12 EFFECT OF HUMAN CHORIONIC GONADOTROPIN (hCG) ADMINISTRATION ON DAY 2 OR DAY 5 AFTER OESTRUS ON PREGNANCY RATE IN HIGH-YIELDING DAIRY COWS

Abstract

In cattle, ~40% of embryonic loss occurs in the period from Day 8 to Day 16 of pregnancy. A significant proportion of embryo loss may be due to inadequate circulating progesterone (P4) concentrations. Low P4 concentrations have also been implicated as a causative factor in the low pregnancy rates (PR) observed in high-yielding dairy cows. Administration of hCG during the early luteal phase stimulates hypertrophy of the original corpus luteum (CL) and, depending on the day of administration, induces ovulation of the first-wave dominant follicle and formation of a functional accessory CL, which increases circulating P4 concentrations. The aim of this study was to examine whether administration of hCG on Day 2 or Day 5 after oestrus after timed AI (TAI) would lead to an increase in pregnancy rates in dairy cattle. Lactating Holstein cows (n = 194) from 12 commercial dairy herds in Southern Spain (37.8833° N, 4.7667° W) with an average milk production at 37.8 L/cow per day and typically with a PR to first AI of ~30% were randomly assigned based on their body condition score (2.65 ± 0.05; mean ± SEM), parity (2.60 ± 0.09), and days in milk (75.06 ± 0.63) to 1 of 3 treatments and administered a single intramuscular injection of 3000 IU of hCG (4 mL of Veterin Corion) either (1) on Day 2 = 36 h after TAI (n = 65; hCG2 group), (2) Day 5 = 108 h after TAI (n = 64; hCG5 group), or (3) 4 mL of saline on Day 2 = 36 h after TAI (n = 65; control group). Cows were synchronized using a 7-day Ovsynch TAI protocol that included a P4-releasing intravaginal device (PRID DELTA 1.55 g). First, gonadotropin-releasing hormone (Cystorelin 100 mg) treatment was administered at PRID insertion (Day 0) followed by 25 mg Dinoprost (prostaglandin F2α: Enzaprost T) on Day 7 at PRID withdrawal. Then, 56 h later, the second gonadotropin-releasing hormone (100 mg) treatment was administered and all cows were inseminated 16 h later. Pregnancy was diagnosed by ultrasonography 28 to 32 days after TAI. Logistic regression model and chi-squared test were used to analyse data. Pregnancy rate to AI was significantly higher in the hCG2 and hCG5 groups than in the control group (43.1 and 45.3%, v. 27.7%; P < 0.05). A treatment-by-parity interaction was observed; while pregnancy rate for primiparous cows was not affected by treatment, multiparous cows from the hCG2 group had greater pregnancies per AI than those in the control group (47.2% v. 21.1%, respectively; P < 0.05). In conclusion, these preliminary results suggest that hCG administration on Day 2 and 5 after oestrus increases PR at first postpartum AI in Holstein cows. In addition, hCG on Day 2 increases the fertility in multiparous cows.This study was funded by the Spanish Ministry of Science and Innovation (AGL2012–37510) and partially supported by Ceva Salud Animal S.A., Spain – synchronization protocol and DFV, Spain – hCG treatments.