Summary. Follicular growth rates were studied in 5 Hereford-Holstein cross heifers on Day 14 of the oestrous cycle. The granulosa cell mitotic index (MI) was measured in non-atretic antral follicles of various diameters (0·13–8·57 mm) from Bouin-fixed ovaries collected before (199, control) and 2 h after colchicine treatment (189, treated). In control ovaries, follicles of 0·68–1·52 mm had a higher MI than those of other size classes (P < 0·05). In colchicine-treated ovaries, the MI of follicles ranging from 0·68 to 8·57 mm increased more than that of other sized follicles, so that the mitotic time was shorter (0·78 h vs 1·32 h) in medium and large sized follicles (0·68–8·57 mm) than in smaller follicles (0·13–0·67 mm). Calculations based on the number of granulosa cells in follicles of various classes and from the time required to double the number of cells within a follicle indicate that a follicle takes 27 days to grow from 0·13 to 0·67 mm, 6·8 days from 0·68 to 3·67 mm and 7·8 days from 3·68 to 8·56 mm, indicating that growth rates varied with the size of the follicle. A period equivalent to 2 oestrous cycles would therefore be required for a follicle to grow through the antral phase, i.e. from 0·13 mm to preovulatory size. Increased MI, decreased mitotic time and increased atresia found in follicles larger than 0·68 mm could indicate a change in the follicular metabolism during its maturation.

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

Summary. Dairy heifers were superovulated in the presence (dominant group, N = 8) or absence (non-dominant group, N = 6) of a dominant follicle at the start of a superovulatory treatment on Days 7–12 of the oestrous cycle (Day 0 = oestrus). Daily ultrasonographic observations of ovaries (recorded on videotape) starting on Day 3 were used to assess the presence or absence of a dominant follicle (diameter > 9 mm, in a growing phase or at a stable diameter for < 4 days) and to monitor follicular development before and during treatment. The number of CL estimated by ultrasonography (7·1 ± 1·8 vs 13·5 ± 1·4) or by rectal palpation (6·9 ± 2·0 vs 16·3 ± 1·6) and mean progesterone concentrations (32·5 ± 19 vs 80·7 ± 16 ng/ml) after treatment were lower (P < 0·01) in the dominant than in the non-dominant group. Based on number of CL, two populations of heifers were identified in the dominant group, i.e. those that had a high (dominant–high, N = 4; > 7 CL) or a low (dominant–low, N = 4; < 7 CL) response to treatment. During treatment, the increases in number of follicles 7–10 mm and > 10 mm in diameter occurred sooner and were of higher magnitude in the non-dominant than in the dominant–high or dominant–low groups (P < 0·01). At the expected time of ovulation 6–7 days after the start of treatment, there was a rapid decrease in number of follicles 7–10 mm and > 10 mm in diameter in the dominant–high and non-dominant groups but not in the dominant–low group. Compared with the dominant–high group, differences in profiles of changes in diameter of largest (F₁) and second largest (F₂) follicles indicated that emergence of the dominant F₁ follicle before treatment was delayed by 1–2 days in the dominant–low group. These results suggest that the presence of a dominant follicle before superovulation treatment may decrease the superovulatory response and/or alter the maturation process of growing follicles during treatment, especially when emergence of the dominant F₁ follicle occurred within 3 days of the start of treatment.

Keywords: follicle; dominance; superovulation; ultrasonography; cattle

The effects of charcoal-extracted bovine follicular fluid (BFF) on endocrine profiles and follicular development in intact and hemiovariectomized postpubertal heifers were examined. Oestrus-synchronized heifers received Norgestomet implants on day 1 and 7 of treatment and were then injected s.c. with 11 ml saline (control) or 11 ml BFF twice a day for 12 days. The ovary bearing the largest follicle (OV₁) was removed on day 7 and the remaining ovary (OV₂) was collected on day 13. Follicles were observed by daily ultrasonography and were classified according to diameter (size 1: 2–3 mm; size 2: 4–6 mm; size 3: 7–10 mm; size 4: > 10 mm). After ovariectomy they were classified by diameter and histologically as normal or atretic. Intact control heifers had increased numbers of size 4 follicles on OV₁ on days 6 and 7; no increase was observed in BFF-treated heifers (P < 0.03). In BFF-treated heifers, the mean basal LH concentration was higher (P < 0.05) and that of FSH was lower (P < 0.04) than in controls. FSH concentrations in BFF-treated heifers decreased from 0.60 ± 0.08 ng ml⁻¹ (day 1) to 0.22 ± 0.05 ng ml⁻¹ (day 7; P < 0.04). The concentration of oestradiol increased in control heifers, but not in BFF-treated heifers (P < 0.001). After hemicastration, OV₂ underwent compensatory hypertrophy in control heifers, with an increase in the number of size 2, 3 and 4 follicles (P < 0.05), whereas BFF-treated heifers did not. Thus, total follicular volume was much lower in BFF-treated than in control heifers on day 13 (92.2 ± 15.4 versus 1393.8 ± 276.6 mm³; P < 0.0002). A transient increase in FSH (P < 0.006) and oestradiol (P < 0.01) concentrations occurred after hemiovariectomy in control but not in BFF-treated animals. In control heifers, an analysis of temporal relationships showed negative correlations between the volume of size 3 and size 4 follicles, and between FSH concentrations and the volume of size 3 and 4 follicles. A positive correlation was found between the mean diameter of the largest follicle and the concentration of oestradiol, whereas negative relationships were found between the concentrations of FSH and oestradiol, and between FSH and the mean diameter of the largest follicle. Analysis of the histological data showed that the number and volume of follicles > 8.57 mm was lower in the BFF-treated OV₁ ovary, whereas no differences were found for follicles ≤ 8.57 mm. An absence of or a lower number and volume of follicles ≥ 3.68 mm was observed in BFF-treated OV₂ compared with controls. We showed that: (i) compensatory hypertrophy in the remaining ovary occurs in heifers when the ovary bearing the largest follicle is removed; (ii) treatment with BFF decreases the FSH concentration and follicular development in intact postpubertal heifers, and prevents compensatory FSH increase and follicular growth after unilateral ovariectomy; and (iii) the histological population of follicles < 3.68 mm in diameter are not altered following 12 days of BFF treatment. Our results suggest that follicular development beyond 3–4 mm in cattle depends on an adequate circulating concentration of FSH.