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J. J. Dufour and G. L. Roy

Summary. Ovaries were obtained at slaughter from 12 Holstein dairy cows at 15, 25 or 35 days after their 4th calving. Non-atretic and atretic antral follicles were separated into 6 size classes according to size and the numbers in each class were expressed as a percentage of the total for each ovary. Non-atretic follicles of diameter 0·1 6–0·28 mm decreased from 27·5% at Day 15 to 1·5% at Day 35 whereas those of 0·29–0·67 mm and 0·68–1·57 mm diameter increased from 37·4 to 47·2% and from 11·5 to 17·3% respectively (all P < 0·05). The proportions of follicles measuring 1·58–3·68, 3·69–8·56 and > 8·56 mm remained almost constant. The atretic follicles of 0·29–3·68 mm varied significantly in number according to the post-partum interval and to whether they were in the ovary containing the CL of pregnancy. It is concluded that the CL of pregnancy and/or the conceptus have a carry-over effect on the rate of growth of the antral follicles even after parturition.

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L. A. Guilbault, G. L. Roy, F. Grasso, and P. Matton

Summary. Luteolysis was induced by an injection of 500 μg cloprostenol (a prostaglandin (PG) analogue) in pregnant (P) Holstein heifers on Days 17 or 24 of gestation and in non-pregnant (NP) Holstein heifers on Day 17 of the oestrous cycle (oestrus = Day 0). Heifers in Groups P-17 (N = 8) and P-24 (N = 8) were inseminated twice whereas those in Group NP-17 (N = 8) were not inseminated. Immediately after PG injection, embryos were recovered by uterine flushing (400 ml) to confirm pregnancy in Groups P-17 and P-24. Uterine flushing with an equivalent volume of physiological saline was also done in Group NP-17. The interval from PG injection to oestrus and to the peak of luteinizing hormone (LH) as well as profile of increase in plasma oestradiol concentrations during that period did not differ (P > 0·1) among the groups. However, the proportion of heifers exhibiting abnormal luteal phases (primarily of short duration) during the oestrous cycle after PG injection was greater (P < 0·01) in Group P-24 than in Groups NP-17 +P-17 pooled (6/8 vs 3/16). These results suggest that the previous presence of a conceptus did not have any effect on the onset of oestrus, or on plasma concentrations of oestradiol and LH after PG-induced luteolysis on Days 17 or 24 of gestation. However, luteal function during the subsequent oestrous cycle was impaired if heifers were 24 days pregnant when luteolysis was induced.

Keywords: pregnancy; luteolysis; conceptus; ovary; cattle