animal Health, Montreal, Quebec, Canada) on Day –11; on Day 0, they were given 100 µg GnRH (Cystorelin®; Merial Canada, Pointe-Claire, Quebec, Canada), with or without concomitant insertion of a CIDR (1.9 g progesterone; Pfizer Animal Health, Montreal, Quebec, Canada). On Day 7, CIDR inserts were removed and cows were given PGF, with or without concomitant injection of 400 IU of eCG (Pregnecol®; Bioniche Animal Health, Belleville, Ontario, Canada). On Day 9 (54-56 h after PGF), TAI was done, with concomitant injection of 100 µg GnRH. Ultrasonographic examination of 147 cows on Day 7 revealed that 62.4% had ovulated in response to the first GnRH. Pregnancy rates (ultrasonographic examination) on Day 38 did not differ between cows with or without a CIDR (52.9 and 51.4%, rspectively; P = 0.64), with or without eCG treatment (53.5 and 50.7%, respectively; P = 0.28), in cycling vs. anestrous cows (51.6 vs 56.7%, respectively; P = 0.76), and in cows that had ovulated (58.1%) or did not ovulate (50.0%) after the first GnRH treatment ( P = 0.58). In Experiment 2, 151 cows were given 500 µg cloprostenol (PGF; Estrumate®; Schering–Plough Animal Health, Pointe-Claire, Quebec, Canada) on Day –12, 100 µg GnRH on Day 0, with or without concomitant insertion of a CIDR. On Day 3, half of the cows were given 400 IU eCG. On Day 7, CIDRs were removed and cows were given PGF; on Day 9 (54–56 h after PGF), all cows were given 100 µg GnRH. On Day 15, ultrasonography was done to select suitable recipients for transfer of frozen–thawed embryos on Day 16 (part of another experiment, balanced across synchronization groups). Recipient selection rates did not differ whether cows received or did not receive a CIDR (93.4% vs 85.5%, respectively; P = 0.27) or eCG (91.0 vs 87.8%, respectively; P = 0.67). In addition, pregnancy rates on Day 43 did not differ whether cows received or did not receive a CIDR (32.3 vs 32.4%, respectively; P = 0.52) or eCG (35.2 and 29.2%, respectively; P = 0.21). In summary, the addition of a CIDR or eCG to a GnRH-based synchronization protocol initiated after PGF presynchronization in lactating beef cattle yielded no improvement in pregnancy rates following TAI, or recipient selection and pregnancy rates following TET.

Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.