Summary. Lactating Friesian dairy cows (2nd–4th parity) which calved in spring (N = 7) or autumn (N = 15) were used. Their ovaries were examined by ultrasound scanning and blood samples were obtained daily for progesterone and oestradiol concentrations from the 5th day after calving until the first post-partum ovulation occurred. Five autumn-calving cows selected at random were bled every 15 min over a 6-h period on 1 day each week for 4 weeks after calving to assess the patterns of LH secretion. Follicular development during the post-partum anoestrous period was characterized by the growth and regression of small (⩽ 4 mm) and medium-sized (5–9 mm) follicles, until a dominant follicle (> 10 mm) was detected. The first detected dominant follicle ovulated in 14 cows, became cystic in 4 cows (all in autumn), and failed to ovulate in 1 cow. It was not possible to detect a dominant follicle in 3 cows due to scanning difficulties. The post-partum interval to detection of the first dominant follicle (mean ± s.d.) was shorter (P < 0·05) in autumn (6·8 ± 1·8 days) than in spring (20 ± 10·1 days). However, there was no significant difference between the respective intervals to first ovulation (autumn 27·4 ± 25·9 and spring 27·3 ± 18·9 days). Autumn-calved cows which had cysts had longer (P < 0·001) intervals to first ovulation (58·2 ± 23·5 days) than did normal cows (12·0 ± 2·5 days). All cows with cysts had twin ovulations at their first post-partum ovulation. A pulsatile pattern of LH secretion was detected in the first week post-partum and LH pulse frequency was 2–3 per 6-h period in Weeks 1 and 2 post partum and increased to 5–7 pulses per 6-h period in the presence of a dominant or cystic follicle. Concentrations of progesterone in plasma during post-partum anoestrus were usually low (< 0·2 ng/ml); oestradiol concentrations were also low (< 5 pg/ml), but higher values (5–110 pg/ml) were observed in cows that had a dominant or a cystic follicle.
Keywords: cow; follicle; oestrus; post partum; ultrasound
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