131 Parity effects in embryo-transfer recipients: pregnancy rate by using in vitro-produced embryos in dairy cattle at high-altitude conditions

Reproductive function seems to be affected by high-altitude conditions in dairy cattle. The aim of the present research was to examine the parity effects on the pregnancy rate in dairy cattle recipients by using in vitro-produced embryos at high-altitude conditions. A total of 67 healthy Brown Swiss cows (parturitions 1–6; bodyweight: ∼450 ± 50 kg) located in Melgar province, Peru (Köppen-Geiger: ET; precipitation: ∼590 mm; relative humidity: ∼50%; M.T.: ∼7.9°C; latitude: 14°46′17″S/longitude: 70°54′11″W; altitude: ∼3,970 m.a.s.l.), were divided into two groups (low parturition [LP; n = 17] group: 1–3 and high parturition [HP; n = 50] group: 4–6 parturitions). All cows were synchronised using a conventional ‘Pre-Synch’ protocol based on two prostaglandin F2α applications (D-cloprostenol, Prostal, Over SRL) of a dose of 2.5 mL/cow/each. Good-quality immature COC were placed in maturation medium (TCM-199 supplemented with 10% [vol/vol] FCS) and cultured at 39°C for 22 h in a humidified atmosphere containing 5% CO₂. Matured COC were washed and placed in drops containing fertilisation medium (25 mM bicarbonate, 22 mM Na-lactate, 1 mM Na-pyruvate, 6 mg mL⁻¹ fatty acid-free bovine serum albumin, and 10 mg mL⁻¹ heparin). In vitro fertilisation was performed with frozen-thawed semen at a final concentration of spermatozoa mL⁻¹ at 39°C under 5% CO₂ during 20 h. Presumptive zygotes were denuded, washed, and transferred to 25-μL culture droplets (SOF + 5% FCS) at 39°C under 5% CO₂, 90% N₂, and 5% O₂ atmosphere with maximum humidity. The day of the embryo transfer (ET), the recipients’ body condition score (BCS; 1–5) was evaluated. Epidural anaesthesia (lidocaine 2%: 4 mL) was applied to the recipients and the corpora lutea diameter (CLD; mm) was scored by ultrasonography using a 6 MHz frequency linear transrectal transducer (4Vet Slim^®, DRAMIŃSKI S. A.). Day 7 in vitro-produced embryos were previously classified (grade I and II) before embryo transfer. The diagnosis of pregnancy was assessed by ultrasonography on Day 30 and 60 after ET for determining the presence or absence of the conceptus. Wilcoxon, chi-squared test, Spearman correlation, and binary logistic regression were performed (SPSS v. 25). Statistical differences were observed between groups regarding CLD (18.05 ± 0.59 vs 16.07 ± 0.53 in LP and HP, respectively; P = 0.02). No differences were observed in BCS between LP and HP groups (2.37 ± 0.08 vs 2.32 ± 0.11; P > 0.05). A significant positive correlation was observed between embryo quality and pregnancy success irrespective of the group analysed (ρ = 0.71; P = 0.01). There was a significant interaction between embryo quality and pregnancy success as well (P < 0.001). Finally, the pregnancy rates differed between parity groups (5/17 [29.4%] vs 8/50 [5.9%] in LP and HP, respectively; P < 0.05). In conclusion, the number of parturitions affected the corpora lutea quality, and the pregnancy rate at high-altitude conditions was better in the LP group (1 to 3 parturitions). Thus, low parity recipients together with high embryo quality were crucial factors for pregnancy success in dairy cattle at high-altitude conditions.