151 Influence of chorionic gonadotrophin, breeding procedure, and gonadotrophin-releasing hormone on pregnancy, embryo viability, and kidding rate of lactating Alpine goats time inseminated during the early transitional reproductive phase

Abstract

Oestrus and ovulation synchronisation (E/OS) regimens containing equine chorionic gonadotrophin (ECG) are used for follicular stimulation during the early transitional reproductive phase and goat anestrus. However, the effects of ECG when applied at different times are unknown. Thus, the objective was to evaluate the influence of ECG, breeding procedure, and gonadotrophin-releasing hormone (GnRH) on pregnancy, embryonic viability, and kidding rate in dairy goats. We used 41 alpine goats (mean ± standard deviation: 3.82 ± 1.2 years old; 57.0 ± 6.0 kg of bodyweight; 1.75-2.25 body condition score; 195 ± 16.7 days in milk; range of 1-4 kiddings). This study took place during the transitional phase (12 h of daylight, 12 h of dark) in September (35.9° N, 97.3° W) using controlled internal drug release intravaginal inserts (300 mg of progestagen; P4) for 11, 12, or 13 days with a 2-mL intramuscular dose containing 10 mg of dinoprost tromethamine at P4 removal. The E/OS protocols differed on the time when the 1-mL intramuscular dose of P.G. 600© (Merck Animal Health; 120 and 60 IU of ECG and human chorionic gonadotrophin, respectively) was given (i.e. early = 24 h before P4 removal; late = 24 h after P4 removal), and the traditional control group did not receive P.G. 600. Time breeding, either by laparoscopic-aided insemination using frozen-thawed semen (n = 28) or natural (n = 13), was scheduled 48 h after P4 removal. To improve embryo survival, 5 days post-breeding (dpb) 21 goats received 1 mL of GnRH analogue intramuscularly as 50 µg mL⁻¹ gonadorelin diacetate tetrahydrate, and 20 control goats received 1 mL of placebo intramuscularly. Pregnancy rate (PR) was evaluated at d 18-24 by non-return to oestrus, d 30 by pregnancy-specific protein B, and d 40 by ultrasound. Kidding rate was determined for females with at least one newborn. Logistic regression was used to establish statistical significance. Days of P4 exposure did not influence any response variable (P > 0.20). Table 1 summarises the results; age influenced all PR evaluations. The number of kiddings and administration of GnRH at 5 dpb had a significant effect on PR, whereas bodyweight, days in milk, E/OS protocol, and breeding procedure were not influential. Embryo viability and kidding rate were not influenced by any variable. In short, P.G. 600 had no bearing on any response variable analysed. Late-lactating goats can be evaluated early in the transitional breeding phase using PR at 18-24, 30, or 40 dpb. As indicated by the lack of effect on embryo viability at 30 and 40 days, the mechanism by which GnRH increases PR was not by decreasing embryo mortality.

Table 1.  Logistic model independent variable probabilities on response variables¹