122 Fertility in lactating dairy cows following timed embryo transfer with fresh in vitro-produced embryos derived from conventional or sex-sorted sperm

The aim of this study was to compare pregnancy rates in lactating dairy cows following timed embryo transfer (TET) with fresh in vitro-produced (IVP) embryos produced using oocytes collected from either beef or dairy donors and fertilized using either conventional (CV) or sex-sorted (SS) sperm cells. Oocytes were collected once per week for up to 4 successive weeks from Holstein-Friesian dairy donors and Angus beef donors using transvaginal ovum pickup. Following in vitro maturation and IVF, single grade 1 blastocysts were transferred fresh to lactating dairy cow recipients in seven herds. If an insufficient number of fresh embryos were available on the day of transfer, frozen embryos from a previous study were used. A total of 638 recipients were synchronized using a 10-day Progesterone-Ovsynch protocol, and randomly assigned to receive CV-DAIRY (n = 64), SS-DAIRY (n = 97), CV-BEEF (n = 156) or SS-BEEF (n = 154) on Day 7 after presumptive oestrus. On the day of scheduled ET, recipient reproductive tracts were examined by transrectal ultrasound and 127 cows (19.9%) were rejected (inadequate or absent corpus luteum, luteal cyst, uterine infection). In total, 510 recipients received TET, of which 471 received a fresh embryo and 39 received a frozen embryo. After removing three cows from the study due to ill health, culling, and missed pregnancy diagnosis, 507 records were in the final data set. For all cows that had not returned to oestrus, pregnancy status was diagnosed on Day 32 to 38, and cows diagnosed pregnant were re-examined on Day 63. Pregnancy outcome data were analysed using generalised linear mixed models. The initial analysis compared fresh vs frozen TET, and thereafter the data set was restricted to cows that received fresh TET to examine effects of embryo breed and type of semen used for fertilization. Embryo breed (beef vs dairy), semen (CV vs SS) and two-way interaction between embryo breed and semen were included as fixed effects, and parity, days in milk and herd were included as covariate effects. Pregnancy per ET (P/ET) was lesser (P = 0.021) on Day 32 following transfer of frozen embryos compared with fresh embryos (37.0% vs 57.3%).In cows that received fresh ET, there were no differences in P/ET between ET-DAIRY and ET-BEEF (Day 32: 56.4% vs 57.4%, P = 0.89; Day 63: 49.5% vs 48.9%, P = 0.93), or between CV and SS (Day 32: 55.9% vs 57.9%, P = 0.68; Day 63: 46.2% vs 52.2%, P = 0.23). In cows that were diagnosed pregnant on Day 32, pregnancy loss between Day 32 and 63 was not different between ET-DAIRY and ET-BEEF (8.5% vs 12.8%; P = 0.46) or between CV and SS (13.6% vs 7.9%; P = 0.13). The interaction between embryo breed and semen was not significant for P/ET on Day 32 or 63, or for embryo loss between Day 32 and 63 (all P > 0.24). In conclusion, P/ET was similar for fresh dairy and beef breed ET and embryos derived from CV and SS.

This research was supported by the Department of Agriculture, Food and the Marine Research Stimulus Fund (Grant 2021R665).