Comparison of artificial insemination versus embryo transfer in lactating dairy cows

Conception rates (CR) are low in dairy cows and previous research suggests that this could be due to impaired early embryonic development. Therefore, we hypothesized that CR could be improved by embryo transfer (ET) compared with AI. During 365 days, 550 potential breedings were used from 243 lactating Holstein cows (average milk production, 35 kg/day). Cows had their ovulation synchronized (GnRH-7d-PGF2α-3d-GnRH) and they were randomly assigned for AI immediately after the second GnRH injection (Day 0) or for transfer of one embryo 7 days later. Circulating progesterone concentrations and follicular and luteal size were determined on Days 0 and 7. Pregnancy diagnosis was performed on Days 25 or 32 and pregnant cows were reevaluated on Days 60–66. Single-ovulating cows with synchronized ovarian status had similar CR on Days 25–32 with ET (n = 176; 40.3%) and AI (n = 160; 35.6%). Pregnancy loss between Days 25–32 and 60–66 also did not differ (P = 0.38) between ET (26.2%) and AI (18.6%). When single (n = 334) and multiple (n = 57) ovulators were compared, independent of treatment, multiple ovulators had greater (P < 0.001) circulating progesterone concentrations on Day 7 (2.7 ng/ml versus 1.9 ng/ml) and there was a tendency (P = 0.10) for a greater CR in multiple ovulators (50.9% versus 38.1%). However, there was no difference in CR between AI and ET cows with multiple ovulations (50.0% versus 51.7%). In single-ovulating cows, CR tended to be lower for AI than ET in cows ovulating smaller follicles (diameter ≤ 15 mm; 23.7% versus 42.3%; P = 0.06) but not average-diameter follicles (16–19 mm; 41.2% versus 37.3%; P = 0.81) or larger (≥20 mm; 34.3 versus 51.0%; P = 0.36) follicles. Thus, although ET did not improve overall CR in lactating cows, follicle diameter and number of ovulating follicles may determine success with these procedures.