Effects of the presence of a dominant follicle and exogenous oestradiol onthe duration of the luteal phase of the bovine oestrous cycle

The presence of a developing dominant follicle may be a factor in the contr ol of the luteolytic cascade mechanism and the number of follicular waves d uring the bovine oestrous cycle. Ln this study, ovaries of all animals were examined once a day by transrectal ultrasonography. It was expected that h eifers (n = 18) would have two follicular waves if the second wave occurred later than day 10 after oestrus (Expt 1) and that cows (n = 14) would have three waves if the second wave occurred on or before day 10 (Expt 2). The objective of Expt 1 was to determine if absence of a large follicle late in the luteal phase delays luteal regression in heifers that are expected to have two follicular waves. Nine heifers were injected i.v. with 10 mi charc oal-treated bovine follicular fluid three times a day for 4 days, starting on the day after initiation of the second follicular wave, to delay growth of the second wave dominant follicle. Nine heifers were injected with 0.9% NaCl as controls. The duration of the luteal phase (calculated as the numbe r of days that serum progesterone was > 0.5 ng ml(-1)) was greater (P < 0.0 1) in the follicular fluid-treated group compared with the controls (18.7 v ersus 14.1 days). FSH and follicular growth were suppressed during the peri od of injection of follicular fluid (P < 0.01 and 0.03, respectively). The objective of Expt 2 was to determine the effect of increased oestradiol on the duration of the luteal phase in cows that were expected to have three f ollicular waves. Seven cows were injected i.m. three times a day for 4 days with 1 mi oestradiol (100 mu g ml(-1) in corn oil) and seven cows were sim ilarly injected three times a day with 1 mi 0.9% NaCl (control) starting th e day after cessation of growth of the second wave dominant follicle. Lutea l phase duration was shorter in oestradiol-treated animals than in the cont rols (14.0 versus 19.0 days; P < 0.04). Serum oestradiol concentrations wer e higher in the oestradiol-treated group during the period of injection (P < 0.01). in summary, luteolysis was delayed when follicular growth was supp ressed with follicular fluid (Expt 1). Exogenous oestradiol administration during the development of uterine oestradiol responsiveness initiated luteo lysis earlier compared with control animals (Expt 2).