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R. A. S. Lawson, R. A. Parr and L. P. Cahill

Summary. The fate of embryos transferred asynchronously in the ewe was investigated when the functional life of the corpus luteum was prolonged by both hemi-hysterectomy and by the presence of a second synchronously transferred embryo. The development of asynchronously transferred embryos was assessed at progressively later stages after transfer. Prolongation of luteal function did not enable asynchronously transferred embryos to persist. Embryos from Day 4 donors were found to be retarded in their rate of development when placed in 'younger' Day 1 or 2 uteri and appeared unable to develop beyond the early blastocyst stage. Conversely, embryos from Day 4 donors placed in 'older' Day 6 or 7 uteri showed accelerated growth and development which was maintained until the uterus reached Day 12. Thereafter further growth of the asynchronously transferred embryos was retarded, although synchronously transferred embryos then entered the phase of rapid blastodermic vesicle elongation. Asynchronously transferred embryos disappeared from the uterus when the ewe entered pro-oestrus.

The experiments demonstrate the existence of an active relationship between the embryo and the maternal environment during mid-cycle and an apparent lack of association between embryo size, growth rate and physiological maturation.

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R. A. Parr, I. F. Davis, R. J. Fairclough and M. A. Miles

Summary. The 330 Merino ewes used in the study were placed with rams at a synchronized oestrus and, on Days 2–14 after mating, the ewes were placed in a feed lot and fed daily a low, medium or high ration (25%, 100% or 200% of maintenance respectively). Progesterone supplement was given to some ewes on Days 8–14 after mating by using a device containing 340 mg progesterone. Blood samples were taken from all ewes on Day 12 for measurement of plasma progesterone concentrations. On Day 14 after mating all ewes were returned to pasture. Pregnancy rate was determined by returns to oestrus and was later confirmed using ultrasound.

There was a decline in the peripheral progesterone concentrations with increasing ration. The pregnancy rate in ewes fed a high ration was significantly reduced when compared with those of ewes fed a medium or low ration (48% vs 68 and 67% respectively; P < 0·05). In ewes fed the high ration exogenous progesterone increased the pregnancy rate from 48 to 76% (P < 0·01). Progesterone treatment did not influence pregnancy rates in ewes fed medium or low rations. The number of fetuses per ewe pregnant was not influenced by level of nutrition or progesterone treatment.

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Intermittent teasing of ewes with a ram has been reported both to increase (Zeltobrjuh & Rak, 1965) and to decrease (Parsons, Hunter & Rayner, 1967) the interval from the onset of oestrus to ovulation.

Parsons et al. (1967) suggested that the presence of the ram influenced the time of ovulation in the ewe by altering the rate of the secretion of the LHreleasing factor. Cumming, Buckmaster, Blockey, Goding, Winfield & Baxter (1971, 1973) investigated the relationship between the preovulatory release of LH and ovulation and found that ewes ovulated between 21 and 26 hr (mean 23·5 hr) after the release of LH. Blockey (1971), however, found this interval to vary from 23 to 55 hr, the mean being 30 hr. While Cumming et al. (1973) removed the ewes from contact with the ram during oestrus, Blockey (1971) used intermittent teasing.

It is possible that association of the sexes during oestrus