1 Time to conceptus attachment and subsequent pregnancy loss in seasonal-calving pasture-based lactating dairy cows following timed artificial insemination with conventional or X-sorted semen or timed embryo transfer with frozen–thawed in vitro-produced embryos

The objective of this study was to determine the timing of presumptive conceptus attachment (pCA) and subsequent pregnancy loss in lactating dairy cows following timed AI (TAI) with conventional (C) or X-sorted (S) semen or timed embryo transfer (TET) with frozen–thawed in vitro-produced (IVP) embryos. Lactating cows (n = 447, predominantly Holstein-Friesian) located on 3 herds were blocked based on parity, calving date and economic breeding index and randomly assigned to receive TAI-C (n = 148), TAI-S (n = 146) or TET with a single, grade 1 frozen–thawed IVP blastocyst (n = 153). Cows were synchronized with a 10-day Progesterone-Ovsynch protocol, and were either inseminated (TAI-C, TAI-S) 16 h after the second GnRH or received TET on Day 7 after the day of presumptive oestrus/TAI. On the day of scheduled embryo transfer, recipient reproductive tracts were examined by transrectal ultrasound and nine cows (5.8%) were removed from the study (absent or poor-quality corpus luteum, uterine infection). Thus, 144 recipients received TET. Serum pregnancy-specific protein B (PSPB) was measured in all cows that had not returned to oestrus on Days 7, 17, and 19 through 28 after expected ovulation. The first day of significant increase in PSPB (pCA) was calculated as an increase of ≥12.5% from a baseline value (on Day 17) followed by two more days of ≥12.5% increase from the previous day. Pregnancy was diagnosed in cows that had not returned to oestrus via ultrasound examination on Days 32, 62, and 120 postovulation, and calving data were recorded. Day of pCA data were analysed using generalized linear mixed models, including service treatment (TAI-C, TAI-S, TET) and cow live-weight as fixed effects and herd as a random effect. Binary outcomes were analysed using chi-squared tests. Mean (95% CI) day of pCA differed (P = 0.031) between TAI-C (20.1; 19.7, 20.5), TAI-S (20.2; 19.9, 20.6) and TET (20.6; 20.2, 20.9). Cows assigned to TET had later pCA than cows assigned to TAI-C (P = 0.027) and similar d of pCA to TAI-S (P = 0.176). Calving/service event (C/SE) was greater (83.1% v. 57.5%; P < 0.001) and pregnancy loss from pCA to calving was less (16.9% v. 42.5%; P < 0.001) for cows with early pCA (≤ d 20, n = 42/188) compared with cows that had late pCA (≥ d 21, n = 17/33). Pregnancy loss was greater for cows assigned to TAI-S and TET than TAI-C (P = 0.021). Calving/cow synchronized was similar (P = 0.632) for TAI-C (47.0%), TAI-S (41.9%), and TET (39.7%; including cows rejected for TET). Consistent with previous studies, time to pCA is an important measurement to understand pregnancy loss in lactating dairy cows and is affected by service type. The timing of pCA was later for TAI-S and TET compared with TAI-C, and may be associated with increased risk of pregnancy loss.

Supported by the Department of Agriculture, Food and the Marine Research Stimulus Fund (2021R665) and Science Foundation Ireland (16/RC/3835; VistaMilk).