Fertility in seasonal-calving pasture-based lactating dairy cows following timed artificial insemination or timed embryo transfer with fresh or frozen in vitro-produced embryos

The objective was to compare pregnancy per service event (P/S) in lactating dairy cows following timed artificial insemination (AI) or timed embryo transfer (ET) using either fresh or frozen in vitro-produced embryos. Oocytes were collected once per week for up to 9 wk using transvaginal ovum pick-up from elite dairy donors (ET -DAIRY; n = 40; Holstein -Friesian and Jersey) and elite beef donors (ET -ELITE -BEEF; n = 21; Angus). Both ET -DAIRY and ET -ELITE -BEEF donors consisted of heifers and cows. In addition, oocytes were collected from the ovaries of beef heifers of known pedigree following slaughter at a commercial abattoir (ET-COMM-BEEF; n = 119). Following in vitro maturation and fertilization, presumptive zygotes were cultured in vitro to the blastocyst stage. Grade 1 blastocysts were either transferred fresh or frozen for on -farm thawing and direct transfer. A total of 1,106 recipient cows (all lactating, predominantly HolsteinFriesian) located on 16 herdlets were blocked based on parity, calving date, and Economic Breeding Index, and randomly assigned to receive AI (n = 243) or ET (n = 863) after estrous synchronization with a 10-d Progesterone -synch protocol. Cows assigned to ET were further randomized to receive fresh (n = 187) or frozen (n = 178) ET -ELITE -BEEF embryos, fresh (n = 169) or frozen (n = 162) ET -DAIRY embryos, or fresh (n = 80) or frozen (n = 87) ET-COMM-BEEF embryos. Pregnancy was diagnosed using transrectal ultrasound on d 32 to 35 after synchronized ovulation and confirmed on d 62 to 65, at which time fetal sex was determined. Pregnancy per service event at d 32 was not different between AI (48.8%) and ET (48.9%) and did not differ between dairy and beef embryos (50.3% vs. 48.1%, respectively). However, P/S was less on d 32 following transfer of frozen embryos (41.6%) compared with fresh embryos (56.1%). Pregnancy loss between d 32 and 62 was greater for ET (15.1%) compared with AI (4.7%), with greater losses observed for frozen beef (18.5%), fresh beef (17.3%), and frozen dairy (19.2%) compared with fresh dairy (6.0%) embryos. Serum progesterone (P4) concentration on d 7 was associated with P/S at d 32 and 62. Cows in the quartile with the least serum P4 concentrations (quartile 1) had less probability of being pregnant on d 32 (33.4%) compared with cows in the 3 upper quartiles for serum P4 (45.7%, 55.6%, and 61.2% for quartile 2, quartile 3, and quartile 4, respectively). Sex ratio (male: female) at d 62 was skewed toward more male fetuses following ET (61.1:38.9) compared with AI (43.2:56.8) and was consistent with the sex ratio among in vitro blastocysts (61.2:38.8). In conclusion, P/S was similar for AI and ET, although pregnancy loss between d 32 and 62 was greater for ET than for AI.