Pregnancy rates following timed embryo transfer with fresh or vitrified in vitro produced embryos in lactating dairy cows under heat stress conditions

Abstract

Timed embryo transfer (TET) using in vitro produced (IVP) embryos without estrus detection can be used to reduce adverse effects of heat stress on fertility. One limitation is the poor survival of IVP embryos after cryopreservation. Objectives of this study were to confirm beneficial effects of TET on pregnancy rate during heat stress as compared to timed artificial insemination (TAI), and to determine if cryopreservation by vitrification could improve survival of IVP embryos transferred to dairy cattle under heat stress conditions. For vitrified embryos (TET-V), a three-step pre-equilibration procedure was used to vitrify excellent and good quality Day 7 IVP Holstein blastocysts. For fresh IVP embryos (TET-F), Holstein oocytes were matured and fertilized; resultant embryos were cultured in modified KSOM for 7 days using the same method as for production of vitrified embryos. Excellent and good quality blastocysts on Day 7 were transported to the cooperating dairy in a portable incubator. Nonpregnant, lactating Holsteins ( n=155) were treated with GnRH (100 μg, i.m., Day 0), followed 7 days later by prostaglandin F 2α (PGF 2α, 25 mg, i.m.) and GnRH (100 μg) on Day 9. Cows in the TAI treatment ( n=68) were inseminated the next day (Day 10) with semen from a single bull that also was used to produce embryos. Cows in the other treatments ( n=33 for TET-F; n=54 for TET-V) received an embryo on Day 17 (i.e. Day 7 after anticipated ovulation and Day 8 after second GnRH treatment). The proportion of cows that responded to synchronization based on plasma progesterone concentrations on Day 10 and Day 17 was 67.7%. Pregnancy rate for all cows on Day 45 was higher ( P<0.05) in the TET-F treatment than for the TAI and TET-V treatments (19.0±5.0, 6.2±3.6, and 6.5±4.1%). For cows responding to synchronization, pregnancy rate was also higher ( P<0.05) for TET-F than for other treatments (26.7±6.4, 5.0±4.3, and 7.4±4.7%). In the TET-F treatment group, cows producing more milk had lower ( P<0.05) pregnancy rates than cows producing less milk. In conclusion, ET of fresh IVP embryos can improve pregnancy rate under heat stress conditions, but pregnancy rate following transfer of vitrified embryos was no better than that following TAI.