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
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