Abstract
Abstract Pregnant mares' serum (PMS, 500-1,500 i.u.) was injected into 149 Romney ewes on days 11, 12, or 13 of the oestrous cycle following induction of ovulation by injections of progesterone. The ewes were mated to fertile rams and recovery in vivo of ova attempted 55-91 hours after onset of oestrus. PMS hastened the onset of oestrus and caused super-ovulation. Marked variation in ovarian response existed, especially after injection of 1.500 i.u. PMS. The percentage of ova recovered was not associated with variation in number of corpora lutea, but the fertilisation rate decreased significantly when more than about 12 ovulations had occurred. The stages of cleavage of ova relative to interval from onset of oestrus were recorded. Recovery of ova at about the 8-cell stage was most successfully attempted 72 hours after onset of oestrus and when the ewes were observed at 8-hourly intervals.
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