Rate of abnormalities in lambs from in vitro produced embryos transferred on Day 2 compared with Day 6 postfertilization.

The effect of transferring ovine IVP embryos on Day 2 versus Day 6 postinsemination was investigated. Oocytes were collected from 35 cull ewes and cultured separately for each donor. Embryos were exposed to serum in the maturation and fertilization media, and then cultured in a serum-free SOF system under serum-conditioned silicone oil. Cleaved Day 2 postfertilization embryos were randomly allocated for immediate transfer versus Day 6 transfer. Parturition was induced (with 15 mg dexamethasone) between Days 141 and 143 of pregnancy; those failing to initiate parturition by Day 146 had a Caesarean section. Stillborn lambs and those dying within several days of birth were necropsied. Lambs were weighed at birth; lambs (n = 132) derived from the same flock (from natural breeding or AI) were used as a control. An estimate of the 99th percentile of the normal population was used as the point above which lambs from three litter-size categories were classified as abnormally large (analysis was repeated for 95% confidence and median estimates of this value). Thirty-eight lambs were born from early transfer, 45 from late transfer and 12 from unobserved lambings (parentage could not be determined). Lambs from early transfer were heavier at birth (P = 6.0 x 10 (-11)). The early treatment group had fewer lambs produced per embryo transferred (P = 0.0002), fewer live lambs per embryo transferred (P = 0.00009) and fewer normal lambs per embryo transferred (P= 0.0002). There was an effect of donor on the number of dead lambs per embryo transferred (P = 0.017). The number of dystocias per transfer was not significantly different for early versus late transfer groups. However, the probabilities of survival for cleaved embryos were 27.3% and 13.1% in the early and late transfer treatment, respectively. There were more lambs produced per cleaved embryo with early transfer (P = 0.004) and there was an effect of donor within ram and replicate (P = 0.04) on the number of lambs produced. The odds ratio for early/late treatment was 2.449 with a confidence interval of 1.368-4.382. Day 2 transfer of cleaved embryos did not prevent the production of oversized or abnormal offspring; however, there was an increased chance of a cleaved embryo producing a lamb when transferred on Day 2 rather than Day 6 postfertilization.