Ten pluriparous mares were used as donors to supply embryos which were transferred into 103 recipients, 31 of which were nulliparous, 34 were pluriparous and lactating, and 38 were pluriparous and non-lactating. The embryos were recovered eight days after ovulation and pregnancy was confirmed by ultrasound six days after the transfer; the length of the embryos was measured ultrasonographically on days 12, 14, 16, 18, 20, 25 and 30 after the embryo transfer. One hundred and fifteen of 200 flushes provided embryos, 12 being degenerate and 103 being viable embryos. From the 103 embryo transfers carried out, 51 pregnancies were confirmed by ultrasound within 30 days; 16 of the 31 nulliparous recipients became pregnant, 16 of the 34 pluriparous lactating recipients and 19 of the 38 pluriparous non-lactating recipients. There were no significant differences between the groups of mares in the mean (sd) rate of growth of their embryos between 12 and 30 days of gestation.
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