Follicular development in long-term anoestrous suckler beef cows fed two levels of energy postpartum

Follicular growth patterns were monitored daily by ultrasonography, from calving until completion of the first ovarian cycle postpartum in beef cows fed either a low (L; n = 11) or high (H; n = 12) energy diet. Mean cow body condition score at calving was 2.1 (range 1.75–2.25). The mean (± standard error of the mean) number of days postpartum to detection of the first follicle wave was 10.4±0.9 and 9.6±0.9 days for the L and H groups respectively (P>0.10). L diet cows tended to have more (P=0.09) medium-dominant follicle waves before first ovulation than H diet cows. Because the first two follicle waves postpartum consisted of the medium follicles, this resulted in a slightly longer (P>0.10) interval to detection of the first dominant follicle wave in the L cows than in the H cows. The total number of follicle waves before first ovulation was 10.6±1.2 anand 6.8±1.2 (P<0.05) for L and H cows, respectively. The growth rate, maximum diameter, persistence and regression rate of anovulatory dominant or medium-dominant follicles were not affected by level of diet (P>0.10) but did change (P<0.05) as wave number postpartum increased. Peripheral concentrations of oestrogen were low and were not related to follicular growth during the non-ovulatory period. The oestrogen concentrations were significantly higher immediately before first and second ovulations, and paralleled the growth of the first and second ovulatory follicles. The mean interval from calving to first ovulation was longer (P<0.05) for the L cows (95 ± 12 days) than for H (70 ± 12 days) cows. All of the cows on the L diet (11/11) had a short cycle following first ovulation while 8/12 of the H cows had a short cycle, 3/12 had a normal cycle and 1/12 had a long cycle after first ovulation. In summary, beef suckler cows with a prolonged postpartum interval to first ovulation have an early resumption of the development of dominant follicles whose growth characteristics are largely unaffected by plane of nutrition; these dominant follicles undergo recurrent development and atresia. In conclusion, the problem of prolonged postpartum anoestrus in beef cows is due to failure of ovulation of dominant follicles rather than a lack of development of dominant follicles.